34 THE ELECTRIC ARC 



experiment showed was that dE/dl was greater than E/I 

 between the gas of the arc and the cathode. In other 



words, that the form of the 

 curve showing the relation 

 between the cathode drop 

 and the current was that 

 shown in Fig. n. This is 

 the same kind of a curve 

 that one finds when examin- 

 ing the relation between 

 the potential difference and 



the current for discharge produced by Rontgen rays. No 

 one ever speaks of a "forward E.M.F." with such discharge 

 and there would appear to be no more reason for using 

 that expression in connection with the arc. 



Pollock 1 explained this "forward E.M.F." of DuddelTs 

 as being due to the tendency of a hot cathode to send 

 negative ions out from it. This tendency undoubtedly 

 exists and in this case it would assist the flow of current, 

 but if it assisted enough to produce a forward E.M.F., 

 there would be a rise in potential at the surface of the 

 cathode instead of a drop, and a change of heat energy 

 into electrical energy, while in reality we find there a change 

 in the opposite direction. It is altogether possible that 

 this change of energy would be greater, if the emission of 

 negative ions did not exist, but we have no evidence that 

 the difference between what the cathode drop would be 

 and what it is, is the same in amount as the "forward 

 E.M.F." which was found by Duddell. 

 Heinke 2 computed the counter E.M.F. from observa- 



1 Phil. Mag., (6), 18, 229; 1909. 



2 Verb. d. V. z. Beford. d. Gewerbfliesses, 83, 403; 1904. 



