1871.] 



of Testing a Telegraph Earth, 



181 



(Appendix I.) 



Equation (II) is necessarily precisely similar to that given by 

 Mr. Schwendler in his " testing instructions" for finding the resis- 

 tance of a line when a natural current exists in it ; but as the 

 proof, for brevity's sake, has been omitted there, I have given it as 

 follows in its simplest form. 



In the following figure, when balance is established, that is, when 

 no current goes through the galvanometer, we have, by KirchhofFs 

 equations, when the earth current tends to help the testing current. 



M A — 0, B » o 



C 2 r — C, W - e \ (VIII) 



(C, + C 8 ) E + C a (B + r) « E + e ) 



where E is the electromotive force of the testing battery, and 

 e that of the earth current, 



