_M Dr. C. Chrec. ////< .>//>/</'' /,,,,.,, OH 



thermometer when the coils and bridge wire are all at temperature T, 

 then, referred to the mean box unit at 20 C., the true resistance is 



by (2) 



P [1 + 0-00026 (T- 20)], 

 and thus the proper temperature correction is 



0-00026 (T - 20)/>. 



Now suppose R the true resistance at t of a platinum thermometer 

 whose true resistance at is RO, and whose fundamental interval is I. 

 Then the platinum temperature is 



pt = 100(Rt-Ro)/I (3), 



and corresponding to any correction (or error) AR,, arising from a 

 cause other than errors in RO or I, we have for the correction (or error) 

 in pt the formula 



Ap/ = AR,(100/I) (4). 



Supposing (2) correct, and the -box at temperature T, we must have in 

 order to balance the bridge 



R, = p [l + 0-00026 (T- 20)] (5). 



If then Ap/ is the correction to pt arising from our application of a 

 temperature correction to the box readings, we have 



Ap/ = (100/I)p x 0-00026 (T - 20) (6), 



where p, like I, is measured in mean box units. 



For one of the ordinary Kew thermometers the following are suffi- 

 ciently near values for purposes of illustration : 



Ice point. Steam point. Sulphur point. 

 P = 258 358 678 



Corresponding to these representative values, with I = 100, we have 



Ice point. Steam point. Sulphur point. 



Apt = 0-067 (T - 20) 0-093 (T- 20) 0-176 (T- 20) 



Using Calendar's formula, 



t-pt = S[(*/100) 2 -*/100] (7), 



we have for the small increment A/ on the air scale corresponding to a 

 small increment A/tf on the platinum scale 



A* = Apt -=- [1-10- 4 S(2<-100)] (8). 



Assuming for the sulphur point on the air scale Callendar and Griffiths' 



