94 EXPLORATION GEOPHYSICS 



To=2n llo (40) 



lo equals the moment of inertia of the magnet which may be determined 

 from its dimensions, as the magnet is of simple shape. Solving these two 

 equations 



T^V^tan^ ^^^ 



I 



H can thus be found and M as well, if desired. 



At the same station the declination can be obtained by measuring the 

 angle between astronomic north, as determined by a solar or an observation 

 on polaris, and the magnetic north. A specially-designed compass called 

 a declinometer is used. 



Also at the station, magnetic inclination, or dip, can be found with an 

 accurate dip needle or dip circle. In this manner the three magnetic 

 elements D, H, and / for a station are measured. 



The instruments used by the Coast and Geodetic Survey for these 

 purposes are of special design to insure accuracy of measurement. The 

 procedure followed is somewhat more refined than that described here, in 

 order to eliminate errors.f Some of the special instruments referred to 

 are described in sections which follow. 



Sine Galvanometers. — The sine galvanometer is an observatory instru- 

 ment us?d for precision determinations of the absolute value of the hori- 

 zontal intensity. The horizontal intensity is determined by the deflection 

 method described above. The deflecting field in this case is produced by 

 an electric current flowing in a coil of known dimensions. When current 

 is passed through this coil, the suspended magnet is deflected from its free 

 position in the magnetic meridian, through an angle which is dependent 

 upon the ratio of the intensity of the field produced by the current in the 

 coil and the horizontal component of the earth's magnetic field. Knowing 

 the constants of the coil and the magnitude of the current, the horizontal 

 intensity can be computed. Sine galvanometers for the determination of 

 the horizontal intensity are described in detail by Barnett| and by Hazard. § 

 Figure 27 shows a photograph of a sine galvanometer designed and con- 

 structed by the Carnegie Institution of Washington for determining the 

 magnetic horizontal intensity. The standard cells and electrical measuring 

 appurtenances used when making observations with this sine galvanometer 

 are mounted in a heat-insulated cabinet. 



t D. L. Hazard. "Directions for Magnetic Measurements," U. S. Coast and Geodetic Survey, 

 Serial No. 166, 3rd. Edition, 1930. 



t S. T. Barnett, Carnegie Institution of Washington, Publication 175, pp. 373-394, Dec. 1921. 

 § D. L. Hazard, loc. cit., p. 38. 



