570 



Vvliicl) has been constnioted according to a whollj different principle. 

 In its construction the two conditions — a prompt heat-exchange 

 and a wide surface exposed to radiation — are satisfied, and conse- 

 quently a high degree of both rapidity and sensitiveness has been 

 attained. 



The thermopile is built np of a great number of metal strips. 



Each of these consists partly {nb) of constantan, and partly {be) 



)f copper, and is soldered at both ends on two copper bars (^ and/). 



lids constantan- copper strip can be exposed 

 to the radiation throughout its entire length. 

 Then, on acconnt of the great difference in heat- 

 capacity of the two junctions a and b, bwi\[ 

 attain a higher temperature than a ; the equili- 

 brium of temperature being attained in a very 

 short time, in consequence chiefly of the 

 good heat-conduction between the junctions. 



As copper and constantan differ considerably in heat- conductivity, 

 the two metals which form the strip are taken of different thickness 

 (and length), so as to have the temperature highest at b, while the 

 strip is exposed to radiation. 



Elements, as desci'ibed, may be readily combined to form a pile. 

 Thus I built a surface thermopile of 80 elements, which, being 

 arranged in three rows (of 24, 32, and 24 elements respectively), 

 practically fill a circular surface of 2 cm. in diameter. The total 

 resistance is about 9 ohms. The sensitiveness and the rapidity of this 

 thermopile may be inferred from the follow iug data. The radiation 

 from a standard candle at a distance of 1 metre produces an electro- 

 motive force of 18 microvolts, the current reaching 9970 of its 

 definitive value within IV2 seconds. 



The heat-capacity of the bars e and ƒ is still further increased 

 by clamping them (with the insertion of a thin sheet of mica) to a 

 heavy piece of copper, this piece of copper occupying the space 

 betw^een the bars as far as just below the strip abc. When the 

 pile has been exposed for some time, and therefore has absorbed 

 heat, the temperature of this metal mass will certainly have risen 

 somewhat, but both the junctions will undergo its influence practically 

 to the same degree. The thermopile therefore displays an uncom- 

 monly great zero-stability. 



Similar elements may also be combined so as to form a thermopile 

 for spectral investigation, but, since in this case the exposed surface 

 must be narrow, such an instrument will be less sensitive. 1 have 



