820 



ELECTBICAL METHODS 



[Chap. 10 



(see Fig. 10-131a) whose secondary does not respond until the detection 

 coils become unbalanced from the presence of conductive or magnetic ma- 

 terial near them. 



Fig. 10-131a. Hughes induction balance. (Pi, Pi, primaries of differential trans- 

 former; S, secondary; B, buzzer; C\, d, detection coils; Vi, F2, service coils; D, slide 

 wire.) 



US I 



Pof. 



p, 



o O 



3 



' 5 



Fig. 10-1316. Modified induction balance. For symbols see Fig. 10-131a. 



^ 



t Pt Pz 5 ohm 



/ 5, 5, m^ V 



ohms ' 



Fig. 10-1316 shows a somewhat improved 

 arrangement, with a potentiometer to 

 facilitate adjustment of the bridge, and an 

 amplifier following the secondary of the 

 differential transformer. In the instrument 

 illustrated in Fig. 10-131c, the inductances 

 are replaced by mutual inductances, with 

 one variable mutual inductance and two 

 small resistors for adjustments. This bridge, 

 developed by the Bureau of Standards dur- 

 ing 1917-1918, is said to have encountered 



practical difficulties because the primaries and secondaries of the mutual 

 inductances were too closely coupled. 



Another group of low-frequency treasure finders employs a transmitting 

 coil and one or two receiving coils close to them. In the arrangements 

 shown in Figs. 10-132 and 10-133, the receiving coil is in the axis of the 

 transmitter coil, so that no induction occurs above barren ground. Trans- 

 mitter and receiver may be interchanged. Fig. 10-132 shows a hori- 



\J 



U. &. Bureau of Standards 



Fig. 10-131C. Mutual-induct- 

 ance balance (after Theodor- 

 sen). 



