WILLIAM .S7/:.J//-:.Y.V, l-'.K.S. 



'43 



metals. Some platinum wire, drawn for me by Messrs. Johnson 

 and Matthey some years since, gave, when measured, a conducting 

 power only 4-7 times that of mercury. Its increase of resistance 

 was from <r>:> units at 20 C. to 1-12 units at 100 C. ; or 22'4 

 ]>'! .vnt. This platinum had been prepared by fusion in a Deville 

 furnace. Platinum recently supplied to me by the same firm, 

 prepared by the old method of forging, had a conducting power 

 of 8 '2, whilst it increased in resistance from 0'97 units at 20 C. 

 to 1-23 units at 100 C., or 88*5 per cent. This led me to believe 

 that the process by which platinum is prepared has much to do 

 with its behaviour as a conductor, owing probably to a slight 

 admixture of iridium and other metals of that class, in the fused 

 metal ; a supposition which is sufficiently proved by the results 

 tabulated below, and from which it follows, that great caution is 

 necessary in selecting platinum-wire for electrical experiments ; 

 and that the fusion of a wire of a given length and diameter for 

 instance, is by no means a test of the strength of an electrical 

 current. 



The percentage increment of increasing resistance of all these 

 specimens was lower than that of pure silver or copper ; but this 

 is really of little practical importance in view of the second part of 

 this inquiry, provided that its coefficient is known, and that it 

 remains constant. A higher coefficient would be of advantage 

 only in so far as by giving greater differences of resistance for 

 given differences of temperature, the readings with it would be 

 proportionately more delicate. 



In carrying out my experimental inquiry regarding the de- 

 pendence of electrical resistance upon temperature, I employed 



