36 THE ELECTRIC ARC 



endeavored to find a corresponding residual E.M.F. He 

 passed a current from a number of cells through the vapor 

 of the arc after the impressed E.M.F. had been removed, 

 passing it first in the direction of the supposed E.M.F. 

 and then in the opposite direction. He found the current 

 in the first direction to be the greater. To compare the 

 E.M.F.'s in the two cases he eliminated the resistance of 

 the circuit by using Ohm's law, but Ohm's law does not at 

 all apply to the case of discharge through hot vapors, and 

 the supposition in this case introduced a very large error. 

 He was led to believe that the counter E.M.F. was equiva- 

 lent to 10 or 15 Bunsen cells. 



But eight years before this Wild 1 had measured this 

 same quantity by throwing the arc with a double-throw 

 switch from the batteries to a high resistance galvanometer. 

 He was not able to determine the E.M.F. accurately, be- 

 cause the resistance of the vapor between the carbons was 

 not known to him, but he concluded that it was more than 

 100 times as great as the E.M.F. produced by a copper- 

 German silver couple having the same difference of tem- 

 perature at its terminals. This statement, indefinite 

 though it is, is more accurate than any statement of resid- 

 ual E.M.F. made for several decades thereafter. 



After Edlund's experiments there was a series of obser- 

 vations by different experimenters, giving all kinds of 

 values from zero to two volts. The fact that many of the 

 earlier experimenters found no residual E.M.F. is less sur- 

 prising, since they were looking for one comparable with 

 the supposed counter E.M.F. of the arc, and we can now 

 say definitely that there is none of this order of magnitude. 

 But that some of the later experimenters should have 



1 Pogg. Ann., in, 624; 1860. 



