26 THE ELECTRIC ARC 



m + nl, where m and n are constants and / is the length of 

 the arc. He believed that the first term of this expression 

 was due to a counter E.M.F. Others did not agree with 

 him, and the discussion then started has continued to the 

 present time. This discussion has been carried on partly 

 because there have been differences of opinion as to facts, 

 but more often because there has been no agreement as to 

 the meaning of the terms used. In the beginning there 

 was too little known to admit of any clear definition of the 

 terms, " counter E.M.F." and " resistance," and though 

 there have since been clear definitions of these terms, there 

 has been no general agreement as to their use, and often no 

 good understanding of how others were using the terms. 



It would appear that the term " counter E.M.F." should 

 mean the same when applied to the arc as when applied to 

 a motor or a storage cell. In those cases we find: first, 

 that the potential difference between the terminals of a 

 cell or motor does not obey Ohm's law; secondly, that 

 there is an E.M.F. remaining after the impressed E.M.F. 

 has been removed and that this is of sufficient magnitude 

 to account for the deviation from Ohm's law; and thirdly, 

 that in the majority of cases the electrical energy is changed 

 into something besides heat energy. 



In the arc only the first of these properties is found to 

 exist. In fact the arc deviates from Ohm's law much as 

 other forms of discharge through gases deviate, where the 

 expression "counter E.M.F." has never, I believe, been 

 applied. 



In the beginning of the investigation on the arc it was 

 not known that this was the only point of similarity be- 

 tween the arc and a cell, and writers using the word no 

 doubt had in mind a counter E.M.F. identical with that of 



