Cathode Fall in Helium and Argon 7 



The purpose of the present investigation is to test the above 

 theory — which is a simpler mathematical expression of Skinner's 

 theory — in the monatomic gases helium and argon. 



Apparatus. — The apparatus used was similar to that used by 

 Skinner.^^ It consisted of a cylindrical tube 3 cm. in diameter 

 with a wire cathode K lying in its axis (Fig. 2). The cathode 



Fig. 2 



was partly covered by a glass sheath which confined the discharge 

 to a definite area. The anode was an aluminium wire placed in a 

 side tube as shown in the figure. The potentials in the gas were 

 measured by means of the probe-wire, W, which could be shifted 

 to any position in the gas, since its axis of rotation is parallel to K. 

 The probe was sheathed by a glass tube .except for the short part 

 of its length parallel to K. The diameter of the circle described 

 by the probe is 20 mm. and its position is obtained by the pointer 

 P moving over a circle of reference 5 — having a diameter of 30 

 cm. 6* is so adjusted that its zero corresponds to the center of K, 

 and thus the distance from the center of K, or the radius of the 

 equipotential surface, is given directly by readings on 5'. 

 11 C. A. Skinner, Phys. Rev., XII, No. 2, p. 143, 1918. 



