340 Effect of small Variations of Temperature on Steel Magnets. 



If F = the decrease in magnetic power for 1° C. expressed as 

 a decimal of the magnet power at 20° C, 



G = the specific gravity at 20° C, 



e = the decrease of specific gravity for 1° C. expressed as 

 a decimal of specific gravity at 20° C, 



P = the number of parts of impurity in 100,000 parts of 

 the steel (all that is not iron being called impurity), 



FG 3 

 then — - = a constant, and the constant is 

 e¥' 7 



4-714. 



It may be worth while to mention that 



P7~ P*/P &C ' 



Regarding it in this form may throw some light on its mode 

 of action. 



We particularly wish to make the reservation that the law 

 holds only for homogeneous bars of the same hardness. 



We hardened our bars by plunging them into water when 

 they were at a red heat, just visible in daylight in the darkest 

 corner of a room. When tempered there is a slight difference, 

 though we cannot say in which direction. When hardened at a 

 full red or approaching a white heat, the magnet power increases 

 and the value of F diminishes considerably. 



We hope to make some measurements of the effects of hard- 

 ness on the magnetic power and the value of F the subject of a 

 future paper. 



When the effect of hardness on the value of F is determined, 

 our constant, which we venture to name the " Thermo -magnetic 

 constant of steel" will merely have to be multiplied by a function 

 of the hardness to make it of universal application. 



Almost all the quantities concerned are absolute, the only 

 measures referred to being the Centigrade scale, and the assump- 

 tion that the specific gravity of water at 4° C. = 1*000. 



We are only beginners in scientific work, and by no means 

 wish our measurements to be received as absolutely correct ; 

 but we think we have shown such a tendency towards a law as 

 may induce some older experimenter to make some determina- 

 tions of the constant with a greater degree of accuracy than we 

 have been able to attain. 



August 21, 1871. 



