A SINE GALVANOMETER FOR DETERMINING IN ABSOLUTE 



MEASURE THE HORIZONTAL INTENSITY OF THE 



EARTH'S MAGNETIC FIELD. 



By S. J. Barnett. 



1. Magnetometer methods devised by Gauss, Weber, and Lamont in the first 

 half of the last century, and brought to a high state of perfection through the labors 

 of many others, have long made it possible to measure the horizontal intensity of the 

 Earth's magnetic field with considerable precision. By comparing with this intensity 

 the intensity due to a coil of wire traversed by an electric current, the current can be 

 determined in electromagnetic units, provided that proportionality exists between the 

 current in the coil and the intensity of its field, and that the ratio of the two can be cal- 

 culated from the fundamental theory. 



Many absolute determinations of electric currents have been made in this way, but 

 it has in comparatively recent years become possible to make independent determina- 

 tions of current with much greater precision and facility than that with which it is 

 possible to measure the horizontal intensity of the Earth's field with magnetometers. 

 Hence many students of the Earth's magnetism have considered making use of the same 

 comparison to determine this intensity in terms of an electric current. 



Probably the simplest instruments by which this comparison of intensities can be 

 made are sine and tangent galvanometers, whose prototypes were introduced by Pouillet 

 in 1837 1 , and either one of which can be constructed in such a way as to satisfy all neces- 

 sary requirements. Of the two instruments, when so constructed, the sine galvanometer 

 is somewhat the simpler, can be made the more sensitive, and has been preferred by at 

 least most of those who have considered this subject. 



2. The essential parts of a sine galvanometer are a simple magnetometer, a divided 

 circle, and a coil of insulated wire producing a magnetic field preferably symmetrical 

 about an axis, together with their adjuncts. 



In the ideal instrument the circle is mounted with its axis vertical and coincident 

 with that of the suspension of the magnetometer magnet; and the axis of symmetry 

 of the coil, which is then horizontal, passes through the axis of the circle and the center 

 of the magnet. Both magnetometer and coil can be rotated together about the vertical 

 axis of the circle. All materials except the magnet must have magnetic susceptibilities 

 differing by negligible amounts from that of air. 



If the axis of the coil is initially in the magnetic prime vertical and that of the 

 magnet in the magnetic meridian, and if a suitable current J is passed through the coil, 

 producing throughout the region occupied by the magnet a magnetic intensity GJ along 

 the coil's axis, the magnet (and mirror or lens) will be deflected through an angle i/< 

 such that 



tan ^ = -^- (1) 



where H is the horizontal component of the intensity of the Earth's magnetic field. But 

 it can be brought back to its initial position relative to the coil by turning the latter, 

 in the direction of the magnet's motion, through an angle d, such that 



sin = -jj- = tan 4> (2) 



1 Compfes Rendus, vol. 4, 1S37, p. 267. 

 373 



