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IX. Experiments on the Calorific Conductibility of Solids. By 

 F. Neumann, Professor of Physics in the University of Kb- 

 nigsberg*. 



I WILL first describe in a few words the method which I 

 have used. First, I do not observe the stationary, but the 

 variable condition of temperatures; and it is the rapid convergence 

 of the trigonometric series by which this variable state is repre- 

 sented, which enables it to serve for the determination of con- 

 ducting powers ; the first terms of the series are sufficient, when 

 the time t is tolerably large : secondly, the bars used have not to 

 undergo any deformation which escapes calculation : thirdly, this 

 condition is realized by introducing into the bar to be studied 

 very slight thermo-electric piles, formed of an iron wire and a 

 german silver wire, which are soldered in the bar; as long as 

 the temperatures of the solderings are between 0° and 100°, the 

 intensity of the current is proportional to the differences of tem- 

 peratures ; and this law differs so little from the truth, that it 

 may be assumed to be exact : fourthly, the absolute values of the 

 internal and external conduclibilities may be determined simulta- 

 neously by my method. 



The bars used were 3 or 4 lines in the sides. The two thermo- 

 electric rods were soldered at a short distance from the end 

 of each bar, and their wires were connected with a differential 

 galvanometer of a special construction, by means of which the 

 sum and the difference of the two currents which traverse the 

 piles may be exactly measured. 



One of the ends of the bar thus prepared, was heated by a 

 flame until the temperatures attained the state of equilibrium ; 

 the flame was then removed, and at the expiration of a certain 

 time the sums and the differences of the intensities of the two 

 currents were observed; they were thus obtained as a function 

 of the time, the isochronous oscillations of a magnetized needle 

 serving as chronometer. The duration of the oscillation of my 

 apparatus was eight seconds, and I could therefore determine 

 the intensities of the currents every eight seconds. It will be 

 already seen that the differences of the intensities depend essen- 

 tially on the internal conductibility, while the sums correspond 

 to the external conductibility ; and analysis leads in fact to this 

 result, that the ratios of the sums successively observed are ex- 

 pressed by the same constant, and the successive differences by 

 another constant. The combination of these two constants gives 

 then the absolute values of the two conducting-powers. 



In some experiments I also replaced the bars by rings, the 

 rods being then fixed in two points diametrically opposite. 



■ * Translated from the Annales de Chimie et de Physique, October 18G2. 



