136 EXPLORATION GEOPHYSICS 



end of the auxiliary magnet toward the magnetic north. M = the moment 

 in gammas of the known auxiliary magnet, and r = the distance between 

 centers of the magnetic system and the auxiliary magnet. 



When calibrating a magnetometer with a known auxiliary magnet, 

 the situation is fundamentally similar to the use of a calibrating coil in 

 that a field of known strength is applied to the instrument. The scale 

 division deflection thus produced is determined. Calibrating coils are 

 preferable to auxiliary magnets for establishing the scale value of a mag- 

 netometer in that they are free from temperature effects which change 

 the moment of calibrating magnets, as well as gradual loss of moment with 

 time. 



Carrying out the same procedure on a horizontal intensity instrument, 

 using the same auxiliary magnet at the same distance r, gave the following 

 results : 



Scale readings 



Ss = 22.3 Using Equation 58 as given : 



So = 19.3 E = ^T^'l^'?? = 16.66 gammas / scale division. 



1,000,000 '6.0 

 Sn = 16.3 The significance of Ss and ^'a' as symbols is changed as ex- 

 plained above. 



Ss-Sn= 6.0 



Determination of the Moment of an Auxiliary Magnet. — It is a 



simple matter to determine the moment of an auxiliary or standard magnet, 

 using either a vertical or a horizontal magnetometer, the scale value of 

 which is accurately known. A series of deflection readings is taken, the 

 magnet to be tested being placed at measured distances from the magnetic 

 system of the magnetometer by means of the graduated extension tube and 

 magnet holder. The procedure followed is in all respects similar to that 

 just described for finding the scale value using an auxiliary magnet, except 

 that Equations 57 and 58 are solved for the magnetic moment M of the 

 magnet used. These formulae thus solved are : 



For the vertical magnetometer : 



j^f^AjllSA^Jyl (59) 



For the horizontal magnetometer : 



j^^^AjHSaTlIA. (59,) 



Example: using vertical magnetometer No. 751414, scale value 9.9 gammas per 

 scale division. To find the moment of a large standard magnet, the magnet is placed at 

 a distance of 70 cm. beneath the instrument. 



