718 Mr. A. Campbell on Direct 



(B) The Rectifying Loop Method. 



The arrangement of the measuring arm in this method is 

 shown in fig. 4, in which a contact F can be moved along a 

 loop of fixed resistance (n). (The resistance of the contact 

 is assumed small and constant.) 



Fm. 4. 



B 

 i/ 



As before, let us assume the parabolic formula 



R*=R (1 +**-/3**), 



and let us choose R = l, 



makin g Ri=l + at-p. 



In fig. 4 let x+y= constant =n, and let resistance AB be 

 proportional to R^ when a balance (in the bridge) is obtained. 



TheA m- *■« = «("-*) 



x-\-y n 



n 



Now suppose that x is altered in proportion to the rise of 

 temperature of R, 



i. e. x=x (l + mi). 



Let x =l, and therefore x=l+mt 

 Then 



n n 



° r *Cl + «*-^)=(l-J)+m(l-?)«-^. 



If this is true for all values of *, we have 

 7 _ -. 1 n — 1 



n n 



— 9: 



(6) 



a = ,n G£i) ; w 



* = Z=T (8) 



