THE LOCALISATION OF BEEAKS AND FAULTS. 399 



the break. Take off current and a second or two later open 

 galvanometer short-circuit key and note false zero deflection. 

 Balance the bridge to this zero, noting the zero again after 

 each adjustment, and adjusting bridge to balance as nearly as 

 possible to it. 



Having got the balance with the high current, change to 

 lower battery power on key, and obtain second balance in the 

 same way. Read the currents on the mllammeter immediately 

 after each balance. One balance with the high current followed 

 by one with the low current form a pair. Take several pairs 

 until there is very little change and the readings are regular. 

 If the regular resistance runs up try a new set of pairs with 

 lower currents. 



Let A be the balance with the low current c. 



„ B „ „ „ ,, high „ no. 



„ X „ „ distance of break in ohms. 



„ n „ „ ratio of testing currents used. 



„ f „ „ resistance of break. 



Then 



A:=X+f. (1) 



B=x + J-^ (2) 



By subtraction we obtain — 



ijn - 1 



Let _^=P. 



fjn — 1 



then /=(A.-B)P. 



Now, by (1), 



x=A-f, 



and substituting value of / we have 



jc=A-(A-B)P (3) 



The values of P are given in the annexed table for any 

 ratio n. 



