MAGNETIC METHODS 137 



Scale readings 



6^8 = 63.0 r' = 343,000 ; E/4 = 2.48 



So = 15.6 M =_ 2.48 • 343,000 •_ 94.8 • 10"^ = 806 c.g.s. 



Sn = 31.8 A series of determinations are made in order to 



arrive at an average value for the moment M. 



Ss - Sn = 94.8 



Standard magnets should be carefully handled and are usually carried 

 in a padded box to prevent them from being jolted. They should, of 

 course, be kept away from strong direct currents or pow^erful magnets. 

 If more than one magnet is carried in the same case, they should be laid 

 with their poles reversed to minimize their effect on each other. 



The moment of an auxiliary magnet may be determined by using a 

 standard magnet of known moment. The deflections of a magnetometer 

 by magnets (of the same size) placed at the same distance from a magneto- 

 meter (with like poles up) are directly proportional to the moments of the 

 two magnets. 



If Mst = the magnetic moment of the standard magnet (which is known) 

 Ma; = the magnetic moment of the unknown magnet 



(Ss — SN)st = the deflection produced by the standard magnet with its 

 South and North ends up respectively, at a distance r cm. 



(Ss — Sn)!v = the deflection produced by the unknown magnet as above 

 and at the same r distance. 



The direct proportion is : 



Ma> : Mst = (Ss - S^)^ : (Ss - Si,)st 



M, = M,, ^.l'~l''\^ (60) 



The magnetic moment of an auxiliary magnet may also be found by 

 using a calibrating coil and a magnetometer. This is done by noting the 

 deflection produced by the auxiliary magnet at a given distance and position 

 from the instrument compared to the deflection produced by a given current 

 flowing in the coil. The coil constant is an expression of the magnetic 

 moment applied to the instrument by the coil. In the examples used, C — 

 173.2 c.g.s. per milliampere, expressed in gammas. A direct proportion 

 would be involved. 



Determination of Temperature Coefficient. — The temperature 

 coefficient of a magnetometer is determined by placing the instrument in 

 a heating box (or heating it by other suitable means) and observing the 

 scale reading and temperature at about 5-minute intervals. If electrical 

 heating is employed, the box must be non-inductively wound. Alternating 

 current should be used with a tapped transformer and rheostat to control 

 the amount of current and thus the rate of heating. Circulating water 

 may be employed by providing a copper tube coil, through which hot or 

 cold water can be circulated. The box should be provided with a detachable 



