KANSAS CITY PUBLIC LIBRARY 0000102080645 M pflggf. fcfl'g Keep Your in This Pocket PUBLIC LIBRARY Kansas City, No* * ffWUWffljff njlWr ^fWf ^p p| ^pp^f p ^Pl^^^pp' KANSAS CITY, MO PUBLIC LIBRARY RADIATION, LIGF;: AND ILLUMINATION A SERIES OF ENGINEERING LECTURES DELIVERED AT UNION COLLEGE BY CHARLES PROTEUS STEINMETZ, A.M., PH.D. COMPILED AND EDITED BY JOSEPH LaROY HAYDEN THIRD EDITION McGRAW-HILL BOOK COMPANY, INC. 239 WEST 39TH STREET. NEW YORK LONDON: HILL PUBLISHING CO., LTD. 6 & 8 BOUVERIE ST.. E. C. 1918 COPYRIGHT, 1909, 1918, BY THE McGRAW-HILL BOOK COMPANY, INC. AUTHOR'S PREFACE. THE following lectures were given as a course of instruction to the senior students in electrical engineering at Union University. They are however intended not merely as a text-book of illuminating engineering, nor as a text-book on the physics of light and radiation, but rather as an exposition, to some extent, from the engineering point of view, of that knowledge of light and radiation which every educated man should possess, the engineer as well as the physician or the user of light. For this purpose they are given in such form as to require no special knowledge of mathematics or of engineering, but mathematical formalism has been avoided and the phenomena have been de- X scribed in plain language, with the exception of Lectures and XI, which by their nature are somewhat mathematical, and are intended more particularly for the illuminating engineer, but which the general reader may safely omit or merely peruse the text. The lectures have been revised to date before publication, and the important results of the work of the National Bureau of Standards, contained in its recent bulletins, fully utilized. CHARLES PROTEUS STEINMETZ. SOHBNBCTADY, September, 1909. COMPILER'S PREFACE. A SERIES of eight experimental lectures on "Light and Radia- tion" were delivered by Dr. Steinmetz in the winter of 1907-8 before the Brooklyn Polytechnic Institute. Unfortunately no stenographer was present and no manuscript prepared by the A lecturer. far more extended course of experimental lectures was however given by Dr. Steinmetz at Union University in the winter of 1908-9, on "Radiation, Light, Illumination and Illu- " minating Engineering, and has been compiled and edited in the following. Two additional lectures have been added thereto by Dr. Stein- metz to make the treatment of the subject complete even from X the theoretical side of illuminating engineering: Lecture on "Light Flux and Distribution" and Lecture XI on "Light Intensity and Illumination." These two lectures give the element^ of the mathematical theory of illuminating engineering, With the exception of the latter two lectures the following book contains practically no mathematics, but discusses the subjects in plain and generally understood language. The subject matter of Lecture XII on "Illumination anc Illuminating Engineering" has been given in a paper before th< Illuminating Engineering Society; the other lectures are ne^ in their form and, as I believe, to a considerable extent also ir their contents. In describing the experiments, numerical and dimensiona data on the apparatus have been given, and the illustration! drawn to scale, as far as possible, so as to make the repetitioi of the experiments convenient for the reader or lecturer. Great thanks are due to the technical staff of the McGraw-Hil Book Company, which has spared no effort to produce the boo! in as perfect a manner as possible. JOSEPH L. R. HAYDEN. SCHENECTADY, September, 1909. CONTENTS. I. NATURE AND DIFFERENT FORMS OF RADIATION. 1. Radiation as energy. 1 2. Measurement of the velocity of light. 2 3. Nature of light. 4 4. Difference of wave length with differences of color. Meas- urement of wave length and of frequency. Iridescence. The ether. 6 5. Polarization proving light a transversal vibration. Double refraction. 7 6. The visible octave of radiation. Ultra-red and ultra-violet radiation. 9 7. The electric waves. 15 8. The spectrum of radiation covering 60 octaves. 16 VECTURE II. RELATION OF BODIES TO RADIATION. 9. Electric waves of single frequency, light waves of mixed frequency. 20 10. Resolving mixed waves into spectrum. Refraction. 21 11. Relation of refractive index to permeability and dielectric constant. 24 12. Spectrum. 25 13. Continuous spectrum. Line spectrum. Band spectrum. Combination spectra. 26 14. Reflection, absorption and transmission. 29 15. Conversion of absorbed radiation into heat and light. 30 16. Transmitted light. 31 17. Opaque colors and transparent colors. 32 18. Objective color and subjective color. 33 19. Effect of excess and of deficiency of certain wave length of the illuminant on the opaque and the transparent colors. 34 vii viii CONTENTS. LECTURE III. PHYSIOLOGICAL EFFECTS OF RADIATION. PAGE Visibility. 20. The eye. 37 21. Dependence of sensitivity of the eye on the color. Mechan- ical equivalent of light. Comparison of intensities of different colors. 40 22. Sensitivity curves of eye for different intensities. 43 23. Change of shape of sensitivity curve with intensity. 45 24. Harmful effect of excessive radiation power. 48 25. Protective action of eye. 50 26. Specific high frequency effect beginning in blue. 51 27. Perception of ultra-violet light. Harmful effects of ultra- violet. 52 28. Arcs as producers of ultra-violet rays. 55 Pathological and Therapeutic Effects of Radiation. 29. Power effect and specific high frequency effect. 57 30. Light as germicide and disinfectant. 59 LECTURE IV. CHEMICAL AND PHYSICAL EFFECTS OF RADIATION. Chemical Effects. 31. Indirect chemical action by energy of radiation. , Direct chemical action. 03 32. Chemical action of rod and yellow rays in supplying the energy of plant life. Destructive notion of high frequency on plant life. 64 Physical Effects. 33. Fluorescence and phosphorescence. 66 LECTUKE V. TEMPKHATUKK RAWATION. 34. Production of radiation by boat, 70 35. Increase of intensity and frequency with temperature. 73 36. Efficiency and temperature, 70 37. Carbon incandescent lamp. 78 38. Evaporation below boiling point. Allotropic modifications of carbon. HI 39. Normal temperature radiation, M 40. Colored body radiation. 85 41. Measurement of temperatures by radiation, 89 42. Colored radiation and heat luminescence. 90 CONTENTS. Ix LECTURE VI. LUMINESCENCE. PAGE Fluorescence and Phorphorescenee. 43. Radioluminescence. Electroluminescence. Thermolumi- nescence. Physical phosphorescence. Chemical phos- phorescence. Biological phosphorescence. 94 44. Pyroluminescence. Chemical luminescence. 96 45. Electroluminescence of gases and vapors. 98 Disruptive Conduction. 46. Geissler tube and spark. Disruptive voltage. 47. Change from spark to Geissler glow. " 101 105 Continuous Conduction. 48. Nature of continuous or arc conduction. 49. Distinction between arc and spark discharge. 50. Continuity at negative. 51. Rectification of alternating voltages by arcs, 52. Efficiency and color. 53. Most efficient light producer. 54. Electro-conduction from negative, long life, non-consuming positive, limitation in the available materials. 55. Arc most efficient method of light production. 106 Ill 113 117 122 123 125 126 LECTURE VII. FLAMES AS ILLUMINANTS. 56. Hydrocarbon flames. 128 57. Effect of rapidity of combustion and of flame shape on smokiness. 130 58. Effect of oxygen atom in the hydrocarbon molecule on luminosity. 132 59. Mixture of hydrocarbon with air. 133 60. Chemical luminescence. 134 61. Flames with separate radiator. 135 LECTURE VIII. ARC LAMPS AND ARC LIGHTING. Volt-Ampere Characteristics of the Arc, 62. Arc length and voltage. 63. General equations of the arc. Stability Curves oj the Arc. 64. Instability on constant voltage. 65. Equations of the vapor arc. Arc Length and Efficiency. 66. Maximum efficiency length of carbon arc, 67. Maximum efficiency length of luminous arc. 137 140 142 145 146 148 X CONTENTS. LECTURE VIII. ARC LAMPS AND ARC LIGHTING (Continued). Arc Lamps. 68. The elements of the arc lamp. 69. Differential arc lamp. 70. Series arc lamp. 71. Luminous arc lamp. PAGE 151 153 157 160 Arc Circuits. 72. Constant potential and constant current. The mercury arc rectifier system. The arc machine. 160 73. The constant current transformer. The constant current reactance. 163 LECTURE IX. MEASUREMENT OF LIGHT AND RADIATION. 74. Measurement of radiation as power. 360 75. Light a physiological quantity. 107 76. Physiological feature involved in all photometric methods. 109 77. Zero method photometers. 170 78. Comparison of lights, .172 79. Flicker photometer. 178 80. The luminometer. 175 81. Primary standards of light. 177 82. Proposed primary standards. 178 83. Illumination and total flux of light. Incandescent lamp photometry. 179 84. Arc lamp photometry. 182 85. Discussion. Mean spherical, horizontal, downwards, maxi- mum, hemispherical candle power. 184 LECTURE X. LIGHT FLUX AND DISTRIBUTION. 86. Light flux, light flux density, light intensity. 180 87. Symmetrical and approximately symmetrical distribution. 187 cum 88. Calculation of light flux from meridian of Asymmetri- cal radiator. 188 Distribution Curves of Radiation. 89. Calculation of distribution curves. Point or sphere of uniform brilliancy. 90. Straight line or cylindrical radiator, 91. Circular line or cylinder. 92. Single loop filament incandescent lamp as illustration* 190 tOfi 197 200 CONTENTS. xi LECTURE X. LIGHT FLUX AND DISTRIBUTION (Continued). PAGE Shadows. 93. Circular shade opposite and symmetrical to circular radia- tor. 94. Calculation of the meridian curves of a circular radiator, for different sizes of a symmetrical circular shade, and for different distances of it. 95. Circular shade concentric with end of linear radiator. 202 206 210 Reflection. 96. Irregular reflection. 97. Regular reflection. 98. Reflector with regular and irregular reflection. 212 215 218 Diffraction, Diffusion and Refraction. 99. Purpose of reducing tho brilliancy of the illuminant. 100. Effect of the shape of the diffusing globe on the distribu- tion curve. 101. Prismatic refraction and reflection. 221 223 224 LECTURE XI. LIGHT INTENSITY AND ILLUMINATION. Intensity Curves for Uniform Illumination. 102. Calculation of intensity distribution of illuminant for uniform total, horizontal and vertical illumination. .103. Uniform illumination of limited area. 226 229 Street Illumination by Arcs, 104. DincuHsiou of problem. 105. Combined effect of successive lamps. 234 238 Room Illumination by Incandescent Lamps. 106. Distribution curve of lamp. Calculation of resultant total intensity of direct light. 107. Reflection from walls and ceiling. 108. Total directed and diffused illumination. 242 246 251 Horizontal Table Illumination by Incandescent Lamps. 109. Location of lamps, 253 xii CONTENTS. LECTURE XIL ILLUMINATION AND ILLUMINATING ENGINEERING, 110. Physical and physiological considerations. 111. Light flux density. Illumination. Brilliancy. 112. Physical problems. Ceilings and walls. Reflectors, diffus- ing globes, diffracting shades, etc. t 113. Objective illumination. Subjective illumination. Con- traction of pupil. Intrinsic brilliancy. Direct and in- direct lighting. 114. Fatigue. 115. Differences in intensity and in color. Control of color differences. Shadows and their control. Directed and 256 259 260 201 203 diffused light. 116. Direction of shadows. 205 207 117. Color sensitivity in relation to required intensity of illu- mination, 20t) 118. Domestic lighting. 119. The twofold problem of domestic lighting: daylight mid artificial light. 120. Street lighting. 121. Defects of present Htreet lighting. 122. Tower lighting. 270 271 272 27't 27<1 LECTURE XIII. PHWIOMXWM^ PHOUUOMB OF IU,UMINATW< KNGINHBHING, 123. Physical Hide of illuminating engineering, 1'hysiologieal problems, 32*1. Physiological difleroneo between 10 sec. and 195,000,000 mikvs in 3 (MO sec. thus givas a velocity of ~ .. , ,195,000,000 light of i or , 00 AAA 188,000 mil-ios per sec. IIMU Later, the velocity of light was measured directly in a number D of different ways. For instance, let, in Fig. 2, be a clwk per- A forated with holes at its periphery, lamp L wends its light H through a hole in tho dink to a mirror Af located at a con- siderable distance, for instance 5 miles; there the light is reflected NATURE AND DIFFERENT FORMS OF RADIATION. 3 H and the mirror is adjusted so that the reflected beam of light passes through another hole i of the disk into the telescope T. If the disk is turned half the pitch of the holes the light is blotted out as a tooth stands in front of both the lamp and the telescope. Again turning the disk half the pitch of the holes in the same I U-=--r rznnii f 5j*ILJ? FIG. 2. direction the light reappears. If the disk is slowly revolved, alter- nate light and darkness will be observed, but when the speed increases so that more than from 10 to 20 holes pass per second, the eye is no longer able to distinguish the in-lividual flashes of light but sees a steady and uniform light; then increasing the speed still more the light grows fainter and finally entirely disappears. H This means when a hole Q is in front of the lamp, a beam of light passes through the hole. During the time taken by the light D to travel the 10 miles to the mirror and back, the disk has moved, and the hole ffv which was in front of the telescope H when the light from the lamp passed through the hole has Q, moved away, and a tooth is now in front of the telescope and intercepts the light. Therefore, at the speed at which the light disappears, the time it takes the disk to move half the pitch of a hole is equal to the time it takes the light to travel 10 miles. Increasing still further the velocity of the disk D, the light appears ag dn, and increases in brilliancy, reaching a maximum at twice the speed at which it had disappeared. Then the light reflected from the mirror At again passes through the center of H a hole into the telescope, but not through the same hole l through which it would have passed with the disk stationary, but through the next hole f/ 2, that IB, the disk has moved a distance equal to the pitch of one hole while the light traveled 10 miles. D Assume; for instance, that the disk has 200 holes and makes 4 RADIATION, LIGHT, AND ILLUMINATION. 94 rev. per sec. full brilliancy at the moment when the light has again reached In this case, 200 X 94 = 18,800 holes pass the telescope per second, and the time of motion by the pitch of one hole is Sec., and as this is the time required by the light 18 800 ; ^ to travel 10 miles, this gives the velocity of light as 10 -*- -^ or 188,000 miles per sec. The velocity of light in air, or rather in empty space, thus is 188,000 miles or 3 X 1010 cm. per sec. For electrical radiation, the velocity has been measured by Herz, and found to be the same as the velocity of light, and there is very good evidence that all radiations travel with the same velocity through space (except perhaps the rays of radioactive substances). 3. Regarding the nature of radiation, two theories have been proposed. Newton suggested that light rays consisted of extremely minute material particles thrown off by the lightgiving bodies with enormous velocities, that is, a kind of bom- bardment. This theory has been revived in recent years to ex- plain the radiations of radium, etc. Euler and others explained the light as a wave motion. Which of these explanations Ls correct can be experimentally decided in the following manner: Assuming light to be a bombardment of minute particles, if wo combine two rays of light in the same path they must add to each other, that is, two equal beams of light together give a beam of twice the amplitude. If, however, wo assume light is a wave motion, then two equal beams of light add to one of twice the amplitude only in case the waves are in phase, as A l and /^ in A Fig. 3 add to (7r If, however, the two beams and 7i are not 2 2 C in phase, their resultant is 2 less than their sum, and if the two beams A 3 and B3 in Fig. 3 happen to be iu opposition (180 degrees apart), that is, one-half wave length out of phase with each other, their resultant Ls zero, that is, they blot each other out. A Assuming now we take a plain glass plate (Fig. 4) and a slightly curved plate 5, touching each other at C, and illuminate them by a beam of uniform light as the yellow light given by coloring the flame of a bunsen burner with somo sodium Halt a part of the light 6, is then reflected from the lower surface of NATURE AND DIFFERENT FORMS OF RADIATION. 6 the curved glass plate B, a part c, passes out of it, and is reflected A from the upper surface of the plain glass plate A. beam of \ Ba 7 \ FIG. 3. reflected light a, thus is a combination of a beam 6 and a beam c. The two beams of light which combine to a single one, a, differ from each other in phase by twice the distance between the two glass plates. At those points d i9 d 2, etc. at which the distance 4, between the two glass plates is } wave length, or , f, etc., the two component beams of a would differ by i, f , |, etc. wave lengths, and thus would blot each other out, producing darkness, 6 RADIATION, LIGHT, AND ILLUMINATION. while at those points where the distance between the glass plates is |, 1, Ij, etc. wave lengths, and the two component beams a thus differ in phase by a full wave or a multiple thereof, they would add. If, therefore, light is a wave motion, such a structure would show the contact point C of the plates surrounded by alternate dark rings, d, and bright rings, y. This is actually the case, and therefore this phenomenon, called "interference" proves light to be a wave motion, and has lead to the universal acceptance of the Eulerian theory. Measuring the curvature of the plate 5, and the diameter B A of the dark rings d, the distance between the plates and at the dark rings d, can be calculated and as this distance is onequarter wave length, or an odd multiple thereof, the wave length can be determined therefrom. The wave length of light can be measured with extremely high accuracy and has been proposed as the absolute standard of length, instead of the meter, which was intended to be 10~7 of the quadrant of the earth. 4. It is found, however, that the different colors of light have different wave lengths; red light has the greatest wave length, and then in the following order: red, orange, yellow, green, blue, indigo, violet, the wave length decreases, violet light having the shortest wave length. If in experiment (Fig. 4) instead of uniform light (monochromatic light), ordinary white light is used, which is a mixture of all colors, the dark and bright rings of the different colons appear at different distances from each other, those of the violet nearest and those of the red the furthest apart, and HO superimpose upon each other, and instead of alternately black and light ringH, colored rings appear, so-called interference rings. Wherever a thin film of air or anything else of unequal thickness in interposod between two other materials, such interference colors tlmn appear. They show, for instance, between sheets of mica, etc. The colors of soap bubbles arc thus produced. The production of such colors by the interference of ray of light differing from each other by a fractional wave length i called iridescence. Iridescent colors, for instance, are those of mother-of-pearl, Df opal, of many butterflies, etc. Light, therefore, Is a wave motion* NATURE AND DIFFERENT FORMS OF RADIATION. 7 The frequency of radiation follows from the velocity of light, and the wave length. The average wave length of visible radiation, or light, is about lw = 60 microcentimeters,* that is, 60 X 10~6 cm. (or about = innyffe in.) and since the speed is S 3 X 10 10 cm. the frequency o = = X is / r- 500 10 12 , or 500 millions of millions of cycles per LW second, that is, inconceivably high compared with the frequencies with which we are familiar in alternating currents. If, as proven, light is a wave motion, there must be some thing which is moving, a medium, and from the nature of the wave motion, its extremely high velocity, follow the properties of this medium: it has an extremely high elasticity and extremely low density, and it must penetrate all substances since no vacuum can be produced for this medium, because light passes through any vacuum. Hence it cannot be any known gas, but must be essentially different, and has been called the "ether." Whether the ether is a form of matter or not depends upon the definition of matter. If matter is defined as the (hypotheti- cal) carrier of energy (and all the information we have of matter is that it is the seat of energy), then the ether is matter, as it is a carrier of energy: the energy of radiation, during the time be- tween the moment when the wave leaves the radiator and the moment when it strikes a body and is absorbed, resides in the ether. 5. If , light is a wave motion or vibration, it may be a longitudi- nal vibration, or a transversal vibration. Either the particles of the medium which transmit the vibrations may move in the direction in which the wave travels, as is the case with sound B waves in air. If in Fig. 5 sound waves travel from the bell in BA m the direction the air molecules t vibrate in the same direction, A to B. Or the vibration may be transversal ; that is, if the beam * As measures of the wave length of light, a number of metric units have survived and are liable to load to confusion: The micron, denoted by a, equal to one thousandth of a millimeter. The ftp, equal to one millionth of a millimeter. The Angstrom unit, equal to one ten-millionth of a millimeter. As seen, the basis of those units is the millimeter, which was temporarily used as a standard unit of length before the establishment of the present absolute system of units, the (C,G,S), which is based on centimeter length, gram mass, and second time measure. A radiation of the wave length of 60 microcentimeters thus can be expressed also as: 6000 Angstrom units, or 0.6 /x, or 600 pp. 8 RADIATION, LIGHT, AND ILLUMINATION. of light moves in Fig. 6 perpendicularly to the plane of the paper, the vibrating particles move in any one of the directions oa, ob, etc. in the plane of the paper, and thus perpendicular to the ray FIG. 5. of light. In the former case (a longitudinal vibration, as sound) there obviously can be no difference between the directions at right angles to the motion of the wave. In a transversal vibra- tion, however, the particles may move either irregularly in any of the infinite number of directions at right angles to thc^ray (Fig. 6) and thus no difference exists in the different directions perpendicular to the beam ; or they may vibrate in one direction only* ,' as the direction boa wave (Fig. 7) . 11 i called In "// the latter case, -i i j) polarized" andi the i, _ _ has is .li-cr differ- ent characteristics in three direc- tions at right angles to each other: one direction is the direction of FIG. 6, FIG. 7. propagation, or of wave travel; the ^ second is the direction of vibration ; and the tjlir(j is direction per- pendicular to progression and to vibration. For instance, the electric field of a conductor carrying alternating current is a polarized wave : the direction parallel to the conductor is the direction of energy flow; the direction concentric to the conductor is the direction of the electromagnetic component, and the direction radial to the conductor is the direction of the electrostatic component of the electric field. Therefore, if light rays can be polarized, that Is, made to ex- hibit different properties in two directions at right angles to each other and to the direction of wave travel, thin would prove tfee light wave to be a transversal vibration* This is actually the case. For instance, ,if a beam of light is reflected a number of times under a fairly sharp angle, as shown in Fig. 8, this beam becomes m polarized; that is, for instance, the reflection from the mirror o; m m set like the mirrors v 2 * , . which produced the polarization, NATURE AND DIFFERENT FORMS OF RADIATION. 9 is greater, and the absorption less than from a mirror set at right angles thereto, as w/. Some crystals, as Iceland spar (calcium carbonate) , show "double refraction/' that is, dissolve a beam of light, a, enter- ing them, into two separate beams, b and c (Fig. 9) which are polarized at right angles to each other. K In a second crystal, 2J beam & would then enter as a single K K beam, under the same angle as in the first crystal v if 2 were K^ K in the same position as while if z were turned at right angles K to v beam 6 would enter I 2 under the same angle as beam c in K crystal r 6. As seen, light and radiation in general are transversal wave motions of very high speed, S 3 X 10 10 cm. per sec. in a hypo- thetical medium, ether, which must be assumed to fill all space and penetrate all substances. "Radiation is visible, as light, in a narrow range of frequencies only: between 400 X 1012 and 770 X 1012 cycles per sec. cor- responding to wave lengths from 76 X 6 10~" cm. to 39 X 10~ c cm.* All other radiations are invisible and thus have to be observed by other means. I have here a pair of rods of cast silicon (10 in. long, 0.22 in. in diameter, having a resistance of about 10 ohms each), connected X X * The visibility of radiation IB greatest between the wave lengths 50 X 10"* to 60 lO""8 and good between the wave lengths 41 X 10"8 to 76 10""% but extends more or loss indistinctly ovor the range of wave lengths from X X X 33 10~ to 77 10"* and faintly even as far as 30 X 10~8 to 100 NT*. 10 RADIATION, LIGHT, AND ILLUMINATION. in series with each other and with a rheostat of about 40 ohms resistance in a 120-volt circuit. When I establish a current through the rods, electric energy is converted into heat by the resistance of the rods. This heat energy is converted into and sent out as radiation, with the exception of the part carried off by heat conduction and convection. Reducing the resistance, I increase the heat, and thereby the radiation from the silicon rods. Still nothing is visible even in the dark; these radiations are of too low frequency, or great wave length, to be visible. By holding my hand near the rods, I can feel the energy as heat, and show it to you by bringing the rods near to this Crookes' radiometer, FIG. 9. which is an instrument showing the energy of radiation. It con- sists (Fig. 10) of four aluminum vanes, mounted in a moderately high vacuum so that they can move very easily. One side of each vane is polished, the other blackened. The waves of radiation are reflected on the polished Hide of the vane; on the blackened side they are absorbed, produce heat, thus raise the temperature of the air near the vane ; the air expands and pushes the vanes ahead, that is, rotates the wheel As you see, when I bring the heated rods near the radiometer, the wheel spins around at a rapid rate by the radiation from the rods, which to the eye are invisible* NATURE AND DIFFERENT FORMS OF RADIATION. II Increasing still further the energy input into the silicon rods, and thereby their temperature, the intensity of radiation increases, but at the same time radiations of higher and higher frequencies appear, and ultimately the rods become visible in the dark, giving a dark red light; that is, of all the radiations sent out by the rods, a small part is of sufficiently high frequency to be visible. Still further increasing the temperature, the total radiation increases, but the waves of high frequency increase more rapidly than those of lower frequency; that is, the average frequency of radiation increases or the average wave length decreases and higher and higher frequencies appear, orange rays, yellow, green, blue, violet, and the color of the light thus gradually changes to bright red, orange, yellow. Now I change over from the silicon rods which are near the maximum tem- perature they can stand to a tungsten lamp (a 40-watt 110-volt lamp, connected in series with a rheostat of 2000 ohms resistance in a 240- volt circuit)* For comparison I also turn on an ordinary 16 c. p. carbon filament incandescent lamp, running at normal voltage and giving its usual yellow light. Gradually turn- ing out the resistance, the light of the tungsten lamp changes from orange to yellow, yellowish white FIG. 10. and ultimately, with all the resistance cut out and the fila- ment running at more than double voltage, is practically white; that is, gives a radiation containing all the frequencies of visible light in nearly the same proportion as exist in sunlight. If wo should go still further and very greatly increase the temperature, because of the more rapid increase of the higher fre- quencies (violet, blue, green) than the lower frequencies of light (red, orange and yellow) with increase in temperature, the light 12 RADIATION, LIGHT, AND ILLUMINATION. should become bluish. However, we are close to the limit of temperature which even tungsten can stand, and to show you light of high frequency or short wave length I use a different apparatus in which a more direct conversion of electric energy into radiation takes place, the mercury arc lamp. Here the light is bluish green, containing only the highest frequencies of visible radiation, violet, blue and green, but practically none of the lower frequencies of visible radiation, red or orange. A\ SJJJLQJU ....240-VOLT8-00 CYCLES FIG. 11. In the tungsten lamp at high brilliancy and more still in the mercury arc, radiations of higher frequencies appear, that in, shorter wave lengths than visible light, and these radiations arc again invisible. As they arc of frequencies boyomi the violet rays of light, they arc called " ultra-violet rays/' while tho radia- tions which we produced from tho heated silicon rods at moderate temperatures were invisible because of too low frequency and are thus called " ultra-red rays/' or "infra-red rays/' as they are outside of and below the red end of the range of visible radiation. To produce powerful ultra-violet rays, I use a condenser clis** charge between iron terminals, a so-called ultra-violet arc lamp. Three iron spheres, / in Fig. 11, of about f in. diameter, are mounted on an insulator B. The middle sphere is fixed, the NATURE AND DIFFERENT FORMS OF RADIATION. 13 outer ones adjustable and set for about 3 TF in. gap. This lamp is connected across a high voltage 0.2-mf. mica condenser C, which is connected to the high voltage terminal of a small'step-up trans- former T giving about 15,000 volts (200 watts, 110 + 13,200 volts). The low tension side of the transformer is connected to R the 240-volt 60-cycle circuit through a rheostat to limit the current. The transformer charges the condenser, and when the voltage of the condenser has risen sufficiently high it discharges through the spark gaps I by an oscillation of high frequency (about 500,000 cycles), then charges again from the transformer, discharges through the gap, etc. As several such condenser dis- charges occur during each half wave of alternating supply voltage the light given by the discharge appears continuous. You see, however, that this iron arc gives apparently very little light; most of the radiation is ultra-violet, that is, invisible to the eye. To make it visible, we use what may be called a frequency converter of radiation. I have here a lump of willemite (native zinc silicate), a dull greenish gray looking stone. I put it under the iron arc and it flashes up in a bright green glare by convert- ing the higher frequency of ultra-violet rays into the lower frequency of green light. This green light is not given by the iron arc, as a piece of white paper held under the arc shows only the faint illumination given by the small amount of visible radia- tion. I now move a thin sheet of glass, or of mica, between the iron arc and the lump of willemite, and you see the green light disappear as far as the glass casts a shadow. Thus glass or mica, while transparent to visible light, is opaque for the ultra-violet A light of the iron arc. thick piece of crystallized gypsum (sel- onitc) put in the path of the ultra-violet light does not stop it, hence is transparent, as the lump of willemite continues to show the green light, or a piece of cast glaSxS its blue light. I have here some pieces of willemite in a glass test tube. They appear dull and colorless in the ultra-violet light, as the glass is opaque for this light. I shift them over into a test tube of fused quartz, and you sec them shine in the green glare. Quartz is trans- parent to ultra-violet light. When investigating ultra-violet light, quartz lenwes and prisms must, therefore, be used. Still higher frequencies of ultra-violet light than those given by a condenser discharge between iron terminals are produced by a low temperature mercury arc. Obviously this arc must not be 14 RADIATION, LIGHT, AND ILLUMINATION. operated in a glass tube but in a quartz tube, as glass is opaque for these rays. These ultra-violet radiations carry us up to frequencies of about KT 3000 X 10 12 cycles per sec., or to wave lengths of about 10 X 8 cm. Then, however, follows a wide gap, between the highest frequencies of ultra-violet radiation and the frequencies of X-rays. In this gap, radiations of very interesting properties may some- times be found. At the extreme end of the scale we find the X-rays and the radiations of radio-active substances if indeed these radiations are wave motions, which has been questioned. Since at these extremely high frequencies reflection and refraction cease, but irregular dispersion occurs, the usual methods of measuring wave lengths and frequencies fail. The X-rays apparently cover quite a range of frequency and by using the atoms of a crystal as diffraction grating, their average wave length has been measured as 0.1 X 10~6 cm., giving a frequency of 0.3 X 1018 cycles per sec. In comparing vibrations of greatly differing frequencies, the most convenient measure is the octave, that is, the frequency scale of acoustics. One octave represents a doubling of the frequency ; n octaves higher then means a frequency 2n times as high, n octaves lower, a frequency n (|) as high. By this .scale all the inter- vals are of the same character; one octave means the amo relative increase, which ever may be the absolute frequency or wave length. As the perceptions of our senses vary in proportion to the pcr- centual change of the physical quantity causing the perception (Fechner's law), in the acoustic or logarithmic scale the steps are thus proportional to the change of sensual perception caused by them. The visible radiation covers somewhat leas than one octave; ultra-violet radiations have boon observed beyond this for about two more octaves. Nine octaves higher is the estimate* I frequency of X-rays. On the other side of the visible range, towards lower frequencies or longer waves, ultra-red rays, observations have been extended over more than eight octaves up to wave lengths as great as 0*03 cm. length, or frequencies of only 10 12 cycles per sec. The ultrared rays given by the heated silicon rods of our experiment do not extend to such low frequencies, but such very low frequencies NATURE AND DIFFERENT FORMS OF RADIATION. 15 have been observed in the radiations of bodies of very low temperature, as liquid air, or in the moon's rays. 7. Very much longer waves, however, are the electric waves. They are used in wireless telegraphy, etc. I here connect (Fig. 12) jmq Iju&J X FIG. 12. the condenser C of the apparatus which I used for operating the G ultra-violet arc, to a spark gap v of which the one side is con- B nected to ground v the other side to a vertical aluminum rod A v about 8 feet long. The charge and discharge of the aluminum A rod l by the oscillating condenser current, send out an electric, wave of about 50 feet length. This wave passes through you, and when striking the aluminum rod A 2 back of you, induces therein B an electric charge. A 2 is separated from ground 2 by a narrow spark gap C?2 between graphite terminals, and the arrival of the electric wave at A a causes a small spark to jump across the gap 6 which closes the circuit of the tungsten lamp L, thereby 2, lighting it as Jong as the wave train continues. 16 RADIATION, LIGHT, AND ILLUMINATION. The electric waves used in wireless telegraphy range in wave lengths from 100 feet or less to 10,000 feet or more, corresponding to 10 7 to 10 5 cycles per sec. or less. Still very much longer waves are the fields of alternating cur- rent circuits: the magnetic and electrostatic field of an alterna- ting current progresses as a wave of radiation from the conductor, But as the wave length is very great, due to the low frequency, ^ _ X 3 10 1Q ^ a 60-cycle alternating current gives a wave i length ott X 500 10 6 cm. or 3100 miles the distance to which the field of the circuit extends is an insignificant fraction only of the wave length, and the wave propagation of the field thus is usually not considered. Electric waves of higher frequencies than used in wireless telegraphy are the Herzian waves, produced by electric oscilla- tors, "that is, a moderately long straight conductor cut in the middle by a gap and terminated by spherical as condensers^ shown in Fig. 13. On these waves the velocity of propagation o < BNERGY-SU PPUY" - * -o o FIG. 13, o has been measured by Herz by producing standing waves by combination of main wave and reflected wave. Still much higher frequoncieH arc the oscillations between the cylinders of multi-gap lightning arresters, and the limit of fre- quency of electric waves would probably bo given by tho oscilla- ting discharge of two small separated by a narrow gap. spheres against each other when It probably is at about 5 X 10 l cycles, or 0.0 cm. wave length. The blank space between the shortest electric wave and the longest ultnwecl light wave thun has become fairly narrow from 0.0 to 0.03 cm., or only about four octavos 8. In tho following tables, tho different known forms of radia- tion are arranged by their frequency and wave length, and are given also in octaves, choosing as aero point the middle c of the piano, or a frequency of 128 cycles per sec. NATURE AND DIFFERENT FORMS OF RADIATION. 17 SPECTRUM OP RADIATION. Zero point chosen at Speed of radiation S c = = 128 3X cycles per lo10 cm. second. These radiations are plotted graphically in Fig. 14, with the octave as abscissa). As seen, the total range of frequencies of radiation is enormous, covering nearly GO octaves, while the range of sound waves is only about nine octaves, from 15 to 8000 cycles. There are two blank spaces in the range of radiation, one between electric and light waves, and a second and longer one between light and X-rays. It is interesting to note that the range of electric waves is far greater than that of light waves. Only a very narrow range of radiation, less than one octave out of a total of 60, is visible. It is shown shaded in Fig. 14. This 18 RADIATION, LIGHT, AND ILLUMINATION. exhibits the great difficulty of the problem of efficient light production: it means producing as large a part of the total radiation as possible within this very narrow range of visibility. Regarding the range of frequencies covered by it, the eye thus is much less sensitive than the ear, which hears over ten octaves as sound waves. While the visible radiations are the most important ones, as light, the total range of radiation is of interest to the electrical engineer. The ultra-red rays are those radiations which we try to avoid is far as possible when producing light, as they consume power SOUND WAVES FIG, 14. and so lower the efficiency; the ultra-violet rays are of importance in medicine as germ killers. They are more or loss destructive to life, appear together with the visible radiation, and where they are of appreciable amount, as in the arc, protection against them becomes desirable. The X-rays have become of importance in medicine, etc., as they penetrate otherwise opaque bodies and thus allow seeing things inside of other bodies. The total range of electric waves, between tho frequencies of alternating currents and the limits of electric waves, has been of importance to the electrical engineer a# harmful and destructive phenomena in electric circuits, which are to be guarded against, and only in recent years, with tho development of wireless telegraphy, some such electric waves have found a useful commercial application. The main object of their study which IB the study of transient electric phenomena, is still, however, to guard against their appearance in electric circuits and discharge them harmlessly when they appear. Considering the great difference which already exists between alternating currents of low frequency; 25 or 15 cycles, and of high NATURE AND DIFFERENT FORMS OF RADIATION. 19 frequency, 133 cycles, and realizing that the total range of waves, which may appear in electric circuits, is many hundred times greater than the difference between high and low frequency alternating currents, it can be realized that the differences in the character of electric waves are enormous between the low frequency surges of near machine frequency and the high frequency oscillations of a multi-gap lightning arrester, near the upper limits of electric wave frequencies, and the problem of protecting circuits against them thus is vastly more difficult than appears at first sight and the conclusions drawn from experimental investigations of electric waves may be very misleading when applied to waves many octaves different from those used in the experiment. This explains the apparently contradictory evidence of many experi- mental investigations on the protection of electric circuits. LECTUEE II. RELATION OF BODIES TO RADIATION. 9. For convenience, the total range of known radiations can be divided into two classes, the electric waves and the light waves, which are separated from each other by the blank space in the middle of the spectrum of radiation (Fig. 14). Under light waves we here include also the invisible ultra-red radiation and the ultra-violet radiation and the non-refrangible radiations, as X-rays, etc., separated from the latter by the second blank space of the radiation spectrum. In the following, mainly the light waves, that is, the second or high frequency range of radiation, will be discussed. The elec- tric waves are usually of importance only in their relation to the radiator or oscillator which produces them, or to the receiver on which they impinge, and thus are treated in connection with the radiator or receiver, that is, the electric conductor, in the theory of transient electric phenomena and oscillations.* The radiation may bo of a single frequency, that is, a single wave; or a mixture of different frequencies, that is, a mixture of different and frequently of an infinite number of waves. Electric radiation usually is of a single frequency, that is, of the frequency or wave length determined by the constants of the electric circuit which produces the radiation, mainly the induct- ance L and the capacity C. They may, however, have different wave shapes, that is, comprise, in addition to the fundamental wave, higher harmonics or multiples thereof, just as the sound waves which represent the same tone with different musical instruments are of the same frequency but of different wave shapes, that is, contain different higher harmonics. Light radiations usually are a mixture of a number of waves of different frequencies, and very commonly a mixture of an infinite number of frequencies, as is, for instance, the case with the * "Theory and Calculation of Transient Electric Phenomena and Oscilla- tions," 20 RELATION OF BODIES TO RADIATION'. 21 radiation of an incandescent body as a lamp filament, which contains all the frequencies from long ultra-red waves over visible light waves to ultra-violet waves. In the action of vibrations on our senses there is a characteristic difference between the perception of sound waves by the ear and that of light waves by the eye: the ear is analytic, that is, can separate the individual waves in a mixture of different 'sound waves, as an accord on the piano, and distinguish the individual components of the mixed sound which reaches the ear. Thus we can hear and distinguish an individual voice amongst a mass of other noises. The eye, however, perceives only the resultant of all the visible radiations which reach it, but cannot separate their components, and very different mixtures of radiations thus make the same impression upon the eye: thus, for instance, numerous mixtures of blue and yellow light appear alike to the eye and the same as green light, that is, appear green, while physically, it is obvious that mixtures of blue and yellow light are essentially different from green light. It is interesting to imagine how nature would look to us if the eye were analytic, that is, could separate the different component radiations, and if it could perceive waves over as great a range of frequency as the car, about ten octaves instead of less than one octave as is now the case. The information given to us by the sense of sight would be infinitely increased, and we would see many differences and changes which now escape us. 10. However, while the eye cannot distinguish the different component radiations but sees only their resultant, the specific effects of the component radiations, as the physiologically harmful action of an ultra-violet component of light, still remain, even if the oye docs not see the components, and in the study of radia- tion for the purpose- of its engineering use for illumination it is therefore necessary to analyze the mixed radiation given by a source as a lamp, by resolving it into its component waves. , This is done by using some feature of the radiation which varies with the frequency. Such is the case with the velocity of propagation. The velocity of light in empty space is 3 X 1010 cm. per sec. It is practically the same in air and other gases. In denser bodicB, however, as water, glass, etc., the velocity of light is less arid, as will be seen, is different for different frequencies. 22 RADIATION, LIGHT, AND ILLUMINATION. B Assume then, in Fig. 15, a beam of light striking under an angle the boundary between two media, as air A and water W, the vibration of the ether particles in the beam of light is at right angles to the direction of propagation BC, and successively the D waves thus reach a l b lj a z b 2 . . . As soon, however, as the back edge of the beam reaches the boundary at its speed changes FIG. 15. W by entering the medium decreases in the present instances Let then 8 i = speed of propagation in medium A, S 2 npeed of propagation in medium W. Then, while the center of tho txuim moves the distance EC, the back edge, in the denser medium, a moves only the distance D/ = EC, and the wave front of the i back half of the beam thus changes to 01 while that of tho front half of the beam, which is still in the medium A, romahw GC. Then, while the front edge of the beam movon from G to //, tho center and the whole back half of the beam moves in tho denser O[ medium W, only the distance CK ** ^GH, and tho wave front &i W of the beam, in the medium now is EL. } That is, due to the W A difference in velocity in the two media and the wave front , of the beam, and thereby its direction of propagation, IB changed RELATION OF BODIES TO RADIATION. 23 when traversing the boundary between the two media, and the beam BC continues its motion in the direction CM. Let then = 1 angle of incidence, that is, the angle between the incident beam BC and the perpendicular CN on the boundary, = and a 2 angle of refraction, that is, the angle between the out- CM going or refracted beam and the perpendicular CP on the boundary. It is then: FDH = a and LED = a 1 2; hence, FH - DH sin and DL - DH sin t r (1) FH The front edge of the beam moves the distance in medium A, while the back edge moves the distance DL in medium W; ' that is, + SI s 3; (2) hence, substituting (1) into (2), gives: That is, the ratio of the sines of the angle of incidence and the angle of refraction equals the ratio of the speed of propagation iu the two media, hence the ratio of the sines of these two angles IH constant. This is the law of refraction, and this ratio of sines A is called the refractive index between the two media and W. As the refractive index of one medium W, then, is understood its re- fractive index against empty space or against air : * sm c^ $ x (4) = X where S is the velocity of light in empty space 3 10 10 and , S l the velocity in the medium, of which VLC (5) where L is the inductance, C the capacity of the conductor per unit length (the length measured in the same measure as the speed S). The inductance L is proportional to the permeability /*, and C the capacity proportional to the dielectric constant, or specific capacity K of the medium surrounding the conductor, that is, the medium through which the electric wave propagates; that is, A A where is a proportionality constant. The ratio of the speed of propagation of an electric wave in two media 1 and 2 thus is : for empty space, /i hence, 1 and K. S 1 ; _ (8) where S t is the speed of propagation in tho mcdiuni of constants /^ and KV Comparing equation (S) with (4) it follows: '/V, gives the total radiation : " thus, for a 5; P = cAT*; or, Stefan's law, as discussed above. The maximum energy rate at temperature T occurs at the wave length lw - lm given by : dP (lw) __ ' dlw which gives : lmT -1-0.284; or, _ 0.284 I'm m> lm = 50 X 10"8 thus gives: T = 284- ^p- = 5680 deg. abs. With normal temperature radiation the efficiency of light production is thus merely a function of the temperature and does not depend upon the material of the radiating body, provided that the material is such as to withstand the temperature. As the efficiency maximum of normal temperature radiation is far beyond the attainable, within the range of temperature available up to the boiling*point of carbon, the efficiency of light production by incandescence continuously increases, but even then the octave of visible radiation is at the far upper end of the radia- tion curve, and thus the problem of efficient light production is to operate the radiator at the highest possible temperature. The efficiency of light production is rather low even at the maximum efficiency point, that is, with the average frequency of radiation in the visible range, since this visible range is less than one octave; under these most favorable conditions the visible 76 RADIATION, LIGHT, AND ILLUMINATION. energy probably does not much exceed 10 per cent of the total radiation, the rest falls below and above the visible frequencies. 36. At the highest attainable temperature, the boiling point of carbon, the efficiency is much lower, probably below 10 per cent and this would be the highest efficiency attainable by normal temperature radiation. It is utilized for light production in the carbon arc lamp. The carbon arc flame gives practically no light, but all the light comes from the incandescent tips of the carbon electrodes, mainly the positive, which are at the boiling point of carbon and thus give the most efficient temperature radiation. Obviously, in the carbon arc lamp a very large part of the energy is wasted by heat conduction through the carbons, heat convection by air currents, etc,, and the total efficiency of the carbon arc lamp, that is, the ratio of the power of the visible radiation to the total electric power input into the lamp, thus is much lower than the radiation efficiency, that is, the ratio of the power of the visible to the total radiation. Thus the efficiency of the carbon arc is considerably increased by reducing the loss by heat conduction, by the use of smaller carbons the life of the carbons, however, is greatly reduced thereby, due to their more rapid combustion. The carbon arc lamp thus gives the most efficient incandescent light, as it operates at the highest temperature, the boiling point of carbon. But by doing so the radiator is continuously consumed and has to be fed into the arc. This requires an operating mechanism and becomes feasible only with large units of light. To attain the highest possible efficiency of light production by temperature radiation with a permanent radiator, thus requires the use of extremely refractory bodies, since the efficiency increases with the increase of the temperature, and is still very low at the melting point of platinum. To exclude all the losses of energy by heat conduction and heat convection, the radiator is enclosed in a vacuum, so that all the power input is converted into radiation. Even in this case the efficiency of light production is still relatively low. The vacuum used in the incandescent lamp, thus, is not only for the purpose of protecting the filament from combustion. Filling the globe with some gas which does not attack the carbon would do this and yet it would very greatly lower the efficiency, TEMPERATURE RADIATION. 77 as can be seen by admitting air into the lamp bulb, when the filament drops down to dull red heat, before it burns through. However, the presence of an indifferent gas of low heat capacity may lower the evaporation of the filament and so permit operation at higher temperature, and the gain in efficiency more than makes up for the increased losses, as in the gas filled tungsten lamps. A search, thus, has been made and is still being made, through- out the entire range of existing bodies, for very refractory mate- rials. Such materials may be chemical elements or compounds. However, the combination of a refractory element with one of very much lower melting point lowers its melting point, and very refractory compounds, thus, may be expected only amongst the combinations of very refractory elements with each other. The chemical elements, arranged in order of their atomic weight, exhibit a periodicity in their properties which permits FIG. 28. a systematic study of their properties. In diagram Fig, 28 the elements are arranged in order of their atomic weight in the 77 "periodic system. The height of their melting point is indicated by the darkness of the background. That is, the most refractory elements, wolfram and carbon, are shown on black background. The elements of somewhat lower melting point are shown on cross shaded background. Inversely, the elements of the lowest melting point, mercury under the metals and helium under metal- 78 RADIATION, LIGHT, AND ILLUMINATION. loids, are shown on white background, and the easily fusible metals and gaseous metalloids on lightly shaded background. As seen, there are two peaks of refractoriness, one amongst the metalloids, in carbon, and one under the metals in wolfram (or tungsten), and around these two peaks all the refractory elements are grouped. Inversely, there are also two depressions, or points of minimum melting point, in helium under the metalloids, around which all the gaseous elements are grouped, and in mer- cury under the metals, around which all the easily fusible metals are grouped. It is interesting to note that the melting point rises towards wolfram from both sides, as diagrammatically illustrated at the top of Fig. 28, in such a manner that the maximum point should be expected in the space between wolfram and osmium and the unknown element, which belongs in this space of the periodic system, thus should be expected to have still a higher melting point than wolfram, and thus give a higher efficiency of light production. As metal alloys almost always have lower melting points than their most refractory element, very refractory compounds thus may be expected only in the compounds between the very refrac- tory elements, in which at least one is a metalloid, that is, amongst the carbides and borides and possibly silicides and titanides. 37. Some of the earliest work on incandescent lamps was carried out with metal filaments. Platinum and iridium, how- ever, were not sufficiently refractory to give good efficiencies, and the very refractory metals were not yet available in sufficient A purity. small percentage of impurities, however, very greatly lowers the melting point, especially with metals of very high atomic weight. For instance, wolfram carbide contains only 3 per cent of carbon and 97 per cent of wolfram and even 0.1 per cent of carbon in wolfram metal thus means that over 3 per cent of the metal consists of the easily fusible carbide. Very soon, therefore, metal filaments were abandoned and carbon used as lamp filament. While carbon is the most refractory body, remaining solid up to 3750 deg. cent., it was found that the carbon filament could nbt be operated much above 1800 deg. cent, without shortening the life of the lamp below economic limits by the evaporation of the carbon and the resulting blackening of the lamp globes. All bodies evaporate below their melting point. TEMPERATURE RADIATION. 79 Thus water evaporates considerably below the boiling point and even below the freezing point : ice and snow gradually disappear by evaporation even if the temperature never rises above the melting point. Considerable differences, however, exist between different bodies regarding their rate of evaporation. Thus water and benzine have practically the same boiling point, but at the same distance below the boiling point, benzine evaporates much faster than water; that is, has a much higher vapor tension. Carbon has a very high vapor tension, that is, shows a very rapid evaporation far below the boiling point, and since in the incandescent lamp the carbon vapor condenses and is deposited on the globe and carbon is black, it blackens the globe and obstructs the light. Also, the decrease of the filament section by evaporation increases its resistance and thereby decreases the power consumption and so still further lowers the efficiency. While, therefore, carbon remains solid up to 3750 deg. cent., at about 1800 deg. cent, its rate of evaporation is such as to lower the candle power of the lamp by 20 per cent in 500 hr. life, and at this temperature it gives only an output of one candle power for 3.1 watts input. Operating the carbon filament at higher temperature would increase the efficiency and thus reduce the cost of energy for the same amount of light, but would decrease the useful life of the lamp and, therefore, increase the cost of lamp renewals, and the most economical operation, as determined by balancing the cost of lamp renewals against the cost of energy, is reached by operating at such temperatures that the candle power of the lamp decreases by 20 per cent within 500 hr. life. The life of a lamp down to a decrease of candle power by 20 per cent, thus, is called the useful life, and when comparing the efficiencies of incandescent lamps it is essential to compare them on the basis of the same length of useful life: 500 hours with the carbon filament, since obviously by shortening the life higher efficiencies could be reached in any incandescent lamp. The operating temperature of the carbon filament lamp, thus, was limited by the vapor tension of carbon and not by its boiling poi^itl This limitation of carbon lead to the revival of the metal fila- ment lamps in recent years. First arrived the osmium lamp, with 1.5 watts per candle power. The melting point of osmium is very high, but still very much below that of carbon, but the vapor tension of osmium is very low even close to its melting point, so 80 RADIATION, LIGHT, AND ILLUMINATION. that osmium could be operated at temperatures far closer to its melting point without appreciable evaporation; that is, without blackening and falling off of candle power, or ; in other words, could be run at a temperature from which carbon was excluded by its too rapid evaporation. Osmium, however, is a very rare metal of the platinum group, and found only in very limited quantities in very few places and is one of those substances of which no search could very greatly increase the supply, and while one pound of osmium is sufficient for some 60,000 filaments, the total amount of osmium which has ever been found on earth would not be sufficient for one year's supply of incandescent lamps. Osmium, therefore, was excluded from general use by its limited supply. The metal tantalum does not seem to have quite as high a melt- ing point as osmium, as it can be operated only at 2 watts per candle power. Tantalum also is a very rare metal, but, unlike osmium, it is found in very many places, though in small quan- tities, but it is one of those substances, like the rare earth metals used in the Welsbach mantle, of which it seems that the supply could be indefinitely increased when required by the industries and the prices thus would go down with the demand, just as has been the case with the rare earths of the Welsbach mantle. Last of allj however, was made available the most refractory of all metals, wolfram or tungsten, and permitted to lower the specific consumption to 1 to 1.25 watts and finally, in the gas filled lamp, to less than 0.5 watts per candle power. Wolfram melts far lower than carbon, probably at about 3200 deg. cent., but far above the temperature to which the carbon filament is limited by evaporation, and having practically no vapor tension below its melting point, it can be operated far above the temperature of the carbon filament, and thus gives a much higher efficiency. Tungsten (or rather wolfram, as the metal is called, tungsten is the name of its ore) is a fairly common metal, its salts are industrially used to a very large extent for fire-proofing fabrics and its supply practically unlimited. These metal filaments thus differ from the carbon filament in that their temperature is limited by their melting point and not by evaporation, as is the case with the carbon filament, and thus their useful life is usually ended by the destruction of the filament by melting through at some weak spot, but not by blackening. TEMPERATURE RADIATION. 81 These filament lamps do not blacken the globe, except when the vacuum is defective or becomes defective, and by the residual gases in the lamp globe volatile compounds are formed, as tungsten oxides, which then deposit on the globe and terminate the life of the lamp. Even then their blackening is characteristically different from that of the carbon filament, in that it occurs very rapidly, and the lamp, after running possibly for hundreds or thousands of hours without blackening, suddenl}7 blackens within a few days and thereby becomes inoperative, while with the carbon filament the blackening is gradual throughout the life. 38. By the use of these refractory metals the efficiency of light production by temperature radiation has been greatly increased, by permitting the use of higher temperatures in the radiator than were permissible with the carbon filament clue to its evaporation. However, regarding the rate of evaporation, different modifications of carbon show very different characteristics. The carbon filaments first used in incandescent lamps were made by carbonizing vegetable fiber, as bamboo, or by squirting a solution of cellulose through a small hole into a hardening solution and carbonizing this structureless horn-like fiber. These filaments had a very high vapor tension, thus could not be run as hot as the modern carbon filament and so gave a lower effi- ciency. They are now used only as base filaments, that is, as core on which a more stable form of carbon is deposited. Such a form of carbon was found in carbon deposited on the filament by heating it in the vapor of gasolene or other hydrocarbons. This carbon deposit is of much lower electric instance than the base on which it was deposited, its negative temperature coefficient of electric resistance is lower and its vapor tension so much lower as to make it possible to operate the lamp at a specific consumption of 3.1 watts per candle power. Of late years a still more stable form of carbon has been found in the so-called "me- tallic carbon," produced from the gasolene deposited carbon shell of the filament, by exposing it for several minutes to a temperature at the boiling point of carbon; that is, the highest attainable temperature in an electric carbon tube furnace. Hereby the gasolene deposited carbon of the filament shell the inner base does not appreciably change its characteristics acquires metallic characteristics: a low electric resistance, a positive tempera- 82 RADIATION, LIGHT, AND ILLUMINATION. ture coefficient of electric resistance, metallic luster and elasticity and very low vapor tension, so that it can be run at higher temperature corresponding to a specific consumption of 2.5 to 2.6 watts per candle power, with very little blackening. These metallized carbon filament lamps exhibit characteristics similar to the metal filament lamps; their life is largely limited by 'breakage and not by blackening. Whether hereby the possibilities of carbon are exhausted or still more stable forms of carbon will be found, which permit raising the filament temperature as near to the boiling point of carbon as the temperature of the wolfram filament is to its melting point * and thereby reach an efficiency superior to that of the tungsten lamp, remains to be seen, but does not appear entirely impossible. Carbon exists in a number of "allotropic" modifi- cations of very different characteristics (similar to phosphorus in "yellow phosphorus," "red phosphorus" and "metallic phosphorus") to a greater extent than any other element, probably due to the tendency of the carbon atom to join with other carbon atoms into chains and rings, which tendency is the case of the infinite number of carbon compounds. These form two main groups: the chain carbon derivates (methane-derivates) and the ring carbon derivates (benzol derivates). The latter are far more stable at high temperatures, since the breakage of the molecule by temperature vibration is less liable in a ring structure than a chain : a single break splits the molecule in a chain forma- tion, while with a ring formation it still holds together until the break closes again. Chain hydrocarbons at higher temperatures usually convert to ring hydrocarbons. It is, therefore, reasonable to assume that the carbon skeleton left by the carbonization of the hydrocarbons also -may exist in either of the two characteristic atomic groupings : as chain carbon and as ring carbon, and that the latter exhibits a much greater stability at high tempera- ture than the former, that is, a lower vapor tension. Cellulose is a chain hydrocarbon, and as in carbonization it never passes through a fluid state, the molecular structure of its carbon atom probably remains essentially unchanged. Thus the base fila- * As carbon boils, at atmospheric pressure, below its melting point, and the limiting temperature is that at which the filament ceases to be solid, with carbon the limit is the boiling point temperature, while with tungsten it is the melting point.