326 MAGNETIC METHOD [Chap. 8 



is inserted south pole up; if the scale has disappeared to the right, the 

 magnet is inserted with its north end up. Then the additional field 



AZ = ± ?^ (8-246) 



and therefore the correction in scale divisions 



A5=+2M"k^ (8-24c) 



where Ma is the moment of the auxiliary magnet, r its distance, and k is 

 a deflection constant given by 



with L as the pole distance of the deflecting magnet and I that of the 

 magnetic system. For the uncompensated Askania system fZ = 51.7. 

 More accurate results are obtained if the correction in scale divisions is 

 determined experimcntall}^ on a field station where the scale is about to 

 disappear by taking readings with and without magnet. Since the tem- 

 perature of the auxiliary magnets is of some influence and their moment 

 and distance may change in transportation, it is advisable to use them only 

 temporarily for large anomalies. AVhen work is being done for any 

 length of time in areas with regionally different vertical intensities, the 

 latitude adjustment screws should be used. 



As the Schmidt balances are instruments based upon a comparison of 

 the earth's magnetism with gravity, consideration must be given to the 

 problem whether anomalies in gravity or changes of gravity with latitude 

 have any effect. The effect of gravity on the scale value is given by 

 Ctf' = ^trd'/g «o that, if an instrument with 3O7 scale value is taken from 

 the pole to the equator, its scale value would change from 30.00 to 30.15. 



The effect on the reading is approximately Sg — s^' = — ^ ~ , so that 



a large gravity anomaly of the order of 100 milligals would produce a 

 change of 5.47. Hence, in practice, effects of variations in gravity may 

 be disregarded, particularly since most gravity anomalies are accompanied 

 by large magnetic anomalies. 



Variations in temperature produce probably a greater change in magne- 

 tometer readings than any other physical factor; the temperature effect is 

 due to (1) a drop in the magnetic moment of the needle with an increase 

 in temperature, and (2) differential expansion of the metals in the magnetic 

 system, giving rise to displacements of the center of gravity. The first 

 effect may be partially compensated by the second, that is, by a suitable 



