ARC IN AIR BETWEEN CARBON ELECTRODES 31 



ing it was used as one branch of the Wheatstone bridge. 

 Instead of the battery ordinarily used with the bridge a 

 transformer was used and instead of a galvanometer a 

 dynamometer. The three arms of the bridge were ad- 

 justed until no current was shown by the dynamometer. 

 The resistance of the arc could then be determined. The 

 resistance was found to be approximately 2 ohms and the 

 counter E.M.F. 40 volts. Luggin 1 used a different modi- 

 fication of the bridge and Firth 2 still another, both getting 

 results similar to those of Arons. 



"Negative Resistance." In 1895 Ayrton 3 concluded 

 from the result of a few simple experiments that the re- 

 sistance of the arc was negative. What he measured was 

 dE/dl. It had been previously pointed out by Luggin 

 that this quantity was negative, and it has since been 

 shown even more clearly by the curves given by Mrs. 

 Ayrton. 4 If one is to define resistance as meaning dE/dl, 

 there is no question but that the resistance of the arc is a 

 negative quantity, but of all the uses of the word this is 

 perhaps the least justifiable, and to speak of a negative 

 resistance is, to say the least, misleading. 



The word, resistance, means primarily something which 

 hinders the movement of some object. An electrical re- 

 sistance means something which hinders the flow of an 

 electric current, and the most natural meaning of the ex- 

 pression " negative resistance" would be something which 

 helps the flow of the current. It is needless to say that 

 the resistance of the arc does not help the flow of current. 



1 Centralbl. f. Elektrot., 10, 567; 1888. 



2 Mem. and Proc. Manchester Lit. and Phil. Soc., (4), 9, 139; 1895. 

 * Mrs. Ayrton's " Electric Arc," p. 75. 



4 Mrs. Ayrton's " Electric Arc," p. 113. 



