MAGNETIC METHODS 135 



set up and oriented, and the reading So is then taken, without the auxiliary 

 magnet present. The latitude adjustment screws may need to be readjusted 

 to bring the scale reading to approximately the mid-point. The auxiliary 

 magnet of known moment is then placed in position, with the N. end up. 

 (See Figure 58b.) The scale value E may then be calculated using the 

 relationship : 



2M 

 E= „ ,„ ^;-;- (N. endup) (56) 



The auxiliary magnet may then be reversed, using the equation : 



E= „ ,„ ^rT-(S.endup) (56a) 



If desired, the calibration may be done merely by taking readings with 

 the auxiliary magnet having the S, and then the A'^ end up, in which case : 



where 6"s is the reading with S. end up, and Sn with the N. end up, for 

 the auxiliary magnet. 



The relationship between auxiliary magnet and magnetic system in 

 calibrating the vertical component magnetometer is that of the first position 

 of Gauss or the "end-on" position where the magnetic force is 2M/r^. 



Examples : Determination of the scale value of vertical intensity magnetometer, 

 using an auxiliary magnet of known moment M = 500 c.g.s., placed at a distance 

 r = 100 cm. vertically below the instrument. 



Scale readings Using Equation 57 above : 



^^ = 24-6 E = 200Q-10O>Q00 = 21.97 gammas / scale division. 



So = 20.0 1,000,000.9.1 



<-. _ 1 e q Note : 1 gauss (c.g.s.) = 100,000 gammas. The figure 7 in the 



second decimal place has no particular significance except in 



■ connection with an average of a number of determinations. 



5"^ 6'j\r == 9.1 Scale values are ordinarily used to 0.1 of a gamma only; in 



this case the scale value would be 22.0. 



For calibrating the horizontal magnetometer, the same procedure is 

 applied except that the second position of Gauss, or "broad-side" position, 

 is involved. The auxiliary magnet is set perpendicular to the magnetic 

 axis of the magnetic system as shown in Figure 58a and the force 

 H = M/rK 



The scale value formula for the horizontal magnetometer is : 



2M 



In this formula S^ — the reading with the auxiliary magnet horizontal and 

 with its N. end toward the magnetic north. S^ = the reading with the S. 



