Chap. 8] 



MAGNETIC METHOD 



365 



4. Sine galvanometers are combinations of unifilar magnetometers and 

 deflection coils so arranged as to give a horizontal comparison field in the 

 magnetic prime vertical or at right angles to the needle. In the second 

 case, the instrument is used as the "sine" galvanometer proper; in the first, 

 as a "tangent" galvanometer. The horizontal intensity 



H 



CI 



tan v5 



or H = 



sin (p' 



(8-48) 



where C is a coil constant and I the current, measured by standard re- 

 sistance coil, standard Weston cell, and potentiometer. For field instru- 

 ments the coil constant may be determined by measuring deflections at an 

 observatory where H is known. Large instruments for absolute deter- 

 minations of the horizontal intensity are described by 

 Barnett and Hazard; a smaller field instrument is il- 

 lustrated by Nippoldt.^ 



5. Compass variometers are instruments intended for 

 the measurement of the horizontal intensity on moving 

 support. They consist of two compasses mounted some 

 distance apart above each other. The force acting on 

 either compass is dependent on the horizontal intensitj^ 

 and the attraction due to the other needle. Generally, 

 the angle of spread between the two compasses is meas- 

 ured, from which the horizontal intensity 



H = 



2Mk V' 

 7^ ''' 2' 



(8-49) 



Fig. 8-39. In- 

 duction vertical 

 magnetometer 

 (after McNish). 



where M is the magnetic moment of each magnetic needle, 



k a deflection constant, \f/ the spread angle, and r the distance of the 



needles. The instrument is well adapted to the measurement of horizontal 



intensity anomalies on board ship and in aircraft. It may be made into a 



vertical intensity instrument by using two compasses oscillating". in vertical 



planes side bj^ side with their plane of oscillation in the magnetic prime 



vertical. Details are given in publications of L. A. Bauer and others, 



by E. B. Brammer,^^ by G. Angenheister,^" and by K. Luyken." 



" Pub. No. 175, Carnegie Institution, 373-394 (Dec, 1921). 



*6 Op. cit., p. 38. 



''' Nippoldt, op. cit., p. 33. 



*8 Pub. No. 175, Carnegie Institution, V, 339-351 (1926). 



"Zeit. Instr., 45(12), 576-579 (Dec, 1925). 



^oHandb. Exper. Phys., 25(1), 577-578 (1928). 



" Zei't. f. angew. Geophys., 1(6), 163-181 (Oct., 1923). 



