1888.] the Thermo-electric Properties of Bodies. 



227 



time ; after which they require to be brought back to their original 

 temperature. • 



Lastly, to avoid any possible effect from the coal-gas, the end to be 

 heated was inclosed in an iron tube lined with asbestos. 



The temperatures were measured in various ways. In some 

 experiments an ordinary thermometer was used for temperatures 

 below 250° C. ; thermo-electric couples of platinum and copper, silver 

 and copper, were tried, but, although much more tedious, I found the 

 method of platinum wire much less liable to error. 



The wire was given to me by Mr. H. F. Callendar, M.A., and was 

 from the same piece as that used by him in his experiments on " The 

 Practical Measurement of Temperature " (see ' Phil. Trans.,' vol. 178 

 (1887), p. 161). 



The following equations for this wire were used in determining the 

 temperature, and are those obtained by Mr. Callendar in his experi- 

 ments : — 



5^ = 1 + 0-00346 Pt°. 

 ~Rt — resistance of the platinum wire at t° C. 



R°= „ „ „ o°c. 



The wire was arranged as in fig. 5, by which means the resistance of 



Fig. 5. 



A ^ 









C 









jfi ^ — 





B 









7) 



EF alone could be obtained by observing those of AC, BD, CD, and 

 AB ; also AB and CD were known if required, which indeed was the 

 case of one of the later experiments. 



In some cases the insulation was thin tubes of hard glass, in others 

 the wire was wrapped up in thin sheet asbestos. The arrangement 

 is shown in figs. 6 and 6a, where A and B are the carbon rods, C an 



Fig. 6. 



C 



iron tube lined with sheet asbestos, H, H packing of asbestos, D a 

 thermometer for moderate temperature and to test the calculations 



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