V. TEMPERATURE DETERMINATIONS 



159 



The Wheatstone bridge circuit {25) — as used with the four lead 

 platinum resistance thermometer — is shown in Figure 12. In this 

 circuit : 



R1/R2 = Rz/R. 



(6) 



Two observations are required with this circuit to measure the resist- 

 ance of the terminal element. R\ and R2 are the resistances of the 

 equal ratio arms. 7?3 is the resistance required to balance the bridge. 

 Rx is the resistance between the branch points of the thermometer, 

 the four leads of which are designated as A, B, C, and D. By means 

 of the slide wire the ratio of the arms containing Rz and 7?^ may be 

 adjusted to equality within one or two parts in ten million. Once the 



Fig. 12. Bridge circuit for four lead, 

 precision resistance thermometer. 



bridge has been balanced, with the battery connected to thermometer 

 lead C, a commutator with amalgamated contacts is used to switch 

 the battery to lead B. The positions of thermometer leads A and D 

 are interchanged, and the bridge is again balanced. Since it can be 

 shown that: 



R,= {D + A)/2 



(7) 



the reversal of the leads wall cancel out any effects of resistance in the 

 leads and the true resistance of the thermometer will be obtained. 

 A small variable resistor is sometimes placed in one of the thermom- 

 eter leads to equalize the lead resistance in the two positions of the 

 commutator. Unless it is desirable to determine temperature dif- 



