98 



Effect of Magnetisation upon Iron Rings. [May 31, 

 Table II. 



and then slowly increases, until, with a force of 500 units, it is about 

 30 ten-millionths less than at starting. The unannealed ring also at 

 first suffers diminution, but its original volume is recovered with a 

 force of about 90 and with higher values is increased. 



The behaviour of this latter ring may be regarded as probably 

 similar to that of the great majority of rods and rings, the annealed 

 ring used in these experiments being the only specimen of iron that 

 has yet been found to contract along the lines of magnetisation with 

 the smallest forces that produced any effect at all. 



Experiments upon the volume changes produced by magnetisation 

 have been previously made by Joule, Barrett, and Knott., 



Joule* concluded that the volume of an iron bar was altogether 

 unaffected by magnetisation, even though the magnetising current 

 which he employed " was quite equal to saturate the iron." It was 

 at that time believed that " saturation " was produced by a force of 

 from 80 to 100 units, and, assuming that Joule's force was of about 

 that value, an inspection of the curve for unannealed iron in fig. 3 

 will show the probable reason of his having failed to detect any 

 change of volume. There is, in fact, none at all with a force of 

 about 90 units. 



Barrett,t experimenting in the same manner as Joule, " enclosing 

 the bars in a vessel of water terminating in a capillary tube, and 



* Joule's ' Scientific Papers,' p. 236. 

 t ' Nature,' vol. 26, p. 485. 



