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



First Method. 

 The resistance scheme is that of a simple Wheatstone 

 bridge, as will be seen from the connexions shown in fig. 2. 



Fig. 2. 



P is the platinum thermometer with current leads L x and 

 L 4 and potential leads L 2 and L 3 . Q and S are the ratio 

 arnis and R is the adjustable resistance. There will be no 

 current through the galvanometer when 



P + L 3 ^Q(R + L 2 )/S (1) 



When this balance has been made the potential lead L 2 is 

 disconnected from R and joined to Q ; L 3 is joined to R and 

 the battery lead is disconnected from L : aud joined to L 4 . 

 The new balancing condition is 



P + L 3 = Q(R'4L 3 )/S, .... (2) 



R f being the new value of R. From (I) and (2) we have 



2P S =Q(R + ia / )/S + (L s + L 8 )(Q/S-l) . . (3) 



If Q/S = l, then P = (R. + R/)/2. 



However, it is not Well to impose on abridge the condition 

 that the ratio coils shall be exactly equal. Tf we suppose 

 that Q/S is equal to (1 + a) where u is small, a slightly 

 different procedure must be adopted. At the same time as 

 the leads L : and L 4 are reversed in position, the arms P and 

 R are interchanged * (fig, 3). 



Fig, S, 



Li 



* If P and R are interchanged in position instead of Q and S, any 

 current through the galvanometer due to a thermal E.M.F. in V is 

 unchanged in direction. 



