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bright greenish yellow phosphorescence, which changed its position when a magnet was brought near to it. About to years afterwards Hittorf showed that a solid body placed between a pointed electrode and the walls of the tube cast a well-defined shadow of such a shape as to show that the agent producing the phosphorescence travels in straight lines at right angles to the surface of the cathode. The name " cathode rays " for the cause of the phosphorescence was introduced by Goldstein, who made many important investigations on their properties. The opinion held by Goldstein and generally in Germany was that cathode rays were waves in the ether. Varley and Crookes advanced the view that they were electrified molecules shot off at right angles to the cathode. The discovery by Hertz that the cathode rays could pass through thin layers of gold leaf was difficult to recon- cile with this view. The evidence in favour of the cathode rays being electrified particles was much increased by Perrin's dis- covery that when a pencil of the rays entered the opening in a Faraday cylinder they gave a negative charge to the cylinder. One difficulty which had been urged against the rays being nega- tively electrified, viz. that, though they were deflected by a magnetic force, an electric force produced no effect upon their path was removed by J. J. Thomson, who showed that the absence of deflection was due to the gas in the tube acting as a screen and protecting the particles from the electric force. As the gas in the vacuum tube is a conductor of electricity the rays move inside a conductor of electricity, and so will not be affected by an external electrified body. Thomson showed that when the vacuum was very high, so that there was but little gas in the tube, the cathode rays were deflected by an electric and magnetic field, and that the direction of the deflec- tion indicated a negative charge on the particles. The measure- ment of the deflection by known electric and magnetic forces led to a determination of the mass of the particles which carried the charge, and showed that these particles were not atoms or molecules but something with a mass not one-thousandth part of the mass of the lightest atom known, that of hydrogen.

The deflection due to electric and magnetic forces can be calcu- lated as follows. Suppose that the particles are travelling horizon- tally between two parallel horizontal metal plates A, B, maintained at a constant difference of potential, there will be a vertical electric force F acting between the plates, and if the axis of y is vertical the equation of motion of the electrified particle when it is between the plates is

fy ., n> W =Fe.

dy

If y and -? are both zero when the particle enters the region be- tween the plates, then, when it leaves this region, after a time

I Fe dv Fe

y = - f and = /. 2m dt m

Since the electric force is at right angles to the direction of motion of the particles, i> the velocity of the particles will not alter, and if the deflection is small, t=l/v where / is the length of the plates. Thus

i Fe P, dy Fe I y =2 r^ and d* = m ?

Suppose the particles strike a photographic plate or a screen cov- ered with a phosphorescent substance at a distance L from the end of the plates, the y displacement at this plate produced by the elec- tric force is given by the expression

I Fe P, Fe IL

Hence

y =2 * ^ H

m t?

Magnetic Deflection of the Rays. If the rays go through a uni- form magnetic field of length / and strength H, then if the mag- netic force is vertical the force acting on the moving particles will be tin, and will be at right angles to the magnetic force and also to the direction of motion of the rays; i.e. it will be at right angles to the plane of the paper ; if z is the displacement of the particle in this direction

From this we see that the value of z at the screen is given by

_/ = zf m y

m(-

. (26),

"=yH * ' ' (27) '

Thus the measurements of y and z, the electric and magnetic deflec- tions, give the values of e/m and.

The expressions for y and z have been obtained on the supposi- tion that the electric and magnetic fields acted one at a time and not simultaneously. If, however, y and z are small, their values will not to a first approximation be altered if the electric and magnetic deflections occur simultaneously. Thus by making the cathode rays pass through superposed electric and magnetic fields, e/m and v can be got with one exposure by measuring y and z on the screen or photographic plate.

Since from the above equation (26) z'/y is constant as long as e/m is constant, we see that all the particles of the same kind, whatever their velocity, would strike the screen or plate on a parabola, and that if the rays were a mixture of particles of different kinds each kind of particles would trace out a different parabola. Since z/y only depends upon r, all the particles moving with the same velocity will strike the screen or plate in a straight line.

The determination of e/m for the cathode rays led to results of fundamental and far-reaching importance, for it was found that all the cathode rays had the same value for e/m, and that moreover while for a charged atom of hydrogen in liquid electrolytes ejm was equal to io 4, when e was measured in electromagnetic units, the value of e/m for the particles in the cathode rays was considerably more than one thousand times this value. Thus if e were the same for the particle as for the hydrogen atom (and we shall see later that this is the case), the mass of the cathode particle is only ,,^,0 of that of an atom of hydrogen, the smallest mass which hitherto had been recognized. Again it was found that whatever metal might be used for the cathode, or whatever might be the gas in the discharge tube, the value of e/m was unaltered. As those particles must have come either from the electrode or the gas, it follows that the particles of the cathode rays are a constituent of the atoms of all the chemical elements. These particles are called " electrons."

After the electrons had once been detected in the cathode rays, they were soon detected under many other conditions and found to be of very wide-spread occurrence. Thus, for example, it was found that streams of electrons are given out by incandescent metals, the rate of emission increasing very rapidly with the temperature. This has received a very important industrial application in what are known as " hot wire valves," at which a current from a hot cathode passes through a vessel in which the vacuum is so perfect that the gas takes no part in the discharge; the current, in some cases amounting to several amperes, is carried entirely by electrons. Lenard found that they were emitted by metals exposed to ultra- violet light. They are emitted when Rontgen rays strike against matter, and by radio-active substances. The speed of the electrons ejected either by ultra-violet light or by Rontgen rays does not depend upon the intensity of the radiation but only upon the wave length. The energy acquired by the electrons is = hn where n is the frequency of the radiation and h Planck's constant.

Since the cathode rays are deflected by electric and magnetic forces proportionally to the magnitude of these forces, we can use the deflection of the rays as a measure for electric and magnetic forces. As these rays have practically no inertia they are especially adapted to measure very rapidly alternating forces which could not be detected by any index having an appreciable mass. The cathode ray oscillograph, an instrument by which electric and magnetic forces are measured by the deflection of cathode rays, has already been used in many investigations, and is a very important aid to research. Another property of cathode rays is that when they strike against matter they generate Rontgen rays, the hardness of the latter increasing with the speed of the former.

Positive Rays. Goldstein discovered in 1886 that, if the cathode on a highly exhausted tube was perforated, bundles of a luminous discharge streamed through the aperture into the space behind the cathode. The colour of this discharge depends upon the gas in the tube; thus in hydrogen it is rose colour; in air, yellowish. The colour of the light due to these rays is not the same as that produced when cathode rays pass through the gas. In some gases the difference is very striking: thus in neon the light due to the cathode rays is pale blue, while the discharge which streams through the cathode is a gorgeous red. Goldstein called the rays which stream through the hole in the cathode Kanalstrahlen; but as they have been proved to consist of posi- tively charged particles it seems more natural to call them " positive rays." These rays produce phosphorescence when they strike against glass and many other substances, though the phosphorescence is generally of a different colour from that produced by cathode rays. They also affect a photographic