a Conductor the Measure of the Current passing* 353 



reference to the Clark cell and standard resistance ; to get 

 the precision of other resistance measurements ; and to avoid 

 the use of apparatus which is easily put out of adjustment. 



2. To try whether the resistance of a conductor carrying- 

 alternating current could be easily measured, the connexions 

 were arranged as in fig. 1. The mean fall of potential over 

 R carrying a current from the alternating supply A was 

 compared with that over S, a standard ; a small direct 

 current being passed through the system from the accu- 

 mulator H. The potential-differences of R and S were 

 compared on the potentiometer. B, K were regulating re- 

 sistances, and highly inductive coils at M, N the primaries of 



Fig. 1. 



A 

 o^o 



B 



K 



VWV\A 



H 



R 



M 



=Q 



N 



C© 



U- 



0=:_O® O : 



POTENTIOMETER 



small high-voltage transformers reduced the alternating 

 currents in circuits where only direct-current effects were 

 required. If the alternating current in R be I, and the 

 direct current i, the heat developed cc T 2 + i 2 ; so that if I be 

 large compared with i, say 100 times as great, the heating 

 effect of i is negligible. The experiments of fig. 1, in which 

 I was several hundred milliamperes, and i was 1 milliampere, 

 were designed to show whether the resistance of R carrying 

 alternating current could be measured by a superimposed 

 direct current so small that it would not affect the heat 

 developed. It was found that using the Chelmsford town- 

 supply for the alternating source observations could be made 



