Resistance of the Electric Arc. 411 



som? degree of accuracy the resistance of the arc under various 

 conditions. 



Methods. 

 A number of methods were tried using alternating currents, 

 of which the following were most successful : — 



Method I. is represented diagrammatically in fig. 1. D is 



Fig. 1. 



the armature of an alternator, the current from which passes 

 round two circuits in parallel, one of which contains the arc 

 X, and the other an adjustable resistance R. By adjusting R, 

 the alternating currents in the two halves can be made equal. 

 When this is the case the impedances of the two halves to 

 alternating currents must be equal. 



In the diagram the continuous-current circuit is shown to 

 the left. It consists of a battery of accumulators B, the 

 hand-adjusted arc-lamp X, the resistance K, the ammeter A, 

 and (with the commutator C as shown) the resistance S 

 and the alternator D. It will be noticed that the alternator D 

 carries the continuous current, but this of course does not 

 prevent its acting as an alternator. 



In order to measure the small alternating current indepen- 

 dently of the continuous current flowing wo used the air- 

 transformer T, the thick wire coil of which was in series with 

 the alternator D ; the thin wire coil being connected with an 



