134 



EXPLORATION GEOPHYSICS 



Before an instrument is taken into the field, it is advisable to calibrate 

 it and thus check the scale value. In the course of field work it is also 

 desirable to check the scale value at about 30- to 45-day intervals. In cases 

 of unusual jolts to the instrument, or when latitude adjustments are 

 necessary on the magnetic system, the scale value should be checked, as 

 well as the temperature coefficient. 



The scale value of an instrument is the inverse of its sensitivity. A 

 large scale value indicates low sensitivity, while a small scale value shows 

 high sensitivity. 



After determining the scale value of the instrument with the Helmholtz 

 coils, it is possible to determine or check the magnetic moments of the 

 auxiliary magnets, by using the relationships given below. 



Sccde Value Using Auxiliary Magnets. — The second method of deter- 

 mining the scale value of a vertical component magnetometer consists in 



MAGNETIC SYSTEM 



AUXILIARY MAGNET 



Fig. 58. — (a) Method for calibrating a horizpntal 

 component magnetometer, (b) Method for calibrating a 



vertical component magnetometer. 



placing an auxiliary magnet of known moment at a measured distance 

 directly below the instrument and observing the deflection of the scale. A 

 special extension rod carrying a magnet holder may be used for setting 

 the auxiliary magnet at any desired distance below the instrument. The 

 extension rod is graduated in millimeters to give the distance between the 

 center of the auxiliary magnet and the magnet system in the magneto- 

 meter,* 



The magnetic force at a point r cm. along the magnetic axis from the 

 center of such an auxiliary magnet is 2M/r^, where M is the magnetic 

 moment of the magnet. To calibrate a vertical component instrument, it is 



* Care is necessary to see that the distance as measured on the rod is correct. A 

 check should be made with a meter stick or tape. Since the distance enters the calcula- 

 tion as a 3rd power it should be measured with the greatest accuracy practically 

 possible. 



