562 Mr. F. E. Smith on Bridge Methods for Resistance 



measurements of the highest precision are being made, but 

 it has been pointed out to us that for many measurements a 

 dial resistance bridge would be exceedingly convenient. 

 Two such resistance bridges are therefore described here. 



In the Third Method, one of the leads of the platinum 

 thermometer is shunted so as to form a resistance system 

 similar to that of the Kelvin double bridge. The ratio of 

 the two shunt resistances (a + L^, and b (fig. 11) is not, 



Fur. 11. 



however, the same as the ratio of (Q + L 3 ) to S, but differs 

 from it by an amount depending on the ratio of R to S. In 

 the figure (11) P is the platinum thermometer and L l5 L 2 , 

 L 3 , and L 4 the leads. When the bridge is balanced the 

 value of P is given by 



p QR.RLs bL 2 /Q + L 3 a + Ln im 



F -~S~ + S + (a + & + L 1 + L 2 )\ S ~~~b~~} ( W) 



In practice b is made equal to S, and a and Q are made 

 large compared with Lj, L 2 , L 3 , and L 4 . If we at first 

 suppose that L! = L 2 — L 3 = L 4 , then we have as a very close 

 approximation 



P = f + L,{ 



QR 



R ft Q a \ 



S S(a + b) (a + b)J 



, T r a(B-B) + a(B + Q) l 



B + 'I S(a + i) " / 



(11) 



Hence if a is equal to b ^ — —■■ we have the simple relat 



(S-R) 



ion 



P = 



QR 



S 



(12) 



