independent of Changes of Temperature. 



215 



size tested at the Kew Observatory is given by Whipple as O00029.* 

 The change of intensity with temperature is almost strictly linear in 

 these cast-iron magnets, and they are very constant when subjected 

 to blows and shocks. 



Pianoforte Wire. — Lengths of 12 cm. each were cut from a coil of 

 wire, and tested after various treatments. Magnetised in the normal 

 state this material unexpectedly gave a negative coefficient. When 

 heated to bright redness and chilled rapidly or slowly the coefficient 

 became positive. 



As it was thus possible to change the sign of the coefficient, an 

 attempt was made to find the particular temper which would give a 

 zero coefficient. Lengths of the wire were heated severally in oil to 

 200° and 260°, and in air to a temperature producing a film of oxide, 

 and rapidly chilled in water. The coefficient still remained negative, 

 and of nearly the same magnitude. But when heated to dull redness 

 and quenched, the coefficient was very nearly reduced to zero. 

 Heated to higher temperatures and quenched, the coefficient became 

 positive. 



Table II. 



No. 



Condition. 



H = 2lld. 



li. 



I/- 





oc -00. 



16a 





109 



649 -0 



644-6 







•008 



-0-023 





Tempered at 260°. . 





792-1 



769-2 







•029 



-0-018 





Ditto dull red 





883 -0 



863-6 







•022 



-0-002 









892-0 



869-0 







•026 



+ -003 









559-5 



537 -1 







•040 



+ -008 









849-0 



830 -1 







023 



+ '006 



165 





100 



679-0 



633-6 







•067 



-0-055 









593 -0 



497-0 







•163 



-0-017 



Length of each piece, 12 cm. ; weight, about 09 gram; diameterj 16a = - ll cm., 

 16J = - 12 cm. These two specimens are made from different kinds of steel. 



It is a curious coincidence that the intensity of magnetisation 

 attains a maximum for the condition producing minimum tempera- 

 ture coefficient, and this maximum has the exceptionally high value 

 of 892 C.G.S. units. 



The fact that the negative coefficient could not be reproduced if 

 once the wire had been heated above a red-heat indicates that there 

 is some structure physically imposed upon music wire, perhaps in 

 the process of drawing, which partly or wholly contributes in pro- 

 ducing the negative coefficient. Whereas the negative coefficient in 

 the nickel steel is reproducible, and is doubtless a consequence of 



* Whipple, < Koy. Soc. Proc.,' vol. 26, p. 218. 



