Page:EB1911 - Volume 27.djvu/862

over a part of its surface as in fig. 1, while if the electrode is a ring of wire (fig. 2) the luminous boundary resembles that shown in fig. 17 of the article Conduction, Electric (Through Gases). The length of the dark space depends on the pressure of the gas and on the intensity of the current passing through it. The width of the dark space increases as the pressure diminishes, and may, according to the experiments of Aston (Pro. Roy. Soc. 79, p. 81), be represented with considerable accuracy by the expression a+b/p or a+ cλ, where a, b, c are constants, p the pressure and λ the mean free path of a corpuscle through the gas. The thickness of the dark space is larger than this free path; for hydrogen, for example, the value of c is about 4.

Fig. 1.		Fig. 2.

When the current is so large that the whole of the cathode is covered with glow the width of the dark space depends upon the current decreasing as the current increases. In helium and hydrogen Aston (Pro. Roy. Soc. 80 A., p. 45) has detected the existence of another thin dark space quite close to the cathode whose thickness is independent of the pressure. The farther boundary of the Crookes dark space is luminous and is known as the negative glow or the third negative layer. Until the current gets so large that the glow next the cathode covers the whole of its surface the potential difference between the cathode and the negative glow is independent of the pressure of the gas and the current passing through it; it depends only on the kind of gas and the metal of which the cathode is made. This difference of potential is known as the cathode fall of potential; the values of it in volts for some gases and electrodes as determined by Mey (Verh. deuts. Phys. Ges., 1903, v. p. 72) are given in the table.

Cathode Fall

Gas	Electrode
	Pt	Hg	Ag	Cu	Fe	Zn	Al	Mg	Na	Na-K	K
O2	369	. .	. .	. .	. .	. .	. .	. .	. .	. .	—
H2	300	. .	295	280	230	213	190	168	185	169	172
N2	232	226	. .	. .	. .	. .	. .	207	178	125	170
He	226	. .	. .	. .	. .	. .	. .	. .	80	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.

When 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_n+k(C−xpf)^1/2/pf ^1/2,

where K_n 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 cathode fall. The intensity of the electric field in the dark space has been measured by many observers. Aston used very lar e plain cathodes and measured the electric force by observing the dejection 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 1/1700 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 streamito 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 m its mass. It has been found that this stream is somewhat complex and consists of—

α. A stream of neutral particles.

β. A stream of positively electrified particles moving with a constant velocity of 2×10⁸ cm./sec., and having e/m=10⁴. This is a secondary stream produced by the passage of α through the gas, and it is very small when the pressure of the gas is low.

γ. Streams of positively electrified atoms and perhaps molecules of the gases in tli)e 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 iine. This affords a convenient method of investigating the rays; for example, the distribution of the positive stream over the cathode is readily 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: sometimes it is a column of uniform luminosity, at others it breaks up into