494 Prof. Skinner on Conditions controlling the Drop of 



drop at the electrodes, but in using the total drop we assume 

 a mean value for this intensity which should not make a vital 

 difference in the results. The presence of the neutral gas 

 molecules by impeding- the motion of the ions should be 

 marked, especially at the higher gas pressures. On the one 

 hand it should aid in destroying the kinetic energy of the 

 ion, on the other it should increase the time between impacts 

 on the electrode ; the former would accelerate the discharge, 

 the latter tend to impede it. There is another important 

 factor which is not introduced in the equation, namely, the 

 attraction of the metal used as electrode for the charge on 

 the ion. As pointed out in the first paper, the discharge is 

 facilitated by this attraction, in that on comparing different 

 metals under the same conditions it is found that the drop 

 decreases as the attraction of the metal for the charge carried 

 by the ion increases. 



To further test the above view of the cause of the electrode 

 drop the following experiments were performed. The details 

 of the apparatus are given in the first paper. 



Drop at Anode with Potential Gradient in Neighbouring Gas. 



The writer has already called attention (/. c.) to the fact 

 that the increase in anode drop with gas pressure may be 

 simply due to the simultaneous increase in the potential 

 gradient in the positive column — in that with increased 

 gradient the ions must be driven into the anode field with a 

 greater velocity. 



By using discharge-tubes of different diameters, in which 

 the potential gradient in the positive column increases as the 

 diameter decreases*, the variation of the anode drop with 

 the potential gradient alone may be obtained. 



Three tubes of form shown in fig. 2, having diameters of 



18, 12, and 8 mm. respectively, were joined together in open 

 connexion, very carefully prepared by repeated evacuations 

 and drying, then filled to the desired pressure with atmo- 

 spheric nitrogen obtained and purified in the usual manner. 

 * A. Herz, Wied. Ann. liv. p. 244 (189o). 



