Chap. 8] MAGNETIC METHOD 327 



mass adjustment. In the earlier style magnetic systems the steel blades 

 were so arranged in reference to the aluminum block as to effect a displace- 

 ment of the center of gravity to the north side, thus offsetting the rise of 

 the S3'stem on that side with an increase in temperature. In the new 

 compensated systems steel is used for the frame and compensation is 

 effected by an aluminum spindle on the north side and an invar spindle on 

 the south side, which are both provided with suitable masses. 



The drop of the magnetic moment M referred to above with temperature 

 e may be written 



Mq = M2o(1 - UsQ), (8-25a) 



where Hs is of the order of 0.00048 for good magnets of the earlier systems 

 and about 0.00014 for the new systems; 9 is the temperature in degrees C, 

 usually referred to a normal temperature of 20°; and M20 is the magnetic 

 moment at that temperature. If the above relation is applied to eq. (8-22), 

 it is seen that the magnetic temperature effect on scale value is tolerably 

 small. This is not true for the deflection whose change with temperature 



is given by se — S20 = , ' . Thus the temperature coefficient,^* 



mgd 



T.C., in gammas is 



r.C. - 'S'l^J^^ = _^,z. (8-256) 







It follows frem the above (considering the magnetic effect only) (1) that 

 the reading se is less than the reading S20 , (2) that the temperature coeffi- 

 cient is negative and the temperature correction positive,^* and (3) that 

 the temperature correction varies with the vertical intensity. It is also 

 seen that an uncompensated system with a ns of 5.10~* and a normal scale 

 value of 3O7 at Golden, Colo., (Z = 53,1007) would have a temperature 

 coefficient of —0.85 scale divisions. It is obvious that a compensation 

 must be effected which may be done by a suitable mass distribution. The 

 effect of temperature on the latter may be expressed by the equivalent 

 contraction or expansion of the gravity lever arms a and d in formula 

 (8-20), so that oe = 020 (1 + pG) and de = (1 + q6), where p and q are 

 the total expansion coefficients in the horizontal and vertical direction 

 resulting from all metals, their masses, lever arms, and expansion coeffi- 

 cients. Therefore the reading at the temperature is 



2f[MZ(l - M«e) - rngad -f p0)] 

 mgd(l — q0) 



" The temperature coefficient has the opposite sign of the temperature correction. 



