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Originally appearing in Volume V27, Page 837 of the 1911 Encyclopedia Britannica.
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CATHODE FALL ELECTRODE GAS Pt Hg Ag Cu Fe 7.n Al Mg Na Na-K K 02 369 300 295 280 230 213 190 168 185 169 172 H2 N2 232 226 -- 207 1781 125 170 He 226 ---' .. 8o 78 5 69 Arg 167 100 . . The cathode fall of potential measures the smallest difference of potential which can produce a spark through the gas. Thus, for example, it is not possible to produce a spark through nitrogen with platinum electrodes with a potential difference of less than 232 volts, except when the electrodes are placed so close together that with a smaller potential difference the electric force between the terminals amounts to more than a million volts per centimetre; for this to be the case the distance between the electrodes must be comparable with the wave-length of sodium light. \\Then the current is small the glow next the cathode does not cover the whole of the surface, and when this occurs an increase in the current causes the glow to cover a greater area, but does not increase the current density nor the cathode fall. When the current is so much increased that the glow covers the whole of the cathode an increase in current must result in an increase of the current density over the cathode, and this is accomplished by a rapid increase in the. cathode fall of potential. The cathode fall in this case has been investigated by Stark (Phys. Zeit. III, p. 274), who finds that its value K can be represented by the equation K=K„+k(C–xpf)I/pfi, where K„ is the normal cathode fall, f the area of the cathode, C the current through the tube, p the pressure of the gas and k and x constants. The increase in the potential fall is much more marked in small tubes than in large ones, as with small tubes the formation of the negative glow is restricted; this gives rise to a greater concentration of the current at the cathode and an increase in the cathodefall. The intensity of the electric field in the dark space has been measured by many observers. Aston used very large plain cathodes and measured the electric force by observing the deflection of a small pencil of cathode rays sent across the dark space at different distances from the cathode. He found that the magnitude of the force at a point in the dark space was proportional to the distance of the point from the junction of the negative glow and the dark space. This law of force shows that positive electricity must be in excess in the dark space, and that the density of the electrification must be constant throughout that space. The force inside the negative glow if not absolutely zero is so small that no one has as yet succeeded in measuring it; thus the surface of this glow must be very approximately an equi-potential surface. In the dark space there is a stream of positively electrified particles moving towards the cathode and of negatively electrified corpuscles moving away from it, these streams being mutually dependent; the impact of the positive particles against the cathode gives rise to the emission of corpuscles from the cathode; these, after acquiring kinetic energy in the dark space, ionize the gas and produce the positive ions which are attracted by the cathode and give rise to a fresh supply of corpuscles. The corpuscles which carry the negative electricity are very different from the carriers of the positive; the former have a mass of only -r t of the atom of hydrogen, while the mass of the latter is never less than that of this atom. The stream of positive particles towards the cathode is. often called the Canalstrahlen, and may be investigated by allowing the stream to flow through a hole in the cathode and then measuring, by the methods described in CONDUCTION, ELECTRIC (Through Gases), the velocity and the value of e/m when e is the charge on a carrier and in its mass. It has been found that this stream is some-what complex and consists of-- a. A stream of neutral particles. S. A stream of positively electrified particles moving with a constant velocity of 2 X log cm./sec., and having e/m = to'. This is a secondary stream produced by the passage of a through the gas, and it is very small when the pressure of the gas is low. y. Streams of positively electrified atoms and perhaps molecules of the gases in the tube. The velocity of these depends upon the cathode fall of potential. The streams of negative corpuscles and positive particles produce different kinds of phosphorescence when they strike against a solid obstacle. The difference is especially marked when they strike against lithium chloride. The corpuscles make it phosphoresce with a steely blue light giving a continuous spectrum; the positive particles, on the other hand, make it shine with a bright red light giving in the spectroscope the red lithium line. This affords a convenient method of investigating the rays; for example, the distribution of the positive stream over the cathode is readiiy studied by covering the cathode with fused lithium chloride and observing the distribution of the red.glow. Goldstein has observed that the film of metal which is deposited on the sides of the tube through the sputtering of the cathode is quickly dissipated when the positive stream impinges on it. This suggests that the sputtering of the cathode is caused by the impact against it of the positive stream. This view is supported by the fact that the" sputtering is not very copious until the increase in the current produces a large increase in the cathode fall of potential. The magnitude of the potential fall and the length of the dark space are determined by the condition that the positive particles when they strike against the cathode must give to it sufficient energy to liberate the number of cathode particles which produce, when they ionize the gas, sufficient positive particles to carry this amount of energy. Thus the cathode fall may be regarded as existing to make the cathode emit negative corpuscles. If the cathode can be made to emit corpuscles by other means, the cathode fall of potential is not required and may disappear. Now Wehnelt (Ann. Phys., 1904, 14, p. 425), found that when lime or barium oxide is heated to redness large quantities of negative corpuscles are emitted; hence if a cathode is covered with one of these substances and made red hot it can emit corpuscles without the assistance of an electric field, and we find that in this case the cathode fall of potential disappears, and current can be sent through the gas with very much smaller differences of potential than with cold cathodes. With these hot cathodes a luminous current can under favourable circumstances be sent through a gas with a potential difference as small as 18 volts. The dimensions of the parts of the discharge we have been considering—the dark space and the negative glow—depend essentially upon the pressure of the gas and the shape of the cathode, and do not increase when the distance between the anode and cathode is increased. The dimensions of the other part of the discharge which reaches to the anode and is called the positive column depends upon the length of the tube, and in long tubes constitutes by far the greater part of the discharge. This positive column is separated from the negative glow by a dark interval generally known as the Faraday dark space; the dimensions of this dark interval are very variable—it is sometimes altogether absent. The positive column assumes a considerable variety of forms as the current through the gas and the pressure are varied: some-times it is a column of uniform luminosity, at others it breaks up into a series of bright and dark patches known as striations. Some examples of these are given in fig. 17 of CONDUCTION, ELECTRIC (Through Gases). The distance between the striations varies with the pressure of the gas and the diameter of the tube, the bright parts 3eing more widely separated when the pressure is low and the diameter of the tube large, than when the pressure is high and the tube small. The striations are especially brilliant and steady when a Wehnelt cathode covered with hot lime is used and the discharge produced by a number of storage cells; by this means large currents can be sent through the tube, resulting in very brilliant striations. When the current is increased the positive column shortens, retreating backwards towards the anode, and may, by using very low currents, be reduced to a glow over the surface of the anode. The electric force in the positive column has been measured by many observers. It is small compared with the forces which exist in the dark space; when the luminosity in the positive column is uniform, the force there is uniform; when the positive column is striated there are periodic variations in the electric force, the force being greater in the bright parts of the striation than in the dark. Anode Drop of Potential.—Skinner (Wied. Ann. 68, p. 752; Phil. Mag. [6], 8, p. 387) has shown that there is a sudden change in potential between the anode itself and a point in the gas close to the anode. This change amounts to about 20 volts in air; it is thus much smaller than the cathode fall of potential, and it is also much more abrupt. There does not seem to be any region at the anode comparable in dimensions with the Crookes' dark space in which the drop of potential occurs. The highly differentiated structure we have described is not the only way in which the current can pass through the tube. If a large Leyden jar is suddenly discharged through the tube the discharge passes as a uniform, continuous column stretching without interruption from anode to cathode; Gold-stein has shown (Verb. deutsch. phys. Ges. 9, p. 321) that the spectrum of this discharge shows very interesting characteristics. (J. J. T.)
End of Article: CATHODE
CATHETUS (Gr. K60ero3, a perpendicular line)
CATHOLIC (Gr. KcaBoXuK6s, general, universal)

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