54 MODERN SCIENCE OF METALS 



current thus produced, but the resulting electric 

 " thermo-piles " never attained much importance. 

 In another direction, however, great success was 

 achieved by utilising this property of junctions of 

 dissimilar metals for the construction of sensitive 

 instruments for detecting slight changes of tempera- 

 ture. Further study of the subject among others 

 by P. G. Tait in Edinburgh led finally to the 

 invention by Henri Le Chatelier of the thermo- 

 electric pyrometer for measuring high tempera- 

 tures. Le Chatelier introduced the use of a thermo- 

 couple or pair of wires consisting of pure platinum 

 and of platinum alloyed with ten per cent, of 

 either rhodium or iridium. The junctions of these 

 wires can, with suitable precautions, be exposed to 

 temperatures as high as 1600 C. (an intense white 

 heat) without injury or excessively rapid deteriora- 

 tion, such as would occur with metals like copper 

 or iron which would either melt or rapidly burn 

 away. Le Chatelier's thermo-couple can therefore 

 be used for the accurate measurement of high 

 temperatures. At first sight it appeared that the 

 principal use of such an instrument would lie in 

 its direct application to industrial furnace practice, 

 where a ready means of accurately measuring high 

 temperatures would furnish a new method of 

 regulating and controlling those operations. To 

 some extent this has been the case, but the cost 

 and fragility of the instruments required have 

 acted as a serious handicap for this application 



