THE LOCALISATION OF BREAKS AND FAULTS. 



517 



is. in legal ohms, and used with a B.A. bridge, the readings will 

 be in legal ohms. 



Measurement of Battery Resistance (Muirhead). — The in- 

 troduction of milammeters has considerably lightened the 

 accessory measurements which used to be necessary to calcu- 

 late the current passing through the bridge to line. Some 

 methods, however, require the battery resistance to be known 

 and it is necessary to check the measurement over from time to 

 time to see that no excessive rise is taking place, and to weed 

 out faulty cells. 



Muirhead's method for measuring the internal resistance of a 

 cell or battery gives a very accurate determination of the resis- 



FiG. 288. — Muirhead's Battery Resistance Test. 



tance for any given working rate. The battery to be measured 

 is connected in the usual way to the testing key (Fig. 288), 

 and from the latter there are two circuits : one to galvanometer 

 and condenser and the other to the resistance R through a 

 plain contact key. On closing the battery key a throw is 

 observed on the galvanometer due to the condenser charge and 

 proportional to the total E.M.F. of the battery. Keeping this 

 key clamped down the shunt key is closed for a second or two 

 when a discharge throw occurs (in the reverse direction) pro- 

 portional to the drop in potential at the battery terminals. . 



The resistance E, should be adjusted to produce the current 

 usually taken from the battery in the tests for which it is employed . 

 Thus, supposing it is vised on break tests the currents to line 



