822 



ELECTRICAL METHODS 



[Chap. 10 



coins or weapons from being carried in or out by workers or visitors. 

 These installations use large coils, through which the persons under ex- 

 amination have to pass. The great sensitivity of these devices is made 

 possible by beating the frequency of the search coil circuit (which is 

 slightly changed by the presence of metal) against a fixed frequency and 



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Fir.. 10-133. Single eiiergizer (T) and pickup coil (R) combination; reaction to sub- 

 surf£),ce metal object (after Joyce). 



by detecting the beat frequency. This may be considered as a modifica- 

 tion of the induction balance, since the inductance in one circuit remains 

 fixed while the other is being varied. The same idea has been used in 

 treasure finders, one coil being carried over the ground, the other being 

 housed in a shielded box with the fixed oscillator. An instrument of this 

 kind was constructed bv Jovce.^^^ 



125 U. S. Bur. Mines Circ. Inf. No. 6854, Oct., 1935. 



