MAGNETIC METHODS 



131 



Pz =^ pZ ^ force on pole due to vertical component of earth's field. 

 Ph = pH = force on pole due to horizontal component of earth's field. 



= deflection angle of the system. 

 Wi = temperature coefficient adjustment weight. 

 Wt = latitude adjustment weight. 

 Ws = sensitivity adjustment weight. 



Calibration of Schmidt-type Magnetometers 



It is necessary to know two constants of the magnetometer used : 

 namely, the scale value, E, and the temperature coefficient, Tc, to calculate 

 the data from the field measurements. 



Scale Value Determination. — The physical significance of the scale 

 value has previously been given in terms of the mass and magnetic mo- 

 ment of the magnetic system, the gravity force acting, the focal length, and 

 the vertical distance of the center of gravity from the center of rotation. 

 (Equation 50.) It also may be defined as the number of gammas of mag- 

 netic force necessary to deflect the magnetic system one scale division. The 

 scale value usually is determined by either of two methods; (1) using a 

 calibrating coil, or (2) using auxiliary magnets. 



A B 



Fig. 56. — A, vertical component magnetometer, and B, horizontal 

 component magnetometer with calibration coil in position. (Courtesy 

 American Askania Corp.) 



Scale Value Using Calibrating Coil.— A magnetic field of known 

 strength and direction can be created by passing a direct current through 

 a coil of known constants. One special form is known as the Helmholtz 

 coil. It utilizes the straight line magnetic field which exists between two 

 identical coils, with their planes parallel and separated a distance about 

 equal to their radius. Such a calibrating coil is shown in Figure 56. 

 It is positioned on the instrument symmetrically to produce a straight line 



