102 



Profs. Dewar and Fleming on the 



slide-wire reading the temperature of our working thermo- 

 meter P 2 in terms of our standard platinum thermometer P 1? 

 all corrections for connexions being included. Hence, when 

 the working thermometer P 2 is immersed in any liquid or 

 region of sufficiently constant or slowly varying temperature, 

 and when the position of balance of the contact on the slide- 

 wire is found so that the resistance-bridge is balanced, then 

 we know from this scale at once the platinum tempera- 

 ture of the wire of our working thermometer in terms of 

 our standard platinum thermometer. This bridge and ther- 

 mometer has proved of the very greatest use, since it enables 

 us to measure instantly any temperatures between absolute 

 zero and something approaching to 200° C, which is the 

 highest our working thermometer will bear without injury. 

 As an instance of the degree of accuracy in temperature 

 measurement we can obtain with this instrument, we may 

 mention that in the neighbourhood of — 196 0, 7, which is the 

 platinum temperature of boiling liquid oxygen, we can with 

 ease distinguish a change of temperature of one twenty-fifth 

 of a degree. The same degree of accuracy is not obtained at 

 all temperatures ; but the temperatures between 0° C. and 

 100° C. can be determined to within one tenth of a degree 

 with ease. 



The above arrangements therefore enabled us to instantly 

 determine the temperature of the platinum wire of our 

 working thermometer, and therefore, when the temperature 

 was constant or very slowly varying, to determine the tem- 

 perature of the hollow copper cylinder on which the working 

 platinum wire was wound. 



5. The set of bare junctions of the thermo-couples prepared 

 as described above was then inserted into the hollow interior 

 of the little copper cylinder of the working thermometer, and 

 kept tightly pressed against the copper by a small conical 

 wooden peg. We satisfied ourselves by experiment that 

 under these conditions, if the temperature of the region in 

 which the copper cylinder was placed was constant or very 

 slowly varying, the temperature of the platinum wire wound 

 round the copper cylinder was the same as that of the junctions 

 packed into the interior of the cylinder. This being the 

 case, we had at once the means of determining the tempera- 

 ture of the junctions by means of one measurement, made 

 instantly, of the resistance of the platinum wire. We have 

 next to describe the arrangements for determining the thermo- 

 electromotive force set up in the couples when one set of 

 junctions was kept at 0° 0., and the other varied. 



This was accomplished by the employment of a potentiometer 



