28 Dr. C. Chree. / tions on 



their temperature at different rates, and unless the mercury thermo- 

 meter is specially chosen it may have a greater or a smaller lag than 

 any one of the coils. The mercury thermometer used in the Kew 

 box, like most open-range thermometers, contains a considerable 

 quantity of mercury, and when the temperature changes fast it lags very 

 sensibly behind the coils. For instance, in some special steam point 

 observations with KI, when the recorded box temperature rose 2'5 in 

 twenty minutes, the corrected bridge-wire reading fell about 0'06, 

 instead of remaining practically constant as it ought to have done. 

 The figures really show that the temperature of the box thermometer 

 had lagged about 0*7 C. behind that of the coils in use. This of 

 course was an extreme case, and under ordinary experimental condi- 

 tions the difference between the temperatures of the box thermometer 

 and the coils is hardly likely to exceed 0'3, and is probably seldom 

 half this. Necessarily, however, a good deal depends on the depth of 

 the bulb of the thermometer inside the coil chamber, and any alteration 

 of that depth should be carefully avoided. 



26. The temperature of the bridge wire is open to much greater 

 uncertainty owing to its relatively exposed position. The whole bridge 

 wire resistance in the Kew box is fully 30 box units, and if this 

 were all on the platinum thermometer side of the Wheatstone bridge 

 the error arising from the assumption that it possesses the temperature 

 of the box thermometer would be approximately O'OOSr, where T is the 

 error, in degrees Centigrade, in the temperature assigned. 



As T may easily amount to several degrees, an error of several 

 hundredths of a degree might easily creep in from this cause if one 

 used a large fraction of the bridge wire. Theoretically it is possible 

 to keep the bridge-wire reading within 5 units of the bridge centre,* 

 thus reducing the uncertainty to less than one-sixth of that existing 

 in the extreme case supposed. This, however, has not been compatible 

 with our practice of employing the same coil combinations for all the 

 thermometers of the same pattern. 



Even with the drawback of temperature uncertainty it is often 

 advantageous to make a liberal use of the bridge wire. It facilitates 

 finding the balance, and in work such as the comparison of mercury 

 thermometers at high temperatures, where rapid reading is essential, it 

 is a great convenience. 



It is thus highly desirable that the bridge wire should be better 

 protected than in the Kew box, and that its temperature should be 

 measured directly. 



Insufficient Immersion. 



27. Supposing a correct mercury thermometer to be immersed in 

 ice up to the reading - 10 C. on the stem, there is a column 

 * This was the policy recommended by Mr. Griffiths originally. 



