the Determination of "HP 461 



magnetic moment of the subsidiary magnet in the interval 

 elapsing between the two parts of: the experiment is a tem- 

 perature change. The temperature coefficient of magnetic 

 moment is *001. Thus in order to obtain the required 

 accuracy the temperature must be kept constant to within 

 -^0° C. Suitable precautions can easily be devised to ensure 

 this. 



In both me! hods described in the present paper the magnet 

 when in a deflected position lies in a field of strength H ^/2 

 approximately, and hence owing to induced magnetism its 

 magnetic moment will be slightly greater than when in a 

 field H. This, however, introduces no error, for the magnetic 

 moment of the suspended magnet is a factor which does not 

 enter into the problem except in so far as it alters the angle 

 of deflexion due to residual torsion. At the most the change 

 introduced into a is a small fraction, and hence as a is 

 negligible the change owing to altered magnetic moment is 

 also negligible. 



Comparison of the two methods. — Both methods give an 

 accurate result, and both are easier to carry out than the 

 Kew Magnetometer method. Once the constants n, cos a, 

 the angles between the mirrors, and the direction of H are 

 determined the only determination in the first method is that 

 of the current required to give the necessary deflexion. 

 When the solenoid is rotated through 90° the optical apparatus 

 must also be rotated through the same angle, but in the 

 apparatus which is being designed for permanent installation 

 the optical system, consisting of eyepiece-scale and reflecting 

 glass, is to be fitted into the end of the tube and will therefore 

 rotate with it. 



In the second method no rotation of the tube is necessary, 

 but it involves the use of a subsidiary magnet. Moreover, 

 two adjustments are necessary. 



(1) The adjustment of the position of the magnet to give 



the requisite deflexion under the influence of the 

 Earth's field. 



(2) The adjustment of the current to create an oppositely 

 directed field which gives the same deflexion. 



Provided that the solenoid is mounted upon a large circular 

 table whose rotation can be measured, as that of a spectro- 

 scope table is, the first method is extremely simple and 

 accurate, and in the author's opinion provides the best method 

 of determining H with accuracy. The fact that the deter- 

 mination of H by both methods gives substantially the same 



