132 



EXPLORATION GEOPHYSICS 



magnetic field that is either (1) in a vertical direction when used on a 

 vertical magnetometer, or (2) in a horizontal direction when applied to a 

 horizontal magnetometer. The strength of the magnetic field is proportional 

 to the current flowing in the coil. This current, measured in milliamperes, 

 may be supplied by a small dry cell battery. The strength of the field may 

 be varied by a rheostat, to give a multiplicity of points for calibration. A 

 reversing switch is provided for changing the direction of the field. 



The field intensity is a function of the current and the characteristics of the coils, 

 and may be expressed by the relationship 



F = IC = 





(54) 



where 



F = field intensity, in gammas 

 C = coil constant 

 D = diameter of coils, centimeters 

 / = current, in milliamperes 

 n = number of turns in each coil 

 E = mean distance between coils, centimeters 



Fig. 57. — Helmholtz coil and diagram of connections, a, bat- 

 tery; b, switch; c, rheostat; rf, milliammeter; e, reversing switch; 

 f, twisted flexible connecting cord; g and g', identical coils; h, coil 

 holder bracket. 



The diagram of connections is shown in Figure 57. Usually the coil constant C is sup- 

 plied by the manufacturers. The coils are approximately 35 centimeters in diameter, 

 and are spaced approximately 20 centimeters apart. A mounting bracket is fastened 

 to the tripod turntable, and the coil system is placed on the bracket (Figure 56), with 

 the coils in a horizontal position when calibrating a vertical component magnetometer 



