PEIRCE. — TEMPERATURE COEFFICIENTS OF MAGNETS. 553 



na-ned magnet at the temperature t at which the adjustments have been 

 made, and M, the moment of this magnet when, its temperature having 

 become raised to t, the needle is deflected through the angle a, 



M — M _ tan a 



M tan u 



Since the temperature coefficients of seasoned bar magnets of a given 

 length and of given material are in general larger the greater the cross 

 section of the bar, it is necessary in comparing materials to take magnets 

 of nearly the same dimensions. Besides a number of chilled cast iron 

 magnets 18 centimeters long and about 0.95 centimeters in diameter, I 

 had many carefully made steel magnets of the same area of cross section 

 and of almost the same length. In the case of all these, the rate of loss 

 of moment per degree of rise of temperature was greater at higher tem- 

 peratures than at low ; we may, however, for the purpose of comparison, 

 use the mean loss, per degree, of the magnetic moment, when the magnet 

 is heated from about 10° C. to 100° C, expressed in terms of the moment 

 at the lower temperature. These mean losses were found to be 



0.00042 in the case of the seasoned chilled iron magnets. 

 '0.00046 in the case of the seasoned magnets made of " Crescent Steel 



Drill Rod." 

 0.00046 in the case of the seasoned magnets made of Jessop's Round 



Black Tool Steel. 

 0.00070 in the case of the seasoned magnets made of Jessop's Scpuare 



Tool Steel. 



I had bar magnets made of many other materials, for instance of 

 Jessop's and Mushet's self-hardening steels, but none of exactly the di- 

 mensions of the cast iron magnets. No kind of steel that I tested had, 

 however, when proper allowance was made for dimensions, quite so 

 small a temperature coefficient as the chilled iron. 



The mean temperature coefficient of chilled cast iron magnets 18 centi- 

 meters long aud 1.25 centimeters in diameter, as obtained from a number 

 of specimens, was 0.00056, which is very low. 



The forms of some of the magnets which we have used (either singly 

 or with others of the same kind), in various instruments, are shown in the 

 subjoined figure. The shapes marked 1, 2, 3, 6, are employed, with the 

 long way of the opening between the poles vertical, in d'Arsonval gal- 

 vanometers ; two or three castings of the shape marked 4, and a number 

 of thin plates of the shape marked 8, are used together in other instru- 



