126 PROCEEDINGS OF THE AMERICAN ACADEMY 



Impressed with the number of the difficulties which beset the use of 

 the thermopile, the writer set about a systematic study of the condi- 

 tions necessary to their removal. It was soon observed that a great 

 advantage could be gained by employing a single thermal junction, 

 instead of many, and by condensing the heat rays upon this one by 

 means of a concave mirror. All of the advantages of the thermopile 

 are thus retained, while its most serious failing — the slowness with 

 which the large mass of metal presenting the sensitive surface parts 

 with its heat — is obviated altogether. The condensing mirror fulfils 

 the function of multiplied junctions, while the single union of bars 

 employed in the new instrument rapidly attains thermal equilibrium 

 under any series of conditions. 



This preliminary proposition having been laid down, — that a single 

 junction of two metals, suitably supplemented by a concave mirror, 

 can be made as effective as a larger number of junctions without the 

 mirror, — various experiments were next undertaken with a view to 

 ascertaining the best form for the single junction, as well as the most 

 suitable materials to enter into it. With these ends in view a great 

 number of unions were made, including some very delicate combina- 

 tions into which tellurium was introduced. The bars of metal used in 

 these experiments were made very thin, and in attempting to form 

 certain junctions great difficulty was experienced ; for when very thin 

 bars of easily fusible metals are to be soldered together, it is some- 

 times almost impossible to perform the act properly, without melting 

 the bars. In such cases the electro-deposition of silver or copper in 

 the junction succeeds admirably. 



As the outcome of these experiments it was learned that the best 

 results could be obtained from a junction formed by two of the stronger 

 metals, worked into very thin strips, the opposite ends of the metal 

 ribbon thus formed being attached to copper rods — the terminals — 

 with the point of junction midway between. The reasons for such a 

 construction and the advantages especially to be derived from it are 

 briefly these. It is plain that a thin strip of metal will not only 

 take up and part with its heat in a much shorter period of time than 

 will a thick bar, but that it will also, in a given interval, be raised to 

 a much higher temperature than the thick bar. The strong metals 

 are employed for the obvious reason that the brittle metals cannot be 

 made into very thin leaves and at the same time remain self-sustaining. 

 Further, thin strips of even such metals as iron and copper, the thermo- 

 electric force of which is comparatively feeble, brought into conjunc- 

 tion, give a greater electric current when excited by a feeble radiation 



