Loomis —Effects of Changes of Temperature, etc. 179 



Art. XYII. — The Effects of Changes of Temperature on Per- 

 manent Magnets ; by Hiram B. Loomis. 



The following paper is an account of some experiments 

 undertaken to determine more accurately, if possible, the kind 

 of change which takes place when a magnet is heated and 

 cooled after it has been brought to the " permanent state." 

 The subject will be considered under the following heads : 

 I. Historical Sketch. II. Experiments and Calculation of 

 Kesults. III. Discussion of Kesults. 



I. Historical Sketch. 



A. Investigations on the change in magnetic moment due to 

 change in temperature. — About 1825, Kupfer* magnetized a 

 steel bar and placed it in a water bath at the temperature of 

 the room. Near it he suspended a magnetic needle and deter- 

 mined the period of 300 swings. The bath was then heated 

 to 100 C. and the period of 300 swings was again determined. 

 The bar was then alternately heated and cooled between these 

 limits of temperature and similar observations taken. Kupfer 

 thus learned that if a permanent magnet is heated above its 

 temperature of magnetization its magnetic moment decreases, 

 that on again cooling the moment increases but not enough to 

 make up for the first loss, and that this is true for the first 

 three or four times it is heated and cooled. 



In 1851, Lamontf found that when a permanent magnet was 

 alternately heated and cooled fifteen or sixteen times between 

 fixed limits of temperature, it reached a permanent state in 

 which it had a definite magnetic moment for a given tempera- 

 ture, to which it always returned when brought to that tem- 

 perature, provided only it had never passed beyond the 

 temperature limits mentioned above. The higher the tempera- 

 ture, the smaller was the magnetic moment. 



Kiess and Moser^: also experimented on the change in mag- 

 netic moment by swinging magnets in the earth's field and 

 determining the period of vibration. For needles 34 lines 

 long they found the following formula held : 

 F==I [1 — 0000324 (? — t)d], 



in which I and V are the intensities of magnetization at the 

 temperatures t and V on the Reaumur scale, and d is the diam- 

 eter of the magnets. For needles two inches long the numer- 

 ical factor is 0*000432, showing that the proportional change 

 in intensity of magnetization is greater in shorter magnets. 



* Wiedemann's Electricitat, iii, p. 753. 



fLanaont, Pogg. Ann., lxxxii, p. 440, 1851. 



% Riess and Moser, Pogg. Ann., xvii, p. 425, 1829. 



Am. Jour. Sci. — Fourth Series, Vol. XV, No. 87. — March, 1903. 

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