1896. 



on Electric Besearch at Low Temperatures. 



259 



galvanometer in one direction. On lifting one junction out of the 

 liquid air it begins to warm up. The first effect of this heating is 

 to reduce the thermo current in the circuit. At about — 111° on our 

 platinum scale, some distance therefore above that of liquid air, the 

 current in the circuit falls to zero. As the junction continues to heat 



Temperature. 



-200 



up, the current increases again in 

 the opposite direction. At about 

 — 60" the low temperature junc- 

 tion reaches the temperature 

 called the neutral temperature, 

 and the current is a maximum 

 in one direction. It then begins 

 to fall off once more, and finally 

 becomes zero again when the two 

 junctions are both at the tem- 

 perature of melting ice, and it 

 lastly increases in the opposite 

 direction as this variable junction 

 continues to warm up from 0° C. 

 to higher temperatures. 



Having carried out the obser- 

 vations described with all our 

 thermo couples, the results were 

 represented on a chart (see Fig. 

 13) as follows: — A horizontal 

 line was taken on which were 

 marked off divisions representing 

 platinum temperatures. Vertical 

 ordinates were then drawn at 

 various temperatures for each 

 couple, representing the electro- 

 motive force in this couple when 

 the cold junction was at the tem- 

 perature denoted by the abscissa 

 of that ordinate. In this way a 

 series of curves were delineated 

 which all passed through the 

 point representing 0° C. These 

 curves are the curves of thermo- 

 electromotive force. 



In Professor Tait's researches 

 on this subject he adopted a 

 method of representing the facts 

 which has many advantages. Suppose the couple to have one 

 junction at a constant temperature and the other to be vary- 

 ing. At any instant the electromotive force of the couple is 

 varying at a certain rate with the changing temperature of the 

 non-constant junction. This rate measures what is called the 



s 2 



Fig. 14. 



Curves showing the variation with tem- 

 perature of the thermo-electric power 

 of various metals. The thermo-elec- 

 tric lino of lead being represented 

 by the dotted line. 



