454 DR. CHARLES H. LEES ON THE EFFECT OF TKMI'KUATtJRE ON THK 



The mean resistance of the coils and leads when at C. was found = 1*3983, and 

 at -200-6 Pt. = '3672. 



Taking the resistance of the leads (about '04 ohm) to vary uniformly throughout 

 this range, we have, if t p is the temperature of the coil, 



-K I"o98o 



T0311 

 r 



ctnn.t' 



200-6, 



1762 



-1-3983 



. 200-6, 



1-0311 







= (r-2-464) 1-105. 



The mean temperature of the material is obtained by subtracting from this half the 

 observed temperature difference A P , and is therefore equal to t p %&tp. 



Measurement of Energy supplied. 



The current (about 1 ampere) supplied to the heating-wire was measured on a 

 Weston ammeter standardised by comparison with a Kelvin balance, whose constant 

 had been checked by copper deposition. 



The difference of potential at the ends of the leads, which consisted of 78 centims. 

 of No. 22 copper-wire, was measured by a Weston voltmeter standardised by 

 comparison with a Clark cell. 



From the two sets of observations the resistance of each coil and its leads, for the 

 following values of the resistance r of one of the platinum spirals, was found to be : 



Twenty-eight centims. of the copper leads were in air at the temperature of the 

 room, and 50 centims. in the Dewar tube, at a mean temperature which may be 

 taken to be 20 C. below that of the hotter platinum spiral. The resistance of the 

 wire in the air = '0121 ohm, that in the tube '0216 ohm, both at 15 C., i.e., when 

 the temperature of the hotter spiral = 35 Pt. scale. 



The resistance of the copper wire in the Dewar tube will become zero when its 

 mean temperature = 237 platinum degrees on DEW AH and FLEMING'S* standard 



* DEWAII and FLEMING, 'Phil. Mug.,' 36, p. 287 (1893). 



