1896.] 



on Electric Research at Low Temperatures. 



257 



electricity which have engaged our attention. If we construct a 

 thermo-electric couple of two metals and connect this with a 

 galvanometer, and if one junction is kept at a constant temperature, 

 say 0° C, whilst the other junction is heated or cooled to various 

 temperatures, we shall in general, but not always, find an electro- 

 motive force acting in this circuit when the junctions are at different 

 temperatures. This electromotive force depends on three things — 

 the nature of the metals, the temperatures of the junctions, and on 

 a certain temperature called the neutral temperature of the metals. An 

 important matter in the experimental study of thermo-electric action 

 is to discover the position of these neutral temperatures, when different 

 metals are tested with lead as the standard of comparison, and when 

 one junction is kept at 0° C. Elaborate experiments made by 

 Professor Tait many years ago furnished full information on this 

 matter for temperatures lying above 0° C, and we especially desired 

 to extend this knowledge to ranges of temperature between 0°,O. and 

 — 200° C. Accordingly, a number of thermo-electric junctions were 

 prepared of various pure metals and alloys, the comparison metal 



Fig. 11. 

 Potentiometer arrangement for measuring thermo-electromotive forces. 



being always pure lead. These couples were grouped together, and 

 one set of junctions always kept at 0° C. in melting ice. The other 

 set of junctions was cooled to various low temperatures by means of 

 liquid air. The experimental process then consisted in measuring 

 the electromotive force set up in each couple respectively, and at 

 the same instant measuring the temperature of the low temperature 

 junction. After various failures a device was adopted for making this 

 double measurement with great accuracy and expedition. 



The arrangement consisted of a combined potentiometer and re- 

 sistance balance (see Fig. 11). A long uniform wire stretched over 

 a scale had a battery connected to its two ends so as to make a fall of 

 potential down the wire which could be regulated by appropriate 

 resistances. It will be easily seen that we can combine a galvano- 

 meter and resistance coil with this arrangement in such a manner as 

 to form it into a Wheatstone's bridge or a potentiometer. In this 

 latter form of instrument an unknown electromotive force is balanced 



Vol. XV. (No. 90.) s 



