A PLATINUM SURFACE AT HIGH TEMPERATURES. 503 



All electrical measurements have been referred to this standard and to a Clarke's 

 cell (N 5217 A), its value being taken as 1'4331 volts at 16 C. 



Both the current through, and the electromotive force at the terminals of the 

 platinum wire, were measured on a Wolff potentiometer. In none of the coils did 

 the error exceed '02 per cent. The temperature coefficient was less than '001 per 

 cent. 



The radiators used were calibrated at frequent intervals, a slight increase in 

 absolute resistance and in temperature coefficient being noticeable. 



One of these sets of readings is given below : 







Radiator R^. 



R 100 = resistance at 100 C ..... '012252 

 R = resistance at C ...... '009907 



c = ^"^ ........ '00002345 



8= 1-304. 



The value of S is deduced from the resistance of the wire in sulphur vapour by 

 CALLENDAR'S formula, 



where t is the temperature in degrees Centigrade, 



P _ p 



pt = temperature in platinum degrees = - - arid 



c 



S = a constant. 



The platinum thermometer has rarely been used to determine temperatures above 

 1100. It is therefore necessary to obtain some confirmation of the above formula for 

 temperatures up to 1500. For this the melting point of palladium was chosen, and 

 the value given by VIOLLE,* 1500 C., taken as correct. 



A length of about 3 millims. of palladium wire '01 centim. in diameter was 

 held against the platinum radiator. The temperature being gradually increased, the 

 palladium soon became viscous and adhered to the platinum, finally fusing at a very 

 sharply-defined temperature. 



To this method there is an obvious objection. With each fusion there is a certain 

 weight ('0004 gram, or '03 per cent, in this case) of palladium added to the platinum. 

 This would produce an appreciable change in the electrical constants. To obviate 

 this difficulty it is necessary to fuse the palladium on a part of the platinum wire, not 

 between the potential leads, but sufficiently near them to be substantially at the same 

 temperature. 



* ' Comptes Rendus,' vol. 87, p. 983, 1878. 



