ADVANCES IN OIL PROSPECTING II5 
engineering. They differ from the Gish-Rooney equipment chiefly in 
that the potentiometer and the milliammeter are combined in one 

Fic. 11.—Latest CSM model of Gish-Rooney equipment with motor-driven com- 
mutator. 

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Fic. 12.—View and circuit diagram of Megger manufactured by Biddle. 
instrument, the ohmmeter, the moving coil of which has two wind- 
ings, the voltage and the current winding. They also differ from the 
Gish-Rooney equipment as the power is not taken from batteries, 
445 
