552 • PROCEEDINGS OF TBE AMERICAN ACADEMY. 



determine their temperature coefficients. This paper gives the results 

 of measurements made on a number of magnets of this kind. 



Most of the magnets experimented upon were made of castings chilled 

 by Mr. G. W. Thompson, the mechanician of the Jefferson Physical 

 Laboratory, who has had a great deal of experience with the process. 

 They were first heated to a bright red in a gas furnace under a power 

 blast, and then plunged into a cold acid bath kept in violent agitation. 

 The castings thus hardened were relaxed by long exposure to boiling 

 water or steam, then magnetized to saturation, and finally seasoned, 

 after prolonged boiling, by being alternately heated in steam and cooled 

 in tap water. The whole seasoning process reduced the magnetic 

 moment of each specimen by perhaps twenty per cent of the value it 

 had just after the magnetization. If, after a magnet has been seasoned, 

 its temperature be suddenly raised from 0° C. to 100° C. and then as 

 suddenly lowered again, it may not wholly recover its original strength 

 until after the lapse of an hour or two : if, however, the range be only 

 40° or 50° C, I have been unable to detect any lag in the attainment of 

 the whole of the original moment after the heating. 



Although there is no advantage in using cast iron for straight magnets, 

 I had a number made for comparison with fine steel magnets of the same 

 dimensions. The cast iron magnets looked rough in comparison with 

 the others, but the moments of a large number of them seemed to differ 

 less amoncr themselves than the moments of the same number of the steel 

 mafi'nets. The stronsest steel magnet that I tested had a moment about 

 four per cent greater than that of the strongest cast iron magnet, but the 

 average moment of the cast iron magnets was practically the same as 

 (in fact two per cent greater than) the average of the seasoned steel 

 magnets. 



In determining the temperature coefficients, the straight magnet to be 

 experimented upon was fixed firmly in a non-magnetic holder inside a 

 non-ma<^netic tube so as to be in Gauss's A Position east of a mirror 

 magnetometer. By the help of a system of pipes and cocks, tap water, 

 steam, or a stream from a bath water heater at almost any desired tem- 

 perature, between 15° C. and 100° C. could be sent through the tube 

 containing the magnet. On the west of the magnetometer, so placed in 

 Gauss's A Position as to bring the needle back exactly into the meridian, 

 was a short, seasoned, compensating magnet, fixed wholly within a wooden 

 holder and completely shielded from sudden temperature changes. If 

 oo is the needle deflection which the compensating magnet would cause 

 if the magnet to be tested were removed, Mq, the moment of the last 



