2 L. G. Raub 



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 re- 

 duced. 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,^ Graham,* Skinner,^ 

 Wilson,^ and many others. The usual method of measurement is 

 to insert a small wire electrode in the gas and measure the poten- 

 tial at that point. From the potential difference between one elec- 

 trode 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 be- 

 tween 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. 



When the current density is small enough so that the glow does 

 not entirely cover the cathode, the cathode fall is independent of 

 the current and is known as the normal cathode fall. As the 

 current is increased the glow covers more of the cathode surface — 

 and the cathode fall remains constant — until it is entirely covered, 

 after which any further increase of current causes an increase in 

 the cathode fall. 



In the luminous negative glow the potential gradient is very 

 small, gradually rising as one progresses from the negative glow 

 through the Faraday dark space to the positive column, in which 

 the gradient becomes fairly uniform up to a point relatively close 

 to the anode, at which a second abrupt potential fall — by no means 

 as large as that at the cathode — is found which is termed the anode 

 fall. 



The cathode fall has been the subject of much research and no 



•■' Hittorf, Wied. Attn., 20, p. 705,' 1883. 



4 Graham, IVisd. Arm., 64, p. 49, 1898. 



'^ C. A. Skinner, Wied. Ann., 68, p. 572, 1899. 



8 H. A. Wilson, Phil. Mag., 49, p. 505, 1900. 



