328 MAGNETIC METHOD [Chap. 8 



Again, the effect on scale value is small so that q9 may be neglected ; then 

 the temperature coefficient 



T.C. = 'i^^^) = 1? . (, - p) - ZU + ,). (8-25C) 



This equation gives two possibilities of compensation for temperature: 

 (1) by making yu, = — q, and p = q, (2) by omitting compensation in the 

 vertical direction and selecting a material for the frame whose expansion 

 coefficient is small. For steel, y = q = 1.1 X 10~^ which is 13 times less 

 than n» . Thus, if q = 0, 



T.C. = '^"1^^ = -^-^^P - Zm.. (8-25d) 



Since mga/M is the vertical intensity Zq corresponding to the latitude 

 adjustment (s = 20) of the instrument, and since Z may be put equal to 

 Zo with reasonable accuracy, the temperature coefficient 



T.C. = !^!^ZLi?i) = _ZoU + P). (8-25e) 



For complete compensation, p must be made equal to — )u« . 



For any magnetic system whose mass distribution is known, the coeffi- 

 cient p can be calculated beforehand; details are given in C. A. Heiland's 

 and W. E. Pugh's article on this subject.'^ The Hs may be determined by 

 deflection observations. Since the temperature coefficient depends on the 

 vertical intensity, a magnetic system compensated for one locality will no 

 longer be compensated at another. 



Extreme care should be exercised in the transportation of the instrument 

 and in releasing the system, as very slight changes in the horizontal posi- 

 tion of the center of gravity produce large changes in reading. From 

 eq. (8-23) the apparent change in vertical intensity equals d{es)/da = 

 — mg/M = about 67 for a displacement of only 10~ cm. The change in 

 scale value due to a vertical displacement of the center of gravity of the 

 same order is 2f times less than the change in reading. The magnetometer 

 is also quite sensitive to changes in magnetic moment, for d(es)/dM = 

 mga/M . That is, a drop in the magnetic moment of 1 gauss decreases 

 the reading about 35 gammas. Therefore, it is necessary to keep the 

 instrument away from magnets, power lines, and other demagnetizing 

 effects. 



Instrument constants and corrections. The purchaser of a magnetometer 

 usually advises the factory where he expects to use his instrument so that 



i« A.I.M.E. Geophysical Prospecting, 334-372 (1934). 



