230 Mr. Albert Griffiths : Some 



placed at various distances from the needle ; the coils can 

 also be rotated as a whole about a vertical axis passing through 

 the centre of the needle. The needle consists of a steel ring, 

 suspended with the plane of the ring vertical. 



Ohrystal gave a very complete explanation of the phe- 

 nomenon in Phil. Mag. p. 401, vol. ii. ] 876 ; his explanation 

 depends on the assumption — no doubt correct — that the mag- 

 netization of a steel needle can be caused to vary by a 

 magnetic field, however small. 



Suppose that, through any cause, the needle makes an 

 angle with the plane of the galvanometer coils : owing to the 

 fact that a needle is most readily magnetized lengthwise, that 

 component of a magnetic field parallel to the needle produces 

 a decided change in the magnetization lengthwise, whilst the 

 action of the field in any other direction is relatively small. 

 It is unnecessary to go into details, but it is found, on 

 examination, that the field, with lines at right angles to the 

 plane of the coils, which tends to increase the angular 

 deflexion increases the magnetization of the needle : on the 

 other hand, the field which tends to diminish the angular de- 

 flexion diminishes the magnetization of the needle. Hence in 

 the case of alternations the currents that tend to increase the 

 deflexion give greater impulses than those in the other direc- 

 tion, and in consequence the angular deflexion of the needle 

 is increased. The same would happen if the needle were not 

 magnetized. 



Since the change in the magnetization of the needle will 

 vary approximately as the strength of the alternating current, 

 and the action on the needle varies as the product of the 

 change and the strength of the alternating current, it follows 

 that the action on the needle will vary approximately as the 

 square of the alternating current. This I proved indirectly 

 by using an alternating current of constant strength, and 

 placing the coils at various distances from the needle, having 

 previously determined the relative sensibilities of the galva- 

 nometer for the various distances with a steady current. 



Still following Chrystal, it is obvious that if a magnetized 

 steel sphere, or any steel body bounded by a surface of revolu- 

 tion whose axis is that of suspension, be suspended, the super- 

 imposed magnetization will always be parallel to the lines of 

 force in the field, and hence there will be no turning moment 

 due to this cause. 



I suspended a magnetized steel sphere in place of the 

 ordinary needle, and was surprised to find that when the 

 angular displacement of the magnetic axis with respect to the 

 coils was in one direction, an alternating current increased 



