of Electricity through Metals. 



equation (3) oven for very low temperatures, so that 

 temperatures 



(w+ D— Wq) 



201 

 at all 



/■ 



a = 



cpd w?o 



The temperature coefficient of the resistance is proportional 

 to do-ldO, and this as we see is proportional to awjdd. This 

 quantity, the rate of increase or the energy with the tempe- 

 rature, is proportional fco the specific heat at the temperature. 

 As the specific heats of many substances are very much 

 smaller at the low temperatures obtained by the use of liquid 

 hydrogen or helium, than ai normal temperatures, we see 

 that on this (heory the temperature coefficients of metals 

 which have no critical temperature ought to be very small 

 at low temperatures. The experiments of Kamerlingh 

 OnneSj and Dewar and Fleming, -how that this is in sonic 

 cases ;i very well marked ('fleet. Fi^. \ shows the variation 



I . ■. I. 









A 



' 





Pt^ 



Au^ 







1 — — 



10 



15 



20 



of resistance of gold and platinum suspected of not being 

 quite pure : it, will be noticed that at very low temperatures 

 the resistance becomes almost independent of the tempera- 

 ture. Similar effects are shown by many alloys; theywould 

 on this theory be shown at low temperatures by any metal 

 or mixture which bad not too Bmall a value of I) and whose 

 specific heat Eell appreciably at low temperatures. 



Intact the general behaviour of alloys seems to admit of 

 q satisfactory explanation on the supposition thai in them, or 

 at any rate in those whose resistance is considerably greater 

 than the value calculated from their percentage composition, 

 the restoring couple D is much greater than in pure metals. 

 This seems whal we might expect when the alloy is not a 

 mere mixture; for it' it was a definite compound of the two 

 metals, we should expeci that there would be a tendency for 

 the axes of the molecules of one metal to have definite 

 orientation with reference to those of the molecules of the 

 other. The same thing would also apply if the metal- did 



