1890.] On a new Magnetometer. . 261 



plane about its position of equilibrium. The first operation gives the 

 value of M/H, or rather would give it if the distance between the 

 poles of the deflectors were known. To measure and allow for this 

 requires a second experiment, when the distance from the needle is 

 altered, and the new deflection read. In this way two simultaneous 

 equations are obtained, from which, by elimination of the distance 

 between the poles, an equation is obtained involving M/H as the only 

 unknown. The second operation gives the product of M and H, or 

 rather would give it if the moment of inertia of the magnet about its 

 axis of oscillation were known. As this quantity is not directly deter- 

 minable, a second equation has to be obtained by increasing the 

 moment of inertia by a known amount, and determining the new time 

 of vibration. In this way two more simultaneous equations are 

 obtained, from which, by elimination of the unknown moment of 

 inertia, the value of MH is obtained in terms of measurable quan- 

 tities. 



Doubtless Gauss's method is excellent when the moment of inertia, 

 as well as the distance between the poles of the deflector, is wanted, 

 but when these quantities are not required (and they never are) a 

 more direct method is very desirable. In a magnetic survey, no 

 doubt, the determination would be shortened by measuring, once for 

 all, the moment of inertia of the deflector, and possibly the distance 

 between the poles. 



In place of the laborious dynamical method of measuring MH, 

 various statical methods have been suggested and employed, notably 

 by suspending the deflecting magnet bifilarly and approximately east 

 and west. The instrument to be described consists of a deflecting 

 magnet of peculiar form, suspended bifilarly and approximately east 

 and west ; in this case the lower end of the bifilars will be turned 

 through an angle which forms a measure of MH. This magnet at 

 the same time acts at a known distance as deflector to a little magnet, 

 the deflection of which is a measure of M/H. Hereafter the deflecting 

 magnet will be, for brevity, referred to as the "magnet," the little 

 deflected magnet as the " needle." 



The magnet consists of a piece of fine pianoforte wire, some 100 cm. 

 long, and 0'06 cm. in diameter, bent into the form of a circle, or 

 approximately so, the two ends being soldered together " end-on," 

 with no overlap. This is magnetised similarly to a Gramme arma- 

 ture, with a north and south pole at opposite ends of a diameter (by 

 placing it with this diameter between the two opposite poles of a 

 weak electromagnet), and so suspended from the bifilar arrangement, 

 that, when the bifilars are vertical, the magnet lies with its plane 

 vertical, and approximately east and west, and with its magnetic axis 

 approximately horizontal. In its position of equilibrium, the couple 

 which the earth exerts upon it is MH cos 0, where M denotes the 



