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



are maintained constant and balance is always obtained by 

 an adjustment of X. The balancing condition is 



P + L 3 + X = Q(It + L 2 )/S 



(?) 



X and It are now interchanged in position and the 

 current is led into L 4 instead of L x (fig. 7). 



Fig". 



When a balance is obtained we have 



P + L 2 + X' = S(R+L 3 )/Q 



(«) 



X' being the new value of X. If Q/S is nearly unity we 

 have from equations (7) and (8) 



P = R- 



X + X' 



(9) 



Any change in P is therefore balanced by a corresponding- 

 change in the opposite direction in X. If Q differs from S 

 by 2 parts in 10,000 and L 2 and L 3 are about 0*1 ohm in 

 resistance, then the leads of the thermometer will have to 

 vary by 100 per cent, before an error so great as o, 001 C. 

 is introduced. 



Practical Application. — A very useful form of bridge is 

 that shown in fig. 8, in which all the coils are of manganiu 

 and the commutator has mercury contacts. It is convenient 

 to make Q and S of about 190 ohms resistance, and they 

 should be approximately equal. It is a coil of about 10 ohms 

 for temperature measurements from 0° to 700° C. 



The switch C is not an essential part of the apparatus, 

 but it is convenient since it enables the value of It to be 

 obtained in terms of X without disconnecting the platinum 

 thermometer. The reading obtained is the " bridge centre " 



