M. G. Wiedemann oti the Transmission of Heat in Metals. 395 



the tin bar. If a piece of paper were interposed between them, 

 the difference amounted to 5^-5 C. Hence Despretz concludes 

 that heat suffers a resistance in its passage from one metal to 

 another. JSesides the numbers here mentioned, Despretz gives 

 none by which the entire process of the experiment can be reahzed. 

 Angstrom has also instituted only isolated experiments. He 

 heated a bar composed of lead, copper and tin, at the one or the 

 other end and observed the temperatures at different places by 

 means of thermometers sunk in the bar. Angstrom found that 

 the ratio of the temperatures of two parts of the bar, on opposite 

 sides of a place ot contact of two of its metallic components, was 

 greater wnen the heat passed from the worse to the better con- 

 ductor than m the reverse case. In any case this last method 

 ot experiment is far too complicated to allow any direct conclu- 

 sions to be drawn as to the different facilities with which heat 

 passes in eitner the one or the other direction. The bars bein- 

 comparative y short, and hence the temperatures observed &t 

 points equidistant from each other not forming a geometrical 

 series, the mathematical consideration of the question is bv no 

 means simple. •' 



The experiments of Gore also give no certain results as to the 

 passage of heat from one metal to another. 



The following experiments are intended to settle the point in 

 question, and to examine whether heat really experiences any 

 resistance in its passage. 



For this purpose the same apparatus was used which had been 

 constructed for the determination of the relative conductibility 

 of the metals for heat. Two metal bars, 5 millims. thick, whose 

 conductibihty had been already determined, were cut in two 

 their sections accurately fitted upon each other, and carefully 

 joined by means of an extremely thin layer of solder. The bars 

 thus joined were placed in the apparatus and heated. The ex- 

 periments were made in air, and the temperatures read off by 

 the application of a thermo-electric element at points 2 inches 

 asunder, commencing at the colder and proceeding to the warmer 

 parts of the bar. In a second series of observations, the tempe- 

 ratures were read off at points midway between the points at 

 ftrst observed, so that at last the temperature of the bars was 

 obtained from inch to inch. 



Of numbers of experiments made in this manner I will select 

 only a few. In the following Table, the numbers under a: are 

 the distances of the several points of the bars commencing at the 

 place of junction ; those under t are the corresponding tempera- 

 tures represented m degrees of the galvanometer. The points of 

 the warmer bar, mentioned first in the table, are marked with + 

 and those of the colder with --. 



2D2 



