338 MAGNETIC METHOD (Chap. 8 



magnetometer and is given by dH/dO = —K/ia , where H is the field and 

 Ma the temperature coefficient of the auxiUary magnet. Since the absolute 

 sensitivity of a horizontal balance is usually greater than that of a vertical 

 balance, it should be especially well protected against shocks. As before, 

 d(es)/da = —mg/M; a change of a by 25 X 10~ mm is sufficient to pro- 

 duce a change of one scale division if en = ISr. For changes in magnetic 

 moment the equations for the horizontal and vertical magnetometer are 

 the same. 



Procedure in the latitude adjustment of a horizontal magnetometer is 

 the same as in the vertical balance. Scale value determinations may be 

 made (a) with magnets, (6) with coils, (c) by observations in different 

 azimuths. Contrary to the vertical balance, the Helmholtz coils are set 

 up with coil planes vertical, and magnets are used in second instead of 

 first Gauss position. While the coil formula (8-26c) remains the same, 

 the numerical factor and deflection constant, k, change in formula (8-26a), 

 so that the scale value 



en =.-^^3 wh«^'^ k = l+-o(^'-|^')- (8-35) 

 (S2 — Si)r r- \2 8 / 



Scale values may be obtained from observations in different azimuths 

 where the horizontal intensity is known. From eq. (8-29) ch = 



— ^^ r — - , where Sa is the reading in the azimuth a. Observations 



{Sa — s) 



may be made in various azimuths to increase the accuracy, which, how- 

 ever, is dependent on how accurately the horizontal intensity at the station 

 is known. The azimuth cannot be varied more than 12-13 degrees either 

 way from north. 



Moments of auxiliary magnets, temperature correction, base correction, 

 auxiliary magnet correction, and planetary and diurnal variation correc- 

 tions are applied as in the vertical magnetometer. 



Likewise, the operation of the horizontal magnetometer is virtually the 

 same as that of the vertical magnetometer, except that only one operating 

 position is possible. This results in an increased number of readings in 

 this position and greater care in orientation and leveling. 



The general directions given previously for the application of the vertical 

 magnetometer apply, especially in regard to the selection of stations. The 

 steps in the operation of the horizontal magnetometer are as follows: (1) 

 Remove instrument from case so that it acquires the temperature of the 

 air. (2) Set up and level tripod. Orient tripod head with compass in 

 the magnetic meridian. Fasten tripod head with screw. Read and note 

 position of index. (3) Set up instrument on tripod; level with tube 

 levels. (4) Orient instrument in magnetic meridian (N toward north). 



