10 Dr. J. Hopkinson. [May 1, 



observations of resistance, however, are indicated by the crosses in 

 the neighbourhood of the letter A on the curve. The wire was then 

 allowed to return to the temperature of the room, and was subse- 

 quently heated, the actual observations being shown by crosses on. 

 the lower branches of the curve, the heating was continued to a 

 temperature of 680 C., and the metal was then allowed to cool, the 

 actual observations being still shown by crosses. From this curve it 

 will be seen that in the two states of the metal (magnetisable and 

 non-magnetisable) the resistances at ordinary temperatures are quite 

 different. The specific resistance in the magnetisable condition is 

 about 0'000052 ; in the non-magnetisable condition it is about 

 0'000072. The curve of resistance in terms of the temperature of the 

 material in the magnetisable condition has a close resemblance to that 

 of soft iron, excepting that the coefficient of variation is much, 

 smaller, as, indeed, one would expect in the case of an alloy ; at 20 C. 

 the coefficient is about O00132 ; just below 600 C. it is about 0040, 

 and above 600 C. it has fallen to a value less than that which it had 

 at 20 C. The change in electrical resistance effected by cooling is 

 almost as remarkable as the change in the magnetic properties. 



Samples of the wire were next tested in Professor Kennedy's 

 laboratory for mechanical strength. Five samples of the wire were 

 taken which had been heated and were in the non-magnetisable state, 

 and five which had been cooled and were in the magnetisable state. 

 There was a marked difference in the hardness of these two samples ; 

 the non-magnetisable was extremely soft, and the magnetisable 

 tolerably hard. Of the five non-magnetisable samples the. highest 

 breaking stress was 50'52 tons per square inch, the lowest 48*75 ; the 

 greatest extension was 33 per cent., the lowest 30 per cent. Of the 

 magnetisable samples, the highest breaking stress was 88 12 tons per 

 square inch, the lowest 85' 76 ; the highest extension was 8'33, the 

 lowest 6'70. The broken fragments, both of the wire which had 

 originally been magnetisable and that which had been non-magnetis- 

 able, were now found to be magnetisable. If this material could be 

 produced at a lower cost, these facts would have a very important 

 bearing. *As a mild steel, the non-magnetisable material is very fine, 

 having so high a breaking stress for so great an elongation at 

 rupture. Suppose it were used for any purpose for which a mild 

 steel is suitable on account of this considerable elongation at rupture, 

 if exposed to a sharp frost its properties would be completely changed 

 it would become essentially a hard steel, and it would remain a 

 hard steel until it had actually been heated to a temperature of 

 600 1 C. 



F. This sample contains 30 per cent, of nickel. Curve 12 shows 

 the relation of induction to magnetising force at the ordinary tem- 

 perature, and Curve 13 the relation of induction and temperature for 



