442 Dr. J. Hopkinson. [Nov. 20, 



IV. " Magnetism and Recalescence." By J. HOPKINSON, D.Sc., 

 F.R.S. Received October 9, 1890. 



In my experiments the results of which are published, 'Phil. 

 Trans.,' 1889, A, p. 443, I showed that recalescence and the disappear- 

 ance of magnetisability in iron and steel occurred at about the same 

 temperature. The evidence 1 then gave was sufficiently satisfactory, 

 but did not amount to absolute proof of the identity of the tempera- 

 tures. Os,mond has shown that the temperature of recalescence 

 depends upon the temperature to which the iron has been heated, also 

 that it differs when the iron is heated and when it is cooled. He 

 also showed that for some sorts of steel the heat is liberated at more 

 than one temperature, notably that in steel with 0'29 per cent, of 

 carbon heat is liberated, when cooling at 720 C. and at 660 C., and 

 that with steel with 0'32 per cent, carbon there is a considerable 

 liberation of heat before the temperature is reached when this 

 becomes a maximum. It appeared to be desirable to obtain absolute 

 proof that the change of magnetic property occurred exactly when 

 heat was liberated and absorbed, and to examine, magnetically, 

 Osmond's two temperatures of heat liberation. I have not been able 

 to obtain samples of steel of the size I used, showing two well marked 

 temperatures of heat liberation and absorption, but I have a ring in 

 which there is liberation of heat extending over a considerable range 

 of temperature. 



The samples had the form of rings of the size and shape indicated 

 in fig. 1. A copper wire was well insulated with asbestos and laid 

 in the groove running round the ring, and was covered with several 

 layers of asbestos paper laid in the groove. This coil was used for 

 measuring temperature by its resistance. The whole ring was served 

 over with asbestos paper and with sheets of mica. The secondary 

 exploring coil was then wound on, next a serving of asbestos paper 

 and mica, and then the primary coil, and, lastly, a good serving of 

 asbestos paper was laid over all. In this way good insulation of the 

 secondary coil was secured, and a reasonable certainty that the tem- 

 perature coil took the precise temperature of the ring, and that at 

 any time the ring was throughout at one and the same temperature. 

 The whole was placed in an iron pot, and this again in a Fletcher gas 

 furnace. Observations were made of temperature as the furnace was 

 heating, and from time to time of induction. In each case the time 

 of observation was noted. Similar observations were made as the 

 ring cooled, the furnace being simply extinguished. We are thus 

 enabled to compare directly at the same instant the condition of the 

 same ring as regards magnetism and as regards temperature, and, 



