The Earl of Rosse on the Construction of Thermopiles. 371 



bismuth and an alloy of twelve parts of bismuth and one part of tin 

 of different thicknesses, of about equal lengths in each case, and sol- 

 dered about | inch apart, upright, on disks of sheet copper of \ inch 

 diameter. A slip of wood was placed between the two bars, to protect 

 them from injury ; and to it they were fixed with thread. The 

 three piles were compared with a pile of four elements, made by 

 Messrs. Elliott ; and, the deviation due to the latter being taken equal 

 to unity, the following deviations were obtained for the three thermo- 

 pairs : — 





Weight of 

 disk face. 



Weight of 

 two bars. 



Deviation. 



Metals employed. 



I. 

 11. 



III. 



8 grains 

 i grain 



42 grains 

 6 „ 



3 „ 



'676 

 1-35 



3-23 



Bismuth, antimony. 



t»- A i f Bismuth, 1 

 Bismuth i .. , ' } 



xj. ,, [Bismuth, 1 



Bismuth < ,. , ' \ 



I tin i l. J 



A heavy and a light pile were also compared, taking the interval 

 between raising and depressing the screen, first =| minute, and 

 then =2 minutes ; and it was found that, in the first case, 



2-6 



Deviation due to light pair 

 Deviation due to heavy pair 



and, in the second case, 



Deviation due to light pair _n.q , 

 Deviation due to heavy pair 



so that the light pair arrived rather more rapidly at the condition of 

 equilibrium than the heavier pair. 



Although the above experiments are far less complete than I 

 could have wished, they are sufficient to show that the sensibility 

 of thermopiles may be considerably increased by diminution of the 

 section of the bars composing them. Whether they may be with ad- 

 vantage reduced to a greater extent than I have already done I cannot 

 say ; but I am inclined to think that they may. I have ascertained 

 from Messrs. Elliott that the alloys used by them in the construction 

 of thermopiles, at the time when I received mine from them, were 

 32 parts of bismuth + 1 part of antimony, and 14| of bismuth + 1 

 part of tin. If allowance be made for the substitution of the first of 

 these two alloys for pure bismuth, the difference between Elliott's 

 pile and the pairs II. & III. will be rather greater. The pile by 

 Messrs. Elliott^ if made of the same metals as I employed, would 

 have been reduced in power from 1 to 0*9. 



The construction of thermo-couples, on the plan I have described, 

 is comparatively easy. In about two hours I was able to make one ; 



