ARC IN AIR BETWEEN CARBON ELECTRODES 27 



the cell. Later writers have used the term merely to mean 

 that the potential difference of the arc deviates from 

 Ohm's law in a definite manner, and have so defined it. 

 Yet no one seems to have pointed out that the meaning 

 of the term as defined by Duddell, for example, is not the 

 same as the meaning which Edlund had in mind, nor the 

 meaning generally given to the term in physics. This has 

 led to more or less confusion in what has been written on 

 the subject during the past few years. For example, one 

 occasionally sees the statement that it has at last been 

 definitely shown that there is a counter E.M.F. in the 

 arc. The fact is that on the one hand it has neither re- 

 cently nor at any other time been shown that the arc has 

 more than the first of the three peculiarities mentioned 

 above, and on the other it has never been doubted but that 

 there was this one point of similarity between the arc and 

 the cell. 



The methods which have been used to test the existence of 

 a counter E.M.F. may be divided into two general classes: 

 First, methods which attempt to find a counter E.M.F. 

 by measuring the resistance of the arc; and secondly, 

 those attempting to find it by measuring the E.M.F. re- 

 maining between the carbons after the impressed E.M.F. 

 has been removed, or what we may call the residual E.M.F. 

 of the arc. 



Those who attacked the problem by measuring the re- 

 sistance of the arc have assumed that the counter E.M.F. 

 was the difference between the potential difference at the 

 terminals of the arc and that part of it which appeared to 

 obey Ohm's law. In other words that E' = E RI, 

 where E' is the counter E.M.F. , the potential difference at 

 the terminals, R the resistance of the arc, and / the current. 



