780 



ELECTRICAL METHODS 



[Chap. 10 



in the output circuit. Reception coils and amplifiers are common to all 

 receiving arrangements discussed below. 



When the frequency is too low for audio detection, the telephone is 

 replaced by a galvanometer in the plate circuit of the output stage (see 

 Fig. 10-101). The sensitivity of the galvanometer is 10~^ to 10"' amperes 



Fig. 10-lOla. Resistance-coupled amplifier for electromagnetic prospecting, with 

 output meter (adapted from Miiller). 



Bucking Circuit 



Fig. 10-lOlb. Transformer-coupled amplifier for electromagnetic prospecting, with 

 output meter (adapted from Miiller). 



per scale division; that of the reception arrangement, of the order of 10 

 gauss. The amplifiers should remain in good calibration and should have 

 constant gain. The latter may be expressed as galvanometer deflection 

 for a given input voltage. The corresponding field is then 



eV2 



H = 



jSiV 



10 gauss, 



(10-51) 



