:228 



Mr. W. M. Mordey. On Slow Changes [Jan. 17. 

 FIG. 2. 



-O- 



On the occasion of each test, measurements were made at three 

 -densities, viz., B 2500, 2752, 3050. 



The sample was kept in the oven between each set of tests, at the 

 same temperature as the others, that is, an average of about 67 C. 



It will be seen that the hysteresis is still rising, the change being 

 about in the same order as in the other samples, showing that the 

 question of joints has no noticeable effect. The change also follows 

 closely the same order, whether measured at 2500, 2762, or 3050 B. 



Effect of Pressure. An attempt was made to find if the increase 

 was due directly to heat or to a mechanical condition brought 

 about by the heat. 



A possible explanation of the increase of hysteresis suggested itself 

 in connection with the method of construction. In transformers the 

 iron laminae are usually bolted together very firmly, in order to 

 ensure good contacts at the magnetic joints, to prevent noise, and for 

 ordinary purposes of mechanical construction. It appeared possible 

 that the expansion of the iron, due to heat, caused the plates to be 

 pressed together more firmly when hot than when cold, and that this 

 state of compression, if long continued, had a hardening effect on the 

 iron similar to that caused by cold rolling or hammering. 



