Cathode Fall in Helium and Argon 3 



explanation has been entirely satisfactory. It has been found to 

 consist of two distinct, but interdependent, parts: (a) the potential 

 difference between the surface of the cathode and a point in the 

 gas very near the cathode, this point being marked by the outer 

 edge of the cathode glow; and (b) the potential difference between 

 this point and the negative glow, or point of minimum potential 

 gradient. Of these quantities (a) the polarization potential differ- 

 ence due to the accumulation of positive ions at the surface of the 

 cathode is, in general, the larger quantity and increases with in- 

 creasing current density. The cathode glow, within which this 

 drop occurs, is about .2 mm. in thickness and does not seem to 

 vary appreciably with the current density. The magnitude of (a) 

 appears to be proportional to the potential gradient of the positive 

 ions just before they enter the polarization region.'^ The differ- 

 ence in potential between the cathode glow and the negative glow 

 can not be explained as readily, and it is the explanation of this 

 potential difference in which we are interested. 



Stark® has given a theory connecting the cathode fall for any 

 current density with the normal cathode fall, the normal current 

 density — i.e., the current density which is just sufficient to cause 

 the glow to cover the surface of the cathode — the gas pressure, the 

 cathode area, the current, and the absolute temperature. This 

 theory is fairly satisfactory, but does not at all explain the drop 

 of potential through the Crooke's dark space. 



In 191 5 Skinner^ published a theory of the cathode fall in which 

 he obtained expressions for the upper and lower limits, giving the 

 difference of potential between the cathode and a point in the gas 

 between the cathode and the point of minimum potential gradient. 

 In a later article he obtained a simpler and more exact theory^" 

 which is applicable to both plate and wire cathodes. 



A description of the process which is conceived to go on be- 

 tween the cathode and outer edge of the negative glow will give 

 us a basis for the theory obtained. Under the bombardment of 



'' C. A. Skinner, Phys. Rev., IX, pp. 97 and 314, 1917. 



C. A. Skinner, Phys. Rev., XII, p. 136, 1918. 

 ^ J. Stark, Ann. d. Physik, 12, p. i, 1903. 

 ^ C. A. Skinner, Phys. Rev., V, p. 483. 1915. 

 1" C. A. Skinner, Phys. Rev., XII, p. 143, 1918. 



