714 



Then 



Mr. A. Campbell on Direct 

 R t =l+ut-j3t 2 . 



Fieri. 



Now in fig. 1 let the Wheatstone's bridge shown have, at 

 0° C, four equal arms, of which BC is the platinum thermo- 

 meter, and AB consists of a variable resistance x shunted by 

 a fixed resistance s. 



If the temperature of H t is t, and x is altered until a 

 balance is obtained, then 



sx 



Therefore 



>R* 



s + x 



s(l + cd-j3t 2 ) 



R t =l + at-j3t 2 . 



s — R t s — l — at + ftr* 



Let 

 Then 



s-1 



=g 



l + cct-/3t 2 

 = sq[l + (l + q)at+ (1 + q) {qcc 2 -/3)t 2 



+ cq(l + q)(a 2 q^2 l S)t s -{-...] . (1) 



