332 Mr Searle, On a compound magnetometer for [May 16, 



where i is the strength of the current, and n the number of turns 

 of wire upon the solenoid per centimetre of its length. The 

 solenoid is placed near a suitable mirror magnetometer; and a 

 small coil, which is joined up in series with the magnetising sole- 

 noid, is so adjusted that it exactly neutralises the action of the 

 solenoid itself upon the magnetometer. Thus when the specimen 

 of iron is placed inside the solenoid the deflection produced is 

 due entirely to the magnetisation of the specimen. From the 

 observed deflection the value of the intensity of magnetisation, /, 

 can be determined. • The magnetising current also passes round a 

 suitable galvanometer by means of which its strength, i, can be 

 measured. The strength of the current is gradually varied by 

 means of a resistance box in the circuit, and the simultaneous 

 readings of the galvanometer and magnetometer are noted. The 

 values of H and I deduced from these readings are used as abscissa 

 and ordinate in the construction of the hysteresis curve. This 

 process naturally involves a good deal of labour. 



I have endeavoured to construct an instrument which should 

 perform simultaneously the functions of both galvanometer and 

 magnetometer and should cause a spot of light -to trace out -a 

 rrysteresis curve upon a screen. One method of attaining this 

 end is to provide a mirror with two independent motions about 

 two axes mutually at right angles, the motions about these two 

 axes being governed by two small magnets. One of these magnets 

 must be acted on by a magnetic force proportional to the 

 magnetising current, and the other by a magnetic force propor- 

 tional to the intensity of magnetisation of the specimen. 



Fig. 2. 



This idea was put into practice in the following manner. AB 

 (fig. 2) is a thin aluminium wire about 80 centimetres long. This 

 is suspended by one end A by a silk fibre from the support K. 



