150 Mr. S. K. Eoget, Effects of prolonged 



" Effects of Prolonged Heating on the Magnetic Properties of 

 Iron. (Second Paper.)" By S. E. EOGET, E.A. Communi- 

 cated by Professor EWING, F.E.S. Eeceived October 26 

 Eead December 8, 1898. 



In a paper by the author, read before the Eoyal Society on May 

 12, 1898,* the results of some experiments were given showing the 

 change in the value of the hysteresis of soft iron transformer plate 

 when subjected to continued baking at temperatures not exceeding 

 200 C. The experiments there described have since been extended to 

 higher temperatures, and the residts for heating at temperatures up to' 

 700 C. are given below. 



The same specimens were used as in the former experiments, con- 

 sisting of soft Swedish iron transformer plate which had been re- 

 annealed. The baking at these high temperatures was carried on in a 

 specially constructed electric oven heated by coils of platinum wire 

 wound on a mica frame, inside a metal vessel completely surrounded 

 by a lagging of silicate cotton except for a mica tube through which 

 the specimens and thermometer were introduced. By this means, any 

 desired temperature could be maintained up to a bright red heat, so 

 that the apparatus could be also used as an annealing furnace. As the^ 

 specimen and thermometer were situated within the heating coil they 

 could be very rapidly brought up to the desired temperature, which 

 was read direct on a Callendar-Griffiths platinum pyrometer. Eegula- 

 tion was effected by alterations in the grouping of the coils as well as 

 by outside resistances. The measurements of hysteresis were made as 

 before with Professor Ewing's hysteresis tester, the specimens being 

 removed periodically from the oven and tested at atmospheric tempe- 

 rature. 



A number of short runs at various high temperatures were taken. 

 The results given in Table I and fig. 1 represent the means of several 

 independent observations at each temperature. 



The absolute values of the hysteresis are given in ergs per cubic 

 centimetre per cycle (for B = 4000), together with the rise expressed as 

 a percentage of the initial hysteresis to the nearest 1 per cent. The 

 general features of the action are similar to those noticed before at 

 more moderate temperatures. They should be compared with fig. 2 of 

 the former paper. The initial rise of hysteresis is more rapid the 

 higher the temperature, but the subsequent fall takes place sooner, and 

 the final state is one of lower hysteresis the higher the temperature, 

 until at about 700 C., a temperature just short of that required for 

 complete annealing, the hysteresis falls again to quite its original value 



* ' Roy. Soc. Proc.,' vol. 63, pp. 258267. 



