University Studies 



Vol. XXI JANUARY-OCTOBER, i()2i Nos. 1-4 



AN EXPERIMENTAL STUDY OF THE CATHODE FALL 

 IN HELIUM AND ARGON WITH WIRE CATHODES 



BY L. G. RAUB 



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 appli- 

 cable 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 poten- 

 tial 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 thin, 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 fol- 

 lowed 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 of 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 



1 Zeleny, Phil. Mag., Vol. 46, p. 120, 1898. 



2 Child, Wied. Ann., Vol. 65, p. 152, 1898. 



