LECTURE I. 
NATURE AND DIFFERENT FORMS OF RADIATION. 

1. Radiation is a form of energy, and, as such, can be produced 
from other forms of energy and converted into other forms of 
energy. 

The most convenient form of energy for the production of rad- 
iation is heat energy, and radiation when destroyed by being 
intercepted by an opaque body, usuaDy is converted into heat. 
Thus in an incandescent lamp, the heat energy produced by the 
electric current in the resistance of the filament, is converted 
into radiation. If I hold my hand near the lamp, the radiation 
intercepted by the hand is destroyed, that is, converted into heat, 
and is felt as such. On the way from the lamp to the hand, how- 
ever, the energy is not heat but radiation, and a body which is 
transparent to the radiation may be interposed between the 
lamp and the hand and remains perfectly cold. The terms 
"heat radiation " and " radiant heat," which are occasionally 
used, therefore are wrong: the so-called radiant heat is not heat 
but radiation energy, and becomes heat only when, intercepted 
by an opaque body, it ceases to be radiation; the same, however, 
applies to any radiation. If we do not feel the radiation of a 
mercury lamp or that of the moon as heat, while we feel that of a 
coal fire, it is merely because the total energy of the latter is very 
much greater; a sufficiently sensitive heat-measuring instrument, 
as a bolometer, shows the heat produced by the interception of 
the rays of the mercury lamp or the rays of the moon. 

The most conspicuous form of radiation is light, and, therefore, 
it was in connection with this form that the laws of radiation 
were first studied. 

1 


2 RADIATION, LIGHT, AND ILLUMINATION. 

2. The first calculations of the velocity of light were made by 
astronomers in the middle of the eighteenth century, from the 
observations of the eclipses of the moons of Jupiter. A number 
of moons revolve around the planet Jupiter, some of them so close 
that seen from the earth they pass behind Jupiter and so are 
eclipsed at every revolution. As the orbits of Jupiter's moons 
were calculated from their observations by the law of gravita- 
tion, the time at which the moon M should disappear from sight, 


FIG. 1. 

when seen from the earth E, by passing behind Jupiter, 7 (Fig. 1), 
could be exactly calculated. It was found, however, that some- 
times the moon disappeared earlier, sometimes later than cal- 
culated, and the difference between earliest and latest disappear- 
ance amounts to about 17 min. It was also found that the 
disappearance of the moon behind Jupiter occurred earlier when 
the earth was at the same side of the sun as Jupiter, at A, while 
the latest disappearance occurred when the earth was on the 
opposite side of the sun from Jupiter, at B. Now, in the latter 
case, the earth is further distant from Jupiter by the diameter 
ASB of the orbit of the earth around the sun S, or by about 
195,000,000 miles and the delay of 17J min. thus must be due to 
the time taken by the light to traverse the additional distance 
of 195,000,000 miles. Seventeen and one-third min. are 1040 
sec. and 195,000,000 miles in 1040 sec. thus gives a velocity of 


light of » or 188,000 miles per sec. 


Later, the velocity of light was measured directly in a number 
of different ways. For instance, let, in Fig. 2, D be a disk per- 
forated with holes at its periphery. A lamp L sends its light 
through a hole H0 in the disk to a mirror M located at a con- 
siderable distance, for instance 5 miles; there the light is reflected 


NATURE AND DIFFERENT FORMS OF RADIATION. 3 

and the mirror is adjusted so that the reflected beam of light 
passes through another hole Hl 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 


5_MOE_S 


FIG. 2. 

direction the light reappears. If the disk is slowly revolved, alter- 
nate light and darkness will be observed, but when the speed in- 
creases so that more than from 10 to 20 holes pass per second, the 
eye is no longer able to distinguish the individual flashes of light 
but sees a steady and uniform light; then increasing the speed 
still more the light grows fainter and finally entirely disappears. 
This means when a hole H0 is in front of the lamp, a beam of 
light passes through the hole. During the time taken by the light 
to travel the 10 miles to the mirror and back, the disk D has 
moved, and the hole Hv which was in front of the telescope 
when the light from the lamp passed through the hole HQ, has 
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 again, and increases in brilliancy, reaching a maximum 
at twice the speed at which it had disappeared. Then the light 
reflected from the mirror M again passes through the center of 
a hole into the telescope, but not through the same hole Ht 
through which it would have passed with the disk stationary, but 
through the next hole H2, that is, the disk has moved a distance 
equal to the pitch of one hole while the light traveled 10 miles. 
Assume, for instance, that the disk D has 200 holes and makes 


4 RADIATION, LIGHT, AND ILLUMINATION. 

94 rev. per sec. at the moment when the light has again reached 
full brilliancy* 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 •* > 

lo,oOU 

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 light- 
giving bodies with enormous velocities, that is, a kind of bom- 
bardment. This theory has been revived in recent years to 
explain the radiations of radium, etc. Euler explained the light 
as a wave motion. Which of these explanations is correct 
can be experimentally decided in the following manner: Assum- 
ing light to be a bombardment of minute particles, if we com- 
bine 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 intensity. If, however, we assume light is a wave 
motion, then two equal beams of light add to one of twice the 
intensity only in case the waves are in phase, as Al and B^ in 
Fig. 3 add to Cr If, however, the two beams A2 and B2 are not 
in phase, their resultant C2 is less than their sum, and if the 
two beams A3 and B3 in Fig. 3 happen to be in opposition 
(180 degrees apart), that is, one-half wave length out of phase 
with each other, their resultant is zero, that is, they blot each 
other out. 

Assuming now we take a plain glass plate A (Fig. 4) and a 
slightly curved plate B, touching each other at (7, and illuminate 
them by a beam of uniform light — as the yellow light given by 
coloring the flame of a bunsen burner with some sodium salt — 
a part of the light b, is then reflected from the lower surface of 


NATURE AND DIFFERENT FORMS OF RADIATION. 5 

the curved glass plate B, a part c, passes out of it, and is reflected 
from the upper surface of the plain glass plate A. A beam of 


FIG. 3. 


reflected light a, thus is a combination of a beam b 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 dv dv etc. at which the distance 


FIG. 4. 


between the two glass plates is J wave length, or j, J, etc., the 
two component beams of a would differ by \, 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 J, 1, lj, 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 B, and the diameter 
of the dark rings d, the distance between the plates B and A at 
the dark rings d, can be calculated and as this distance is one- 
quarter 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 (monochro- 
matic light), ordinary white light is used, which is a mixture of 
all colors, the dark and bright rings of the different colors appear 
at different distances from each other, those of the violet near- 
est and those of the red the furthest apart, and so superimpose 
upon each other, and instead of alternately black and light rings, 
colored rings appear, so-called interference rings. Wherever a 
thin film of air or anything else of unequal thickness is inter- 
posed between two other materials, such interference colors thus 
appear. They show, for instance, between sheets of mica, etc. 
The colors of soap bubbles are thus produced. 

The production of such colors by the interference of rays of 
light differing from each other by a fractional wave length is 
called iridescence. 

Iridescent colors, for instance, are those of mother-of-pearl, 
of opal, of many butterflies, etc. 

Light, therefore, is a wave motion. 


NATURE AND DIFFERENT FORMS OF RADIATION. 1 

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 10~8 cm. (or about 
^<y^<5-<y in.) and since the speed is S = 3 X 1010 cm. the frequency 

a 

is / = r- = 500 X 1012, 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 essen- 
tially 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; that is, 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 
waves in air. If in Fig. 5 sound waves travel from the bell B in 
the direction BA, the air molecules m 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 lead to confusion: 

The micron, denoted by ft, equal to one thousandth of a millimeter. 

The fJLfJ., equal to one millionth of a millimeter. 

The Angstrom unit, equal to one ten-millionth of a millimeter. 

As seen, the basis of these 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 p, 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 the 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 (Fig. 7). In the latter case, the wave is 

called " polarized" and has differ- 
ent characteristics in three direc- 
tions at right angles to each other : 
one direction is the direction of 
propagation, or of wave travel; the 
second is the direction of vibration; 
IG' 6' and the third is the 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 con- 
ductor 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, this would prove tke 
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 
polarized ; that is, for instance, the reflection from the mirror m0, 
set like the mirrors mv m2 . . . 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 ra/. 

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. 

In a second crystal, K2, beam b would then enter as a single 
beam, under the same angle as in the first crystal Kv if K2 were 
in the same position as Kl ; while if K2 were turned at right angles 
to Kv beam b would enter K2 under the same angle as beam c in 
crystal Kr 

6. As seen, light and radiation in general are transversal wave 


motions of very high speed, S = 3 X 1010 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 10"6 cm. to 39 X 10"' 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 

* The visibility of radiation is greatest between the wave lengths 50 X 10~* 
to 60 X 10~e and good between the wave lengths 41 X 10~e to 76 X 10~8, 
but extends more or less indistinctly over the range of wave lengths from 
33 X 10~6 to 77 X 10~6 and faintly even as far as 30 X 10~fl to 100 X 


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 hold- 
ing 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 side 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. 11 


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 tempera- 
ture, the total radiation increases, 
but the waves of high frequency in- 
crease more rapidly than those vof 
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 tung- 
sten 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 
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 we 
should go still further and very greatly increase the temperature, 
probably 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 


FIG. 10. 


12 


RADIATION, LIGHT, AND ILLUMINATION. 


would 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. 


FIG. 11. 


In the tungsten lamp at high brilliancy and more still in the 
mercury arc, radiations of higher frequencies appear, that is, 
shorter wave lengths than visible light, and these radiations are 
again invisible. As they are of frequencies beyond the violet 
rays of light, they are called " ultra-violet rays/' while the radia- 
tions which we produced from the 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 dis- 
charge between iron terminals, a so-called ultra-violet arc lamp. 
Three iron spheres, 7 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 ^ 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 
the 240-volt 60-cycle circuit through a rheostat R 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 
light of the iron arc. A thick piece of crystallized gypsum (sel- 
enite) 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 glass 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 see them shine in the green glare. Quartz is trans- 
parent to ultra-violet light. When investigating ultra-violet 
light, quartz lenses 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 
3000 X 1012 cycles per sec., or to wave lengths of about 10 X 10~6 
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 their average wave length has been 
estimated as 0.01 X 10~6 cm., giving a frequency of 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 Jn as high. By this scale all the inter- 
vals are of the same character; one octave means the same 
relative increase, which ever may be the absolute frequency or 
wave length. 

As the perceptions of our senses vary in proportion to the per- 
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 less than one octave; 
ultra-violet radiations have been observed beyond this for about 
two more octaves. Ten octaves higher is the estimated 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 ultra- 
red 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 tem- 
perature, 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) 


FIG. 12. 

the condenser C of the apparatus which I used for operating the 
ultra-violet arc, to a spark gap Gv of which the one side is con- 
nected to ground Bv the other side to a vertical aluminum rod Alf 
about 8 feet long. The charge and discharge of the aluminum 
rod Al 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 A2 back of you, induces therein 
an electric charge. A2 is separated from ground B2 by a narrow 
spark gap G2 between graphite terminals, and the arrival of the 
electric wave at A2 causes a small spark to jump across the gap 
Gv which closes the circuit of the tungsten lamp L, thereby 
lighting it as long 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 107 to 105 cycles per sec. 

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, — 

3 X 1010 
a 60-cycle alternating current gives a wave length of ^ = 

500 X 10" 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 condensers, as 
shown in Fig. 13. On these waves the velocity of propagation 


< EN ERGY-SU PPLY- 


o — ^^ — o 


FIG. 13. 

has been measured by Herz by producing standing waves by 
combination of main wave and reflected wave. 

Still much higher frequencies are the oscillations between the 
cylinders of multi-gap lightning arresters, and the limit of fre- 
quency of electric waves would probably be given by the oscilla- 
ting discharge of two small spheres against each other when 
separated by a narrow gap. It probably is at about 5 X 1010 
cycles, or 0.6 cm. wave length. 

The blank space between the longest electric wave and the 
shortest ultra-violet light wave thus has become fairly narrow: 
from 0.6 to 0.03 cm., or only about four octaves. 

8. In the following tables, the different known forms of radia- 
tion are arranged by their frequency and wave length, and also 
given in octaves, choosing as zero1 point the middle c of the piano, 
or a frequency of 128 cycles per sec. 


UNIVERSITY 

OF 


NAT 


IFFERENT FORMS OF RADIATION. 17 


SPECTRUM OF RADIATION. 

Zero point chosen at c = 128 cycles per second. 
Speed of radiation S = 3 X lu10 cm. 


Cycles. 

Wave Length in Air 
(or Vacuum). 

Octave: Q^/ 

£. 

Alternating current 

1> 

field: 

15 

20,000 km. = 12,500 mi. 

25 

12.000 km. = 7, 500 mi. 

3.15 

60 

5, 000 km. = 3, 100 mi. 

133 

2,250 km. = 1,400 mi. 

High frequency cur- \ 

rents, surges and 

oscillations, arcing V 

(9.57) 

31.64 

grounds, lightning 

phenomena, etc. J 

Wireless telegraph ( 

105 

3 km. = 10,000 ft. 

9.63 ) A ~, 

waves : ( 

107 

30 m. = 100 ft. 

16.25 \ 6'62 

Herzian waves: 

107 
109 

30 m. = 100 ft. 
30 cm. = 1 ft. 

16.25 ) 
22.90 I 12.3 

Limit of electric waves : 

6X1010 

0.6 cm. = 0.25 in. 

28.55 ) ) 

First gap : 

[4.25] 

Ultra-red rays : 

10'2 

4X10' 

30,000x10-" =0.03 cm. 
76xlO~8 cm. 

3280 ) ftAQ 
41.48 f b>ba 

Visible light rays : j 

7.7X10' 

76xlO-6cm. 
39xlO~e cm. 

41.48 i ft Q7 
42.45 ] U<y/ 

11.6 

Ultra-violet rays : 

7.7X10' 
30X101 

39X10-6 cm. 
10 x 10"8 cm. 

42.45 ) , Q. 
44.40 f 1'wo 

Second gap: 

[8.0] 

X-rays (estimated) : 

3X10'8 

0.01 xlO~J cm. 

54.4 

Sound Waves : 

Total : 

57.7 octaves 

Lowest audible sound : 

15 

66 ft. in air 

-3.1 

Highest audible sound : 

8000 

1.5 in. in air 

+ 6.0 

Total : 

9.1 octaves 

These radiations are plotted graphically in Fig. 14, with the 
octave as abscissae. 

As seen, the total range of frequencies of radiation is enormous, 
covering nearly 60 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 be- 
tween 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 pro- 
duction : 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 eight 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 
as far as possible when producing light, as they consume power 


FIG. 14. 

and so lower the efficiency; the ultra-violet rays are of importance 
in medicine as germ killers. They are more or less 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 the frequencies of 
alternating currents and the limits of electric waves, has been of 
importance to the electrical engineer as harmful and destructive 
phenomena in electric circuits, which are to be guarded against, 
and only in recent years, with the development of wireless 
telegraphy, some such electric waves have found a useful com- 
mercial application. The main object of their study — which is 
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 times greater than 
the difference between high and low frequency alternating cur- 
rents, 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 ex- 
plains the apparently contradictory evidence of many experi- 
mental investigations on the protection of electric circuits.