ELECTRICAL METHODS 



611 



On substituting the values for //ab, H-qq,, etc., and rearranging terms 



one 



obtains 



H 



_^ f\Af_+y 



X {a — y) y {a — x) 



+ 



^{a-y)^+{a-x)'' 

 {a-x){a-y) 



(16) 



Equation 16 specifies the magnitude of the field at any point situated at 

 a distance x from the side AB and a distance y from the side CB. 



The value of the field at various points along a line parallel to DA 

 {y — constant) may be obtained by assigning a series of values to x. 

 In particular, the value of H along the line MM {y = a/2) is obtained 

 by substituting this value of y into Equation 16. 



H 



y=a/2 



^ 2/ /V4£f+5 , V4 {a-xy + a^ ^ 



a \ 



X 



(16a) 



DtSTAHCC ACROSS COIL - PCKCEUT 



Fig. 382. — Plot showing variation of Ha/I ^ at various distances along 

 a profile inside a square coil of side a. 



A graph of Equation 16a is shown in Figure 382. It will be noticed 

 that a relatively small variation of H occurs for the values of a between 

 0.25a and 0.75a, and the field measurements are usually made within this 

 range. 



Field Operations. — The ground is energized by passing alternating 

 current through a large single turn loop of insulated wire laid on the sur- 

 face of the ground, in the form of an approximate square or rectangle. 

 Usually the current is generated by a gasoline-driven alternator and has a 

 magnitude of 5 to 20 amperes, and for shallow investigations, a frequency 

 of 500 to 1000 cycles per second. The size of the ground loop varies with 

 the nature of the problem, usually being from 1000 to 2500 feet on a side. 



The observational technique consists in determining contours of equal 



