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½ the discharge though the rest of the field were comparatively weak. Such a distribution of electric force requires, however, a great accumulation of charged ions near the cathode; until these ions accumulate the field will be uniform. If the uniform field existing in the gas before the discharge begins were strong enough to make the corpuscles produce ions by collision, but not strong enough to make the positive ions act as ionizers, there would be some accumulation of ions, and the amount of this accumulation would depend upon the number of free corpuscles originally present in the gas, and upon the strength of the electric field. If the accumulation were sufficient to make the field near the cathode so strong that the positive ions could produce fresh ions either by collision with the cathode or with the gas, the discharge would pass though the gas; if not, there will be no continuous discharge. As the amount of the accumulation depends on the number of corpuscles present in the gas, we can understand how it is that after a spark has passed, leaving for a time a supply of corpuscles behind it, it is easier to get a discharge to pass through the gas than it was before.

EB1911 Conduction, Electric - Fig. 15.jpg
Fig. 15.

The inequality of the electric field in the gas when a continuous discharge is passing through it is very obvious when the pressure of the gas is low. In this case the discharge presents a highly differentiated appearance of which a type is represented in fig. 15. Starting from the cathode we have a thin velvety luminous glow in contact with the surface; this glow is often called the “first cathode layer.” Next this we have a comparatively dark space whose thickness increases as the pressure diminishes; this is called the “Crookes’s dark space,” or the “second cathode layer.” Next this we have a luminous position called the “negative glow” or the “third cathode layer.” The boundary between the second and third layers is often very sharply defined. Next to the third layer we have another dark space called the “Faraday dark space.” Next to this and reaching up to the anode is another region of luminosity, called the “positive column,” sometimes (as in fig. 15, a) continuous, sometimes (as in fig. 15, b) broken up into light or dark patches called “striations.” The dimensions of the Faraday dark space and the positive column vary greatly with the current passing through the gas and with its pressure; sometimes one or other of them is absent. These differences in appearances are accompanied by great difference in the strength of the electric field. The magnitude of the electric force at different parts of the discharge is represented in fig. 16, where the ordinates represent the electric force at different parts of the tube, the cathode being on the right. We see that the electric force is very large indeed between the negative glow and the cathode, much larger than in any other part of the tube. It is not constant in this region, but increases as we approach the cathode. The force reaches a minimum either in the negative glow itself or in the part of the Faraday dark space just outside, after which it increases towards the positive column. In the case of a uniform positive column the electric force along it is constant until we get quite close to the anode, when a sudden change, called the “anode fall,” takes place in the potential.

EB1911 Conduction, Electric - Fig. 16.jpg
Fig. 16.

The difference of potential between the cathode and the negative glow is called the “cathode potential fall” and is found to be constant for wide variations in the pressure of the gas and the current passing through. It increases, however, considerably when the current through the gas exceeds a certain critical value, depending among other things on the size of the cathode. This cathode fall of potential is shown by experiment to be very approximately equal to the minimum potential difference. The following table contains a comparison of the measurements of the cathode fall of potentials in various gases made by Warburg (Wied. Ann., 1887, 31, p. 545, and 1890, 40, p. 1), Capstick (Proc. Roy. Society, 1898, 63, p. 356), and Strutt (Phil. Trans., 1900, 193, p. 377), and the measurements by Strutt of the smallest difference of potential which will maintain a spark through these gases.

Gas. Cathode fall in Volts. Least potential
difference required
to maintain a Spark.
Platinum Electrodes. Aluminium
Warburg. Capstick. Strutt. Warburg. Strutt.
Air 340-350 .. .. .. 341
H2 about 300 298 .. 168 302-308
O2 .. 369 .. .. ..
N2 230 if free 232 .. 207 251
  from oxygen        
Hg vapour 340 .. .. .. ..
Helium .. .. 226 .. 261-326
H2O .. 469 .. .. ..
NH3 .. 582 .. .. ..

Thus in the cases in which the measurements could be made with the greatest accuracy the agreement between the cathode fall and the minimum potential difference is very close. The cathode fall depends on the material of which the terminals are made, as is shown by the following table due to Mey (Verh. deutsch. physik. Gesell., 1903, 5, p. 72).

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
Argon 167 .. .. .. .. .. 100 .. .. .. ..

The dependence of the minimum potential required to produce a spark upon the metal of which the cathode is made has not been clearly established, some observers being unable to detect any difference between the potential required to spark between electrodes of aluminium and those of brass, while others thought they had detected such a difference. It is only with sparks not much longer than the critical spark length that we could hope to detect this difference. When the current through the gas exceeds a certain critical value depending among other things on the size of the cathode, the cathode fall of potential increases rapidly and at the same time the thickness of the dark