Percy Phillips

“ A body which is receiving waves from the sun on one side is thereby heated and emits waves in all directions. As it is moving in its orbit it will crowd up the waves which it sends out in front of it and lengthen out those which it sends out behind it. But the energy per cubic centimetre will be greater where the waves are crowded up than where they are drawn out, and therefore the body will experience a retarding force in its orbit. As the body tends to move more slowly it falls in a little towards the sun, and so approaches the sun in a spiral path. ”

“ A second way in which the heat lost by a body has been measured at different temperatures is by heating a conductor such as a thin platinum strip by means of an electric current, and measuring the temperature to which the conductor has attained. When its temperature is steady, all the energy given to it by the current must be lost as heat, and therefore the electrical energy, which can very easily be calculated, must be equal to the heat radiated by the body minus the heat received from the enclosure. ”

“ The energy in the spectrum of even the hottest of terrestrial radiators is mostly in the longer waves of the infra-red, but the position of the maximum of energy moves to shorter and shorter wave-lengths as the temperature rises, and so more of the shorter waves make their appearance. The sun is not a full radiator, but is nearly so, and its temperature is so high that the maximum of energy in its spectrum is in the visible part near to the red end. ”

“ As the wave of light with its alternating electric fields comes up to the prism, the field will first displace the electrons in one direction and then in the other, and so on. If the period of one particular type of electron happens to coincide with the period of the wave, that electron will vibrate violently and will in its turn send out a series of waves in the glass. If the wave is an irregular one it will start all the electrons vibrating, but those electrons will vibrate most violently whose periods are equal to the periods of the Fourier's constituents which have the greatest energy. ”

“ Three Effects of Light Pressure.—We thus have three effects of light pressure on bodies describing an orbit round the sun. The first effect is to lengthen their period of revolution, the second is to make their orbits more circular, and the third is to make them gradually approach the sun in a spiral path. These effects are quite inappreciable for bodies anything like the size of the earth, but for small bodies of the order of one centimetre diameter or less the effects would be quite large. ”

“ The amount of heat radiated will depend on the temperature and character of the body itself, while the amount absorbed will depend upon the condition of the surroundings as well as upon the nature {47} of the body. If the amount of heat radiated is greater than the amount absorbed the body will fall in temperature, and vice-versa. This view of Prévost's is called the Theory of Exchanges, and we can see that it is a necessary consequence of our ideas as to the production of heat and light waves by the agitation of electrons in the radiating body. ”

“ Thus when any body which has a free period of vibration is subject to periodic impulses of the same period as its own, it will vibrate very vigorously and absorb nearly all the energy of the impulses. ”

“ The eye is sensitive only to a very small range of period, while there exists in sunlight a range many times as great.
Those waves of shorter period than the violet end of the visible spectrum will be deviated even more than the violet, and will therefore be beyond the violet. They are called the ultra violet rays, and can easily be detected by means of their chemical activity. They cause a number of substances to glow, and therefore by coating the screen on which the spectrum is received with one of these substances, the violet end of the spectrum is extended by this glow. ”

“ The mass has energy due to its motion, and in order to stop the mass this energy must be converted into some other form of energy and work must therefore be done. The electric field has energy due to its motion—the energy of the magnetic field—and therefore to stop the motion of the electric field, the energy of the magnetic field must be converted into some other form, and work must therefore be done. One consequence of the momentum of a moving mass is well illustrated by the pendulum. ”

“ A large number of quite simple experiments show that whenever the electric field at any point is changing, i.e. whenever the lines of force are moving perpendicular to themselves, a magnetic field is produced at the point, and this magnetic field lasts while the change is taking place. An exactly similar result is observed when the magnetic field at a point is changing—an electric field is produced which lasts while the magnetic field is changing. ”

“ A radiator for heating a room should therefore have a dull, dark surface, while a vessel which is designed to keep its contents from losing heat should have a highly polished exterior.
A perfectly transparent substance would radiate no energy, whatever the temperature to which it is {42} raised, for its absorbing power is zero and therefore its radiating power is also zero. No perfectly transparent substances exist, but some substances are a very near approach to it. ”

“ There are many reasons for believing that there exist in the atoms of all substances, minute negatively electrified particles which may rotate in small orbits or oscillate to and fro within the atom. There also exists an equal positive charge within the atom. As the negative particles rotate or oscillate in the atom, it is evident that the field between them and the positively electrified part of the atom alternates, and so electro-magnetic waves are sent out. ”

“ Each disc is heated by the beam of light, and the gas in contact with it becomes heated and causes convection currents in the gas. At very low pressures a slightly different action of the gas becomes a disturbing factor. This effect is due to the molecules which come up to the disc becoming heated and rebounding from the disc with a greater velocity than that with which they approached it. The rebound of each molecule causes a backward kick on to the disc, and the continual stream of molecules causes a steady pressure. ”

“ THE LAWS OF RADIATION
Absorbing Power.—A perfectly dull black surface is simply one which absorbs all the light which is falling on it and reflects or diffuses none of it back. If the surface absorbs the heat as well as the light completely, it is called a perfect or full absorber. Other surfaces merely absorb a fraction of the heat and light falling on them, and this fraction, expressed usually as a percentage, is called the absorbing power of the surface. ”

“ An incandescent solid or liquid gives a continuous spectrum, i.e. all the different wave-lengths are represented, but the part of the spectrum which has the greatest energy is different for different substances and for different temperatures: cf. arc and gas flame in Fig. 16. This is quite in keeping with the idea already suggested that in solids and liquids there are electrons of almost every period of vibration. When they are agitated by being heated, a mixture of simple waves of all periods will be sent out giving a very irregular wave. ”

“ This would be the upper limit to the temperature of the surface of the moon at a point where the sun is at its zenith.
On the surface of the earth the sunlight has had to pass through the atmosphere, and in perfectly bright sunshine it is estimated that only three-fifths of the heat is transmitted. Any surface is also radiating out into surroundings which are at about 300° absolute. Taking into account these two facts, we find that the upper limit to a non-conducting surface in full sunshine on the earth is about 365° absolute, or only a few degrees less than the boiling point of water. ”

“ Since the distance travelled by a particle in a single period of the wave will be equal to four times the amplitude, the velocity at any point in the wave must be proportional to the amplitude and therefore the kinetic energy is proportional to the square of the amplitude. ”

“ Thus resonance might occur, and the consequent vibrations of the electrons would start a series of secondary waves.
If, however, we assume resonance, it is difficult to see why there is a range of wave-lengths produced and another range of wave-lengths which may produce them. We should have expected one definite wave-length or a few definite ones producing one or a few definite wave-lengths in the glow, while if a whole range of waves will produce the effect it is difficult to see why all bodies do not exhibit the phenomenon. ”

“ It therefore need not move in its orbit with quite such a high speed in order that it may not fall into the sun, and its year is therefore lengthened by about three minutes. The lengthening out of comets' tails as they approach the sun, and the apparent repulsion of the tail by the sun, has sometimes been attributed to pressure of sunlight, but it is pretty certain that the forces called into play are very much greater than can be accounted for by the light. ”

“ When we consider very small bodies, however, we find that the pressure of the light may even exceed the gravitational pull, and therefore these small particles will be driven right away from our system. ”

“ Newton's Experiment.—Newton first discovered this fact with an arrangement like that in Fig. 17.
FIG. 17.
If by any means the fan of coloured rays be combined again into a single beam, white light is reformed, and Newton therefore came to the conclusion that white light was a mixture of the various colours in the spectrum, and that the only function of the prism was to separate the constituents. Of the nature of the constituents Newton had little knowledge, since he had rejected the wave theory, which could alone give the clue. ”

“ Where the actual distribution of energy in the spectrum of any source of heat is to be determined the difficulty becomes more serious, and probably there is some error in the determinations, especially in the longest waves, which are masked almost completely by the overlapping shorter waves. ”

“ Before the spark is {86} produced, the lines of force on the lower side of AB will in form be something like the dotted lines in the figure, but as soon as the air becomes a conductor, the positive ends of the lines will surge from A towards B and on to B, and the negative ends will surge on to A. These to and fro surgings will continue for a little while, but will gradually die out. As the surgings are all up and down AB, the electric vibrations in the electromagnetic waves sent out {87} will all be parallel to AB, and therefore they will be polarised. ”

“ The heating of the body and the consequent re-emission of heat-waves is not, however, the only process which goes on. In a large number of substances, waves are given out under the stimulus of other waves without any heating of the body at all. In most of these cases the emission stops as soon as the stimulating waves are withdrawn, and in these cases the phenomenon has been called fluorescence. ”