Measurements of Precision in Platinum Thermometry. 507 



The thermometer has four leads L l5 L 2 , L 3 , and h±. L 2 is 



shunted by b-\-a + ~Li and L 3 is shunted by /3-{-a-fL 4 . 



Fig. 15. 



When the connexions are as shown in the figure the 



condition for balance is 



O + LJL3 , (a-fTj^Lg 



P + 



+ 



a4-/5 4-L 3 +L 4 a + M-Lj + L 



R + 



bL, 



+ b + L l + L, 



Q + 



/3L 3 



s 



a + $ +- L 3 + L 4 

 and the expression for P is 



P = 



OR 



+ 



PU 





/r + 



6L« 



a 4- 6 + Lj -t- 1^ 2 



S 



a4-L4 



is 



) 



a + b 



t>Lt 2 ( (j « 4- LA 



(16) 



In practice the second and third terms on the right hand 

 side of equation (16) are made negligibly small and then 



P = QR/S (17) 



In setting up the bridge the resistances are so chosen that 



approximately R/S = «//& and Jr =*= r. In the general case 



the bridge is set before commencing measurements of tem- 

 perature. To do this a is disconnected from L 4 , and by 

 adjustment of Q and a a balance is obtained whether L 2 is 

 connected to lib or not. The following two equations then 

 hold good : — 



P + L s + a + L { R + 6 



Q 



B 



(18) 



2 P 2 



(15) 



