12 HEAT. 



are equal, the E.M.F. is the same as if the circuit consisted of A and B 

 only. The thermo-electric thermometer is made in many forms, and 

 with many pairs of metals, according to the purpose for which it is used. 

 It will be sufficient here to describe one form, devised by Le Chatelier, 

 and used by Roberts-Austen for the determination of certain high 

 temperatures (Nature, xlv.. 1891-2, p. 534). 



The active metais in this form are platinum, and an alloy of platinum 

 with 10 per cent, of rhodium. The junction to be inserted in the vessel 

 of which the high temperature is to be measured, consists of a platinum 

 wire round which the platinum-rhodium wire is twisted. The two wires 

 are brought out of the vessel and connected up to a D'Arsonval galvano- 

 meter, the junctions with the galvanometer being kept at the same lower 

 temperature, that of the room. The E.M.F. drives a current deflecting 

 the galvanometer, by an amount depending on the difference of tempera- 

 tures. The instrument is calibrated by inserting the testing junction 

 into vessels of known temperatures, containing in succession, say, boiling 

 water 100 C., melting lead 326 C., and boiling zinc 940, the deflection 

 of the galvanometer being observed for each of these, and other tempera- 

 tures being determined by interpolation. 



Some Important Temperatures. The following table gives a few 



important temperatures determined in various ways. They are put here 

 merely to enable the student to realise the range of measurement possible 

 with the instruments and methods now available : 



TABLE OF TEMPERATURES. 



(Chiefly from " Travers' Study of Gases," and from Callendar, 

 Phil. Mag., xlviii., 1899, p. 519.) 



