MAGNETIC METHODS 141 



the temperature correction is the theoretical reading that would have 

 been obtained there if no temperature change had taken place. 



The base temperature usually adopted in mild weather is 20° C, Using 

 the temperature coefficient previously worked out in the example given, 

 0.225 S.D./ 1 C°, we can assume that at station 1 (of a set of stations) the 

 magnetometer might read 21.3 S.D. at a temperature of 22.0° C. This 

 temperature is 2.0° above the assumed base temperature of 20.0° C. Since 

 the temperature has increased two degrees the instrument would read 2 x 

 0.225 S.D. less (or 0.46 S.D. less) than it would have read had the 

 temperature remained at 20.0° C. 



Therefore a correction of 0.46 scale divisions must be applied to the 

 reading at station 1. Since the temperature was higher and the scale reading 

 consequently less, the correction must be added. Inasmuch as the inherent 

 sensitivity of the instrument is less than one-tenth scale division, the nearest 

 tenth scale value is used; i.e., 0.5 in place of 0.46 scale divisions. The 

 instrument reading, therefore, which would have been obtained at the 20.0° 

 C. temperature is 21.3 + 0.5 or 21.8. This latter value would be used in 

 calculating a set of field results. 



If the temperature at station 1 had been 18.0° C, i.e., 2.0° less than 

 the base temperature, the correction would be subtracted. The reading 

 would be 22.3 at the 18.0° C. temperature. To reduce it to the 20° base, 

 the calculation would be: 22.3° - 0.5° = 21.8°, which is the same as in 

 the preceding case for the base temperature. 



The temperature chosen for a base to which a set of readings is reduced 

 is quite a matter of convenience. It is generally advisable, however, in 

 order to avoid errors due to the wrong algebraic sign, to choose a tem- 

 perature below or above that to be encountered during the work. The 

 instrument readings at all stations of a set of readings must be reduced, or 

 corrected, to those readings that theoretically would have been obtained 

 if the ambient at some temperature had remained constant. If a fully com- 

 pensated zero temperature coefficient system is used, obviously, temperature 

 corrections are not required. 



Magnetic Field Work 



As stated previously, the normal values of the components of the earth's 

 magnetic field vary over the surface of the earth. The vertical component 

 of the earth's field varies from approximately minus 67,400 gammas at the 

 south magnetic pole through zero at the magnetic equator to about plus 

 63,500 gammas at the north magnetic pole. The horizontal component has 

 a maximum value of about 39,000 gammas at the equator and decreases to 

 zero at each of the two poles. This variation of the magnetic field strength 

 is usually unimportant where studies are confined to traverses of only 

 a few miles length. In larger surveys, or when tying-in one survey with 

 another at a different location, latitude corrections average from 10 to 12 

 gammas per mile for the vertical component and from 5 to 8 gammas per 



