Reading Resistance- Thermometers 



717 



Here the main resistance R is shunted by s x -f s. 2 , of which 

 \ is invariable and kept cold, while a small part s 2 is o£ the 



Fig-. 2. 

 ^AAAA/Wv^A/VVvVWVV- 



-AAA/WWVY 

 Iron 



same metal as R and at the same temperature. By suitable 

 choice of s^ and s 2 the resistance from A to B can be made 

 very nearly of the form R (l + »rt). 



To take an actual example of Method (A), — a certain 

 platinum resistance-thermometer follows approximately the 

 formula 



., ,n,aon„ «^^^« . (5) 



R, 



= 1 + O0039466 -O0000005798* 2 . 



Fig. 3 shows the values of the resistances necessary to give 

 direct readings for this instrument. 



Fijr. 3. 



It will be seen that the shunt is here equal to 25# > while 

 the approximate formula gives 27 , 86 < .r . 



I need not discuss the first method further, but will pass 

 now to the second, which is much more exact. 



Phil. Mag. S. 6. Vol. 9 Mo. 53. May 1905. 3 B 



