258 



Professor Fleming 



[June 6, 



against the known fall of potential down a certain length of a gra- 

 duated wire, and a galvanometer employed to ascertain the point on 

 the slide wire at which this is the case. Omitting details, it may be 

 stated that I succeeded in devising an arrangement of circuits in which 

 this change from a potentiometer to a resistance bridge was e£fected by 

 moving two brass plugs from one pair of holes to another. This in- 

 strument formed a most useful combined resistance and electromotive 

 force measurer which enabled us to do two things — first, to measure 

 the electromotive force in any thermo couple ; secondly, to measure 

 the temperature of the low temperature junction by measuring the 

 resistance of a platinum wire wound round that junction and acting 

 as a thermometer. In actual practice the platinum thermometer 

 consisted of a small hollow copper cylinder, in the interior of this 

 cylinder being inserted a number of the thermo junctions, and round 

 the outside of which the platinum thermometer wire was wound. Aided 



-200 -ISO -160 -wo -120 -100 -80 -BO -40 -20 O 20 40 50 



Fig. 12. 



Curve of thermo-electromotive force of a platinum-lead couple at various tem- 

 peratures ; one junction kept at 0° C, the temperature of the other being varied. 

 The sloping dotted line represents the variation of the thermo-electric power of 

 platinum with respect to lead. 



by this device we were able to measure temperatures with an accuracy 

 of ^1^ of a degree at a temperature of —200° C, and to ascertain at 

 the same instant the exact electromotive force acting in the couple. 

 When these arrangements had been perfected the method adopted 

 was to put one set of the junctions in melting ice. The other set, 

 enclosed in the copper cylinder, were imbedded in a mass of paraffin 

 wax, which was then cooled down to the temperature of liquid air. 

 The mass was then removed and inserted in a vacuum vessel, and 

 allowed to heat up very slowly. At frequent intervals during the 

 heating the electromotive force of the couple was taken, and also the 

 temperature of the junction.* 



The events which under such conditions happen in the case of a 

 platinum-lead junction can easily be shown and are very interesting 

 (see Fig. 12). At the first immersion of one junction in liquid air, 

 whilst the other is in melting ice, we get a current as shown by the 



* For fuller information see Dewar and Fleming on the ' Thermo-Electric 

 Powers of Metals and Alloys,' ' Philosophical Magazine,' July 1895. 



