458 Dr. B. Hodgson on the Temperature of the 



These results are shown in curve V. There is, again, a 

 linear relationship between T and i, but the anode tempera- 

 ture is only about \ that of the cathode for the same 

 current. 



The change in pressure produces much greater changes in 

 temperature than in the case of the cathode. 



§ 6. The lack of definite knowledge of the mechanism of 

 the discharge in the cathode dark space introduces diffi- 

 culties in the way of theoretical discussion. 



If we assume the normal cathode fall to be the potential 

 through which a positive ion must fall in order to gain 

 sufficient energy to ionize the cathode metal *, the relative 

 number of positive and negative carriers at the surface of 

 the cathode can be found. 



The mass of the copper cathode was 135 grams, and the 

 rate of cooling at 13°*5 0'. excess temperature (above air 

 temperature) was o, 456 C. per minute. Thus the energy lost 

 per second was 4*1 x 10 6 ergs ; and this quantity was supplied 

 per second by the bombardment of the positive ions. If we 

 now suppose that the majority of the positive ions originate 

 just outside the cathode dark space, and that each ion on 

 striking the cathode produces an electron, we have 



ne = 



r 



where n is the number of positive ions striking the cathode 

 * Stark, WinkelmamCs Hdbh. der Physik, Bd. iv. p. 51G. 



