5 GO Mr. F. E. Smith on Bridge Methods for Resistance 



with great accuracy 



2R = X + X / = a constant* . . . 



(12) 



Now, when the switch is placed in the " in " position we 

 have (see equation (9)) 



2P = 2R-(X" + X"'), .... (13) 



that is, the difference between the mean values of X in the two 

 measurements is the resistance of P, and all subsequent 

 changes in the resistance of the thermometer bulb are equal 

 to the corresponding changes in the mean value of X. 



With this bridge if one accumulator is used for the pro- 

 duction of the current, the current through the platinum 

 thermometer will be about O'Ol ampere, and remains constant 

 at all temperatures owing to the constancy (apart from the 

 small variations in the resistance of the leads) of the arms 

 of the bridge. This has already been shown to be of 

 advantage in precision measurements. 



We have used both Methods 1 and 2 for the measurement 

 of resistance and have found, as will be anticipated, that 

 the measurements are simpler when the leads from P are 

 nearly equal in resistance. When a series of measurements 



* The resistance E, is best chosen so that practically the whole of the 

 resistance X is in series when the switch is in the " out " position. 



