24& Professor Fleming [June 5, 



copper, also higher up. When two pure metals are alloyed together 

 in various proportions there is generally some proportion in which 

 the resultant alloy has a maximum resistivity, and except in the case 

 of alloys of zinc, tin, lead and cadmium with each other, the resistivity 

 of the alloy is greater than that of either of its constituent metals. 

 In the case of many well-known alloys the proportions which give 

 high, if not the highest resistivity are those which correspond to 

 definite and possible chemical combinations of the metals with each 

 other, as, for instance, in the well-known platinum-silver alloy in 

 proportion 33 to 66, which corresponds in proportion with the com- 

 bination PtAg4 ; the iron-nickel alloy in proportion of 80 to 20, which 

 corresponds with the combination NiFe4 ; the platinum-iridium alloy 

 80 to 20, which corresponds with the combination IrPt4 ; and the 

 copper-manganese alloy 70 to 30, which corresponds with the com- 

 pound Cu^Mn; all of which are, as far as valency is concerned, 

 possible compounds. It is, however, found that very high resistivity 

 generally involves in alloys a want of tenacity and ductility, and 

 when we reach such limits as 100 microhms per cubic centimetre we 

 begin to find the solid alloys becoming less useful on account of this 

 deterioration of their useful mechanical quality. 



We have especially studied the electrical resistance at low tem- 

 peratures of a large series of steel alloys containing varying propor- 

 tions of nickel, aluminium, chromium, tungsten and manganese in 

 them. 



We have found that the electrical effect of adding to the iron the 

 other elements of the alloy is usually to shift up the resistance line 

 nearly parallel to itself, so that the resistance lines of all the iron 

 alloys are nearly parallel to that of the iron line, only the absolute 

 value of all the ordinates is increased. This is equivalent to saying 

 that the effect of the added material is to increase the specific resist- 

 ance, but not to alter the slope or form of the resistance curve. 

 Amongst these steel alloys there are two or three that are very inter- 

 esting. A nickel-steel alloy containing 19 per cent, of nickel, sent 

 to us by Mr. R. A. Hadfield, exhibits some very extraordinary proper- 

 ties. Nickel-steel alloys with large percentages of nickel can, as Dr. 

 Hopkinson has shown,* exist over wide limits of temperature in two 

 different physical states, in one of which they are strongly magnetic 

 and in the other of which they are feebly magnetic, and they 

 pass from the non-magnetic to the magnetic on cooling to low 

 temperatures. Here, for instance, is a sample of the 19 per cent, 

 nickel-steel in the non-magnetic condition. If it is cooled in liquid 

 air we can make it pass instantly into a magnetic condition. In the 

 first state it is fairly ductile and plastic, but in the second state it is 

 very hard and brittle. Moreover, its electrical resistance and thermo- 

 electric power are both permanently altered on undergoing this 

 change. In the non-magnetic state it has a high resistivity of about 



* See Proc. Roy. Soc. 1890, vol. xlvii. p. 138. 



