1896.] 



on Electric Besearch at Low Temperatures. 



263 



creasing magnetising forces, this force produces magnetisation in the 

 iron, but the magnetisation does not increase proportionally with the 

 force. It tends to a limit, and the curve which shows this variation 

 is called a magnetisation curve. The number which expresses the 

 ratio of the magnetisation to the magnetising force is called the 

 susceptibility of the iron. Instead of considering the magnetisation 

 of the iron as one of the variables, it is often convenient to con- 

 sider the induction in the iron, and the induction is defined as a 

 quantity, the rate of change of which with time measures the electro- 

 motive force set up in a secondary circuit wound round the iron ring. 



^'' 



Variat 

 Nick 



Temper ature 



ON OF M 



EL St 

 Wi 



ACNETIciiMOMENT 



EEL M ACNET ( 

 TH TeM PERATUR 



IN Decr ees Ce NTICRA 



OF A 

 19-64% 



") 



DE 



-200" -100° 0° +100° +200° +300° 



Fig. 16. 



The ratio between the induction and the magnetising force at any 

 instant is called the permeability of the iron. By tedious experiments 

 with the ballistic galvanometer, it is possible to draw out a complete 

 magnetisation curve of the iron, starting from the lowest induction 

 up to the point at which the iron becomes practically saturated. 

 Assisted by Mr. J. E. Petavel, who has given us most valuable help in 

 these very tedious magnetic observations, as well as in the subsequent 

 reductions of them, a large number of observations have been made 

 on the permeability of a carefully annealed iron ring made of very 

 fine Swedish iron of the highest quality.* The result is to show — 



* It is only right to add that in other portions of this work, especially in 

 the resistance and thermo-electric work, we have been much indebted for careful 

 and persevering assistance to Messrs. J. and D. Morris and, in lesser degree, to 

 Messrs. Jakeman and Tilney for help in other observations requiring severa] 

 simultaneous observers. 



