Chap. 10] 



ELECTRICAL METHODS 



821 





zontal and a vertical transmitter with closely coupled receiving coil. The 

 distance between the coils may be increased to allow the operator to walk 

 between them, as in Fig. 10-133. This arrangement is customary in 

 high-frequency treasure finders. 



Two receiving coils are 

 used in the detector shown 

 in Fig. 10-134a, which was 

 applied successfully by Theo- 

 dorsen for the location of 

 unexploded bombs several 

 feet below the ground. It 

 consists of a cylinder with 

 three coaxial coils. The mid- 

 dle coil is the transmitter, or 

 energizer; the upper and 

 lower coils are the receiving 

 coils, in series opposition. 

 On barren ground the induced 

 fields are equal. With a me- 

 tallic or magnetic body pres- 

 ent, the lines of force are 

 distorted, chiefly in the lower 

 coil. This results in an un- 

 balanced e.m.f., which is de- 

 tected in the head phones. 

 In Theodorsen's original in- 

 strument the coils were 3 feet 

 in outside diameter and about 

 7 inches distant from each 

 other. Undoubtedly the di- 

 ameter of the coils can be 

 reduced, if the sensitivity is 

 increased by tuning the 

 transmitter coil and using 

 an amplifier on the receiving side (see Fig. 10-1346). 



2. High-frequency treasure finders include heat-frequency instruments and 

 combined transmitter-receiver arrangements. The former utilize the 

 change in the inductance of a coil produced by the presence of metal, a 

 principle that was used very much during and after the last war in muni- 

 tions factories and is now applied in mints and penal institutions to prevent 



Transformer 



6tntn^rO ^ 



tnergiz//tgrc/7 



^ Resonafinq Condenser 



I \ Amplifier 



Defecfor 

 (fieadplianes) 



Fig. 10-132. Single energizer (T) and pickup 

 coil (/2), with schematic circuit diagram (after 

 Joyce). 



124 Jour. Frank. Inst., 210, 311-326 (1930). 



