Galvanometric Method of Measuring H. 79 



and the absolute value of resistances are now known with as 

 great a degree of accuracy as any electrical constants ; so that 

 any laboratory possessing a resistance-box and a Clark cell 

 has the means of measuring a current with sufficient accuracy 

 even to replace the Kew magnetometer. 



Starting with these considerations, I was led to conclude 

 that the most practical way of measuring the magnetic force 

 would be by running a measured current through an ordinary 

 sensitive reflecting-galvanometer, used either by the tangent, 

 or, preferably, the sine method. For this purpose, a gal- 

 vanometer and its lamp and scale were fixed on a board and 

 the whole mounted on a goniometer, by which they could be 

 set in any required azimuth ; the instrument was first set 

 with its needle parallel to the plane of the coils ; a known 

 current passed through it ; the goniometer-table rotated till 

 the needle became again parallel to the coils. 



Let G = the principal galvanometer-constant. 

 H = intensity of the earth's field. 

 y = current. 

 8= angle of deflexion from the magnetic meridian. 



G7 = HsinS. (1) 



The first adjustment necessary is to set the needle parallel 

 to the plane of the coils : this can be done, sufficiently nearly, 

 by making the reflexion of the light from the mirror to coin- 

 cide with that from the brass face of the instrument. If this 

 is done, it ensures that the galvanometer shall always be used 

 in the same position ; and though the needle may make a 

 small angle i/r with the mean plane, this will only alter the 

 galvanometer-constant in the ratio of cos y]r ; so that if 

 we find G by comparison with a larger measured coil, by 

 Bosscha's method, using the galvanometer in the same 

 position, we shall not make any error. 



Next, the suspending fibre may have some torsion. Let 

 (j> = angle of torsion, 



_ moment for unit angle of torsion 

 magnetic moment of needle 



Suppose the effect of this torsion is to make the needle lie 

 at an angle 6 (of the same sign as (f>) from the magnetic 

 meridian ; then after the goniometer has been rotated through 

 8 the needle will make with the meridian the angle S + 6, and 

 the first equation becomes 



yG + T0-Hsin(S + 0)=O (2) 



If now we take two observations with the galvanometer in 

 positions at right angles to each other and on opposite sides 



