726 



ELECTRICAL METHODS 



[Chap. 10 



4. A.C. resistivity method. An arrangement for measuring resistivity 

 with A.C. by compensation is illustrated in Fig. 10-61, in which A is an 

 ammeter in the current circuit, I a variable inductance, T a transformer 



Fig. 10-62a. Tagg interpretation diagram {k negative). 



used to supply a reference voltage to the potentiometer, V a voltmeter, S 

 a slide wire, and PH a phone for detecting balance. Another A.C. indicator 

 must be substituted for the phone if currents of lower frequency and hence 

 greater depth penetration are applied. 



