THE LOCALISATION OF BREAKS AND FAULTS. 



409 



standard cell through the galvanometer and high resistance 

 before putting on the testing current. It is also observed 

 again after each set of balances, taking the precaution to earth 

 the cable for at least one minute previously, in order to dis- 

 charge the break potential set up by the testing current. 



The first thing to do in this test is to determine the earth 

 current potential in millivolts by comparison with a cell of 

 known voltage. This is done by observing the respective de- 

 flections of the earth current and the cell through a galvano- 

 meter and high resistance, and is described as "taking the 

 constant." 



Constant. — Connections as in Fig. 240, shunt box or galvano- 

 meter being connected to earth or to earth side of key. 



High Besistance 

 in Circuit 



Fig. 241.— Connections for takino' Earth Current in Schaefer's Test. 



Standard cell on key. Plug between cable and bridge out. 

 Bridge ratio plugs in. Press left-hand key, putting zinc to 

 line. Keep shunt high and obtain as large a deflection as 

 possible through a suitable high resistance (not less than 

 100,000 ohms). For uniformity it is convenient to arrange the 

 galvanometer connections so that this deflection is to the Left. 

 Let the deflection be 5 divisions and the standard cell e mili- 

 volts. Then 



^ millivolts per division (1) 



EqiiivaUnt Voltage and Sign of Earth Current. — Next 

 observe the earth current deflection and find the equivalent 

 voltage. Connections as in Fig. 241. Note that the only 



