ARC IN AIR BETWEEN CARBON ELECTRODES 35 



tions made on the whistling arc. His method was in 

 principle the same as Duddell's, but the alternating current 

 was produced by the arc itself, and not by a separate 

 dynamo. The current in a whistling arc is equivalent 

 to a small alternating current superimposed on a direct 

 current. He says concerning it that "only about one- 

 eighth of the observed potential difference, namely, in this 

 case, about 6 volts, was used in overcoming the ohmic 

 resistance of the arc, while the remainder, namely, about 

 40 volts, is to be considered a polarized potential differ- 

 ence, or counter E.M.F. (cathode and anode drop in 

 potential)." 



We may sum up these results by saying that they show 

 that the potential difference between the terminals of the 

 arc varies when the current is varied in much the same 

 way as the potential difference between the terminals of 

 the cell of a storage battery, but they show nothing, of 

 course, concerning the phenomena occurring after the im- 

 pressed E.M.F. is removed. For this purpose we must 

 examine the second group of experiments. 



Residual E.M.F. The second method of testing for a 

 counter E.M.F. in the arc was to measure the residual 

 E.M.F. after the impressed E.M.F. was removed. There 

 is no question of the meaning of terms involved in these 

 measurements and there is apparently no great experi- 

 mental difficulty, and yet there has perhaps been as great 

 disagreement among experimenters concerning such an 

 E.M.F. as concerning any question in physics. 



Edlund 1 not only suggested a counter E.M.F. as ex- 

 plaining the relation which he found between the potential 

 difference at the terminals of the arc and its length, but 



1 Pogg. Ann., 134, 250 and 337; 1868. 



