ELECTRICAL METHODS 541 



end and connects the input winding of transformer Ti to different seg- 

 ments of the commutator. (The commutator arm may be rotated by a gear 

 train connected to the D.C. generator.) When the contact arms are at 

 positions a-a' , no potential difference is impressed across the transformer ; 

 at positions h-h' , the full load voltage is impressed. Hence, the potential 

 impressed across the transformer varies from zero to a maximum and 

 passes through a complete A.C. cycle for each complete revolution of the 

 rotating arm. 



The voltage between the potential electrodes may be measured by the 

 previously described vacuum tube voltmeter or by a sensitive alternating 

 current galvanometer or millivoltmeter. The power supplied to the ener- 

 gizing circuit is measured bv the milliammeter /o which is equipped with 

 appropriate shunts. If an alternating current galvanometer or millivolt- 

 meter is employed, the potential measuring circuit is current-operated (the 

 current being measured in terms of fractions of a microampere), and a 

 correction must be made for variations in the resistance of the circuit. This 

 can be accomplished by two methods : (a) measuring the actual resistance 

 of the circuit for each set-up of the potential electrodes or (b) selecting 

 some arbitrary circuit resistance, e.g.. 1000 ohms, and thereafter always 

 adjusting the potential circuit to this value before each reading. The latter 

 procedure simplifies the field operations and the computations. 



The circuit shown in Figure 330 illustrates a conventional bridge which 

 may be utilized for the type (b) method of operation. Adjustment of the 

 potential circuit is made by depressing the closed-circuit key K2, closing K^, 

 and then adjusting Ro until the galvanometer G2 indicates a balanced cir- 

 cuit. Flexibility of the equipment for use in different areas is obtained by 

 making the resistor R^ adjustable, with values of 500, 1000, and 2000 ohms. 

 The lowest value is used in damp or wet areas ; the mean value is used in 

 areas where average surface conditions obtain ; and the highest value is 

 used in dry desert areas. Best sensitivity will be obtained by always 

 operating at the lowest value possible in any given area. Should it be 

 deemed advisable to change values during the progress of a survey, the 

 data may all be converted to a common basis by correcting for the circuit 

 resistance changes. 



Methods for the Direct Measurement of the Ratio E/l. — In the 



methods described thus far, separate measurements are made of the voltage 

 and current simultaneously. However, only the ratio E/I is needed for 

 calculating the apparent resistivity, and it is advantageous to use instru- 

 ments which measure this ratio directly because only one reading need be 

 taken, with a resultant increase in speed and accuracy. 



Many types of direct ratio reading instruments have been proposed. 

 Of these, the Megger is the best known at present, although its use in geo- 

 physical measurements is limited, due to its inherently poor sensitivity. 



