536 



EXPLORATION GEOPHYSICS 



When Kz is depressed, a certain fixed potential, depending upon the 

 setting of S2, will be impressed on the circuit causing a deflection of the 

 galvanometer. Key Ki, controlling the energizing current, is depressed 

 simultaneously with K^, and the energizing current is now adjusted by 

 rotating the dial R2 to bring the galvanometer back to balance. (The ener- 

 gizing current is adjusted to the proper value to create a potential equal 

 to and opposite in polarity to the applied fixed potential.) 



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Fig. 326. — Resistivity apparatus for constant potential method. 



The power is supplied by a direct current dynamotor which is con- 

 nected to a storage battery through a heavy duty switch K\. In series with 

 the battery is a variable resistor R^ and a bank of fixed resistors which 

 may be tapped at various fixed values by switch 6^2. The output of the 

 dynamotor passes through a milliammeter having a range of 0-10 m.a. and 

 a shunt having multiplying ratios 5, 10, and 100. A relay-controlled 

 closed-circuit switch Sz automatically breaks the circuit when the power 

 switch Kx is released. Sz is made automatic in order to prevent the con- 

 tinued flow of current which normally would take place due to the inertia 

 of the generator as it loses speed. The dynamotors are usually wound for 

 12 to 32 volts D.C. input and 500 to 1000 volts output. Full load output 

 current should be 500 to 1000 milliamperes, intermittent service rating. By 

 means of a tapped resistor and 5*2, and the variable resistor R^, the input 

 current (and hence the output) may be controlled. 



