1890.] On a new Magnetometer. 263 



Thus H is determinable in terms of a mass, the value of the accele- 

 ration of gravity at the place, certain distances, and the ratio of 

 two deflections. 



A possible modification of the method consists in making accu- 

 rately equal to by varying W, d, or I, or, better, by turning the 

 upper end of the bifilars through a known angle. So far, however, 

 the writer has preferred to adjust the constants of the instrument so 

 that for H = O18, 6 shall be nearly equal to 0. It will be noticed 

 tlmt if telescope and scale be used there is no necessity to deter- 

 mine the distance of the scale from the magnets except very roughly 

 indeed, as we are requiring the ratio of the tangent and the sine of 

 two not very large angles. Thus the necessity of measuring two 

 angles of deflection instead of one as in Gauss's method is really an 

 advantage, as it obviates the necessity of determining either angle 

 absolutely.* 



The special feature of the circular form of deflecting magnet is 

 this that it is a matter of utter indifference what the distribution 

 of magnetism in it may be, provided it be circular and the little 

 needle be at the centre. This can be readily seen, for if we imagine 

 some north-seeking magnetism situated at an angular altitude x re ~ 

 ferred to the centre, the earth's moment on this will vary as cos x> 

 but at the same time the intensity of field at the centre resulting 

 therefrom and measured horizontally varies as cos % too, so that not 

 only is the position of the magnetic axis unimportant, but the dis- 

 tribution of magnetism may even be irregular without invalidating 

 equation (2). Moreover, if we are careful in the magnetisation to 

 get the poles in something like the right positions, it is not necessary 

 that the magnet should be absolutely circular ; all that is necessary 

 is that the magnet should be circular only in the neighbourhood of 

 the poles. In the above equations, then, r will stand for half the 

 polar diameter of the magnet. Again, with a magnet of moderate 

 dimensions the needle need not be placed rigorously at the centre, 

 since it is in a minimum field arising from the action of the two 

 opposite poles on opposite sides. To illustrate this, we may take the 

 case of a magnet, not unduly large, 30 cm. in diameter ; then if the 

 little needle, instead of being at the centre, is displaced horizontally 

 1 cm. on either side, allowing, in fact, a range of 2 cm., the intensity 

 of field at the centre is only increased by V2 per cent., so that H will 

 be too small by 0'6 per cent. The needle can easily be arranged 

 within 2 mm. of the centre, and in this case H will only be affected 

 to 1 part in 11,000. It will thus be seen that even for the most 

 accurate work a comparatively small magnet may be used, and the 

 little needle need not be placed rigorously at the centre. All the ad- 



* It must be understood that the writer is not recommending that small angles 

 of deflection should be used (see Note appended). 



