112 CrA. HETLAND 
There is no reason why alternating current could not be used in 
resistivity work, the same as it is done in the potential-drop-ratio 
method. For instance, the current base could be supplied with A.C. 
of moderate frequency by a buzzer, or vacuum-tube oscillator, or by 

9g 
Fic. 9.—McCollum earth-current meter (after McCollum). 
EARTH-CURRENT METER 
5, milliammeter; 6, voltmeter; 9, commutator switch; 10, short-circuiting switch; 11, 
voltmeter switch; 12, commutator crank; 13, ammeter switch; 14, resistance switch. 

EARTH-CURRENT METER CONTACTORS 
1, trench contactor; 2, cantilever contactor; 3, single terminal electrode with extension 
rod; 4, single terminal electrode (disassembled). 
taking, if available, current from the light circuit, and feeding it to 
the ground through a transformer. The only difficulty would be to 
obta:n an accurate reading of the current; however, the absolute value 
of the current is not of great importance when using the methods JJ, 
IT a, or IJ b. To measure the potential difference between the potential 
electrodes, it would be feasible to have a compensator, and a lead to 
442 
