EDWARDS. 



NOTES ON PLATINUM THERMOMETRV. 



551 



However, by having the resistance S very nearly equal to T, and by 

 interchanging it and the two connectors between 8 and 10 and between 

 9 and 6 with the connector 

 between o and 7, we nearly 

 neutralize the required 

 change in the balancing 

 point on the bridge wire. 

 The resulting connections 

 are given in Figure 2. 

 (The numbers refer partic- 

 ularly to Figure 3, and will 

 be explained later.) 



It is then obvious that 

 as a potentiometer method, 

 where each element of the 

 wire measures an element 

 of resistance in the opposite 

 side of the bridge, the resistance (d) of the bridge wire between the 

 balancing points multiplied by the ratio of the resistance of the two sides 

 of the bridge gives the difference between T -\- j and S. 



As a Carey-Foster method the results may be worked out as follows : 



Figure 2. 



T + j + D ^ S-^ F-\-2j 

 M+w N+w' 



S+2j-\-D T+F + j 



(Figure 1). 

 (Figure 2). 



M + to — d N -{- w' + d 



(T+j + D) N+ (T+J +D)w' = (S+ F+ 2j) Jf + (S + F+ 2j) w 



{S + 2j -{- D) N + {S + 2j + D) w' + {S+ 2 J + D) d 



= {T+ F+j) J/ 4- (T+ F + J) w - (r-f F+j) d. 



{S-\-j - T) N+ M+ w + w' = -d(S-^ T+ D + F+ 3J). 



'S+ T-]- D -^ F+ 3j^ 



T-{S+j) = d 



N+ 3I-\- w + w 



^•) 



The lead resistances D and F only appear in the ratio by which the 

 difference d is multiplied. If, by the ordinary method of directly meas- 

 uring the lead resistance and subtracting, a given percentage error is 



