Measurements of Precision in Platinum Thermometry. 551 



so as to produce a small increase of temperature and the 

 temperature of the wire has become steady, the increase of 

 resistance per ohm of the wire is given by 



0-038 i 2 pajhr\ 



where p is the specific resistance of the conductor, a the 

 temperature coefficient of resistance, h the thermal emissivity 

 of the surface, and r the radius. If we at first choose two 

 wires of different materials but of equal radii, then since h 

 is practically the same for both wires, we have for the same 

 current 



Increase of Resistance per Ohm cc pa. 



It is probable that manganin has the smallest value of pa. 

 of any well-known conductor. The value varies consider- 

 ably, but taking a mean value as unity, the values for certain 

 metals are approximately as follows : — 



Silver 10 



Copper 10 



Platinum 80 



Mercury ......... 800 



Thus the current which causes a manganin wire to increase 

 in resistance by 1 part in 100,000 will bring about an 

 increase in the resistance of a wire of platinum of equal 

 radius of about 80 parts in 100,000. This is on the sup- 

 position that the cooling media are similar. If, however, 

 the manganin is in well-stirred oil and the platinum wire is 

 in air, the rate of cooling in the first case may be from 20 to 

 100 times that in the latter, and the percentage increase of 

 resistance of the platinum wire may be 1600 times or more 

 that of the manganin. It is clear that if the diameter of the 

 platinum is less than that of the manganin, the percentage 

 increase of resistance will be still further increased *, 



When the manganin wire is covered with silk and 

 shellack, and wound so as to form the usual kind of 

 resistance-coil, the effective cooling surface is, of course, 

 appreciably reduced, but it will readily be seen that com- 

 paratively high resistance-coils of manganin of fine wire, 

 immersed in oil, may be placed in series with the usual low 

 resistance B.A. form of platinum thermometer or equivalent 



* In a liquid cooling agent, possibly also in air, h varies as 9~^ 2 , 

 202 



