Chap. 10] ELECTRICAL METHODS 819 



locator). If the detection coil is carried in horizontal position above the 

 ground, the sound will increase as the pipe is approached and will be null 

 immediately above the pipe. With a vertical coil (plane parallel with the 

 pipe) the sound will be at a maximum above the pipe. If no contact can 

 be made with a pipe system, it may still be located by grounding both 

 terminals of a buzzer. Since depth penetration is not important, fre- 

 quencies of the order of 1000-1500 may be used, for which the human ear 

 is most sensitive. 



For small metallic objects (boxes, chests, safes) inductive excitation by 

 an insulated horizontal loop about 50-100 feet square is preferable. For 

 detection, a vertical reception coil is best suited, since the normal loop field 

 (with the exception of a narrow area near the wire) is vertical, while the 

 field of metalUc bodies (due to eddy currents induced to flow along their 

 edges) is horizontal above them and is thus picked up full strength. When 

 a signal is detected, the coil should be used in two positions at right angles 

 to each other to determine the direction of the horizontal field. The 

 vertical frame may also be tilted until minimum sound is obtained. In 

 this manner metallic masses may be located by indications of anomalous 

 field direction. The frequency in this application is again of the order of 

 1000-1500 cycles. However, the higher frequencies also have been em- 

 ployed with small vertical transmission loops and receiving coils in hori- 

 zontal position to eliminate the primary field. A transmitter and receiver 

 built by the Fisher Research Laboratories for such work is illustrated in 

 Fig. 10-1356. 



B. (Self-Contained) Treasure Finders 



Self-contained treasure finders contain their own source of power or 

 cnergizer. They may be divided into low- and high-frequency instruments. 



1. Low-frequency instruments operate in the range of 500-2000 cycles. 

 A device commonly used is the induction balance, that is, an instrument 

 or bridge intended for the comparison of the self-inductances of two coils. 

 The principle was first proposed by Dove in 1841 and further perfected by 

 Hughes in 1879. When applied in prospecting for metallic objects, it is 

 made with two similar coils in opposite arms of a bridge, which is balanced 

 when the inductances are balanced. The two coils may be carried over 

 the ground at the ends of a carrying frame. As one of them comes above 

 a conductive or magnetic body its inductance changes, upsetting the bridge 

 balance. It is also possible to use only one coil and to enclose the other in 

 a shielded case with the remainder of the equipment. Power is derived 

 from a 500- to lOOO-cj'cle buzzer. The two detection coils are connected 

 to the opposing primary windings. Pi and Fa , of a differential transformer 



