Measurements of Precision in Platinum Thermometry. 565 



of connexions. Fig. 13 shows the usual scheme with P con- 

 nected to the bridge. 



With the connexions as in fig. 14 the condition for balance 

 is that a shall equal 6(Q -f 1)/(S — 1). Q (and of course a) 

 may now be rapidly varied from 200 ohms upwards by 

 operating the dials, and if the bridge remains balanced we 

 know that the condition of equality is satisfied for all possible 

 measurements with the bridge. In practice this balance 

 should remain gnod within 1 part in 10,000 for measure- 

 ments within 0°*001 C. Clearly the coils of the first two 

 dials should be carefully adjusted. 



To standardize the bridge, standard resistance-coils with 

 current and potential leads are substituted for the platinum 

 thermometer, the connexions being identical with those 

 when a measurement of temperature is being made. It is 

 of interest to note that it is the ratio QR/S which is 

 standardized and not Q. 



To calibrate the bridge an adjustable resistance is sub- 

 stituted for the thermometer and a method similar to that 

 employed for the Callendar-Grimths bridge is used. 



Phil Mag. S. 6. Vol. 21. iS T o. 142. Oct. 1912. 2 P 



