Date of this Version
UNIVERSITY STUDIES VoL. XXI JANUARY-OCTOBER, 1921 Nos. 1-4
It was recognized rather early in the study of the phenomena of discharge of electricity through gases that the ordinary laws of conduction as found in solid and liquid conductors are not applicable to gases. It was shown by Zeleny and independently by Child that Ohm's law by no means represents the distribution of potential between electrodes in an ionized gas, but that the potential gradient is large in the vicinity of the electrodes, and not uniform throughout the space between them. When the pressure of the gas surrounding the electrodes is reduced to one or two millimeters and a current passed between the electrodes, part of the intervening gas becomes luminous and part does not show any luminosity. In this case the potential gradient through the gas is decidedly irregular.
Covering the surface of the cathode is a thiri, luminous layer of gas called the "cathode glow." Beyond the cathode glow is a layer of non-luminous gas known as the Crooke's dark space followed by a layer of luminous gas, the negative glow. At the surface of the anode is a layer of luminous gas, the positive column, the extent ?f which depends upon the length of the tube and the pressure of the gas-the length of the positive column being made shorter either by lowering the pressure of the gas or making the tube shorter. Between the negative glow and the positive column is a second layer of non-luminous gas known as the Faraday dark space. The extent of the cathode glow is not materially changed by a change of pressure or current. However, the negative glow and the Crooke's dark space increase in length as the pressure is reduced. If the exhaustion is carried to an extreme limit as in an X-ray bulb, the Crooke's dark space fills the entire tube and the glass walls begin to fluoresce. The electric intensity varies greatly in the different parts of the tube and has been investigated by Hittorf,3 Graham,4 Skinner/ Wilson,6 and many others. The usual method of measurement is to insert a small wire electrode in the gas and measure. the potential at that point. From the potential difference between one electrode and some point in the gas or between two points in the gas the electric intensity may be determined. Investigation has shown that there is a large potential drop between the cathode and the edge of the negative glow amounting in general to rather more than two hundred volts. This relatively large drop of potential is known as the cathode fall.