MAGNETIC METHODS 



95 



Compass-Variometers. — The Car- 

 negie Institution of Washington has 

 developed several compass-variometers 

 capable of measuring relative values 

 of the horizontal intensity. § 



One type of compass variometer 

 which has been used for measurement 

 aboard ships is shown in Figure 28. 

 Two disc-shaped magnets of equal 

 magnetic moment are suspended in- 

 dependently one above the other. The 

 distance between the two disc magnets 

 is regulated by a graduated micrometer 

 screw. A fine quartz-rod pointer, or 

 index, is attached to each of the 

 magnet supports. The pointer of 

 the upper magnet is in the vertical 

 plane through the magnetic axis of the 

 upper magnet (a diameter of the disc) , 

 and the pointer of the lower magnet 

 is in a vertical plane making an angle 

 ip with the magnetic axis of the lower 

 magnet. (In general, the angle \f/ is 

 made 60°.) 



On looking down on the instrument 

 through a lens one sees the quartz 

 pointer of the lower support and an 

 image of the pointer of the upper 



support reflected from a mirror which is mounted centrally with respect to 

 the magnet system. The angle ij/ between the two pointers is read off a 

 graduated circle which is photographed on a glass plate mounted ap- 

 proximately in the plane of the reflecting mirror. 



It may be shown that changes in the horizontal intensity are related 

 to changes in the angle if/ by the equation 



Fig. 27. — Carnegie Institution of Wash- 

 ington sine galvanometer. The international 

 standard for determining magnetic horizon- 

 tal intensity. A, suspended magnet; B, 

 telescope; C, scale; D, lamp; E, circle micro- 

 scopes; F, lamp; G, Helmholtz coils; H, 

 marble cylinder; I, copper conductors to 

 power supply; J, torsion wire housing. 

 (Courtesy of U. S. Coast and Geodetic 

 Survey.) 



AH = 



DM . ^ 

 S7J? ^'" 2 



Ai/^ 



where 



AH = change in horizontal intensity 

 D = instrumental constant 

 M — magnetic moment of either magnet 

 e = vertical distance between magnets 



tp = angle between two imaginary vertical planes passing through the 

 magnetic axes of the two magnets 



§ L. A. Bauer, W. J. Peters, and J. A. Fleming, "The Compass Variometer," Carnegie Institu- 

 tion of Washington, Publication 175, Vol. V, pp. 339-357. 



