1870.] 



Construction of Thermopiles. 



555 



cient size to catch all the radiant heat required to be measured, with 

 that of a pile of («) pairs, each of equal dimensions with those of the 

 single pair, the area of face being the same in the two cases. 



By increasing the number of elements from one to n, we increase the 

 number of solderings in that proportion ; consequently the average 



amount of heat reaching any soldering is — as great as that reaching the 



soldering of the single pair ; therefore, if the same percentage of the 

 total heat be lost by conduction, the total electromotive force is the same 

 in the two cases. But inasmuch as the total cross-section of metal to 

 conduct the heat away from the anterior face is n times as great in the 

 pile as in the pair, and the resistance of the pile is n times as great as 

 that of the pair, the pile will be inferior in power to the pair, unless 

 these two causes of inferiority are counterbalanced by the loss due to the 

 greater average distance to the soldering from the points where the heat 

 reaches the face, in the case of the pair, than that of the pile of n pairs. 



The experiments already referred to were made with three different 

 thermoelectric pairs. These consisted each of a pair of bars of 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 soldered about J inch 

 apart upright, on disks of sheet copper of j inch diameter. A slip of wood 

 was placed between the two bars, to protect them from injury, and to which 

 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. 

 II. 



III. 



8 grains 

 4| „ 



2 grain 



42 grains 

 6 „ 



3 „ 



•676 

 1-35 



3-23 



Bismuth, antimony. 

 Bismuth { B /? n ™f h *} 



Bismuth {^if'} 



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

 tween raising and depressing the screen, first = J minute, and then =2 

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



Deviation due to light pair 

 Deviation due to heavy pair 



and, in the second case, 



Deviation due to light pair 

 Deviation due to heavy pair 



