[ 33 ] 



VI. Reports on the Progress of the Physical Sciences. 

 By Dr. John Tyndall, F.R.S. 



On the Conductibility of Metals for Heat. By G. Wiedemann and 

 R. FranZj PoggendorfF's Annalen, vol. Ixxxix. p. 497. 



[With a Plate.] 



TO the sense of touch as a thermoscope in experiments on 

 the conduction of heat succeeded the expedient of Ingen- 

 housz, which consisted in coating rods of the substances to be 

 examined with wax^ bringing one end into connexion with a 

 source of heat and observing the distance to which the wax 

 melted on the surface. Exact numerical results were, however, 

 wanting until Despretz took up the subject nearly thirty years 

 ago. In his experiments the bodies examined were reduced 

 to the shape of prismatic bars ; in each bar, and at equal di- 

 stances apart, cavities were hollowed out for the reception of 

 mercmy, into which dipped the bulbs of a number of sensitive 

 thermometers. One end of the bar was heated by a lamp with 

 a constant flame, and the heating was suffered to continue until 

 the temperature of the bar became constant. This of course 

 took place when the amount of heat received by any cross section 

 of the bar from the section immediately preceding, was equal to 

 the quantity imparted to the following section and lost by radia- 

 tion in the same time. The thermometers under these circum- 

 stances formed a series decreasing in height from the hottest end 

 of the bar onwards. From the temperatures thus observed, it is 

 possible, by means of Fourier's formula, to calculate the con- 

 ductive power of the bar for heat. In these experiments, however, 

 the breach of continuity caused by the introduction of the cavi- 

 ties and their contents must have exerted a disturbing influ- 

 ence. In the subsequent experiments of Langberg this difficulty 

 was avoided by the application of a thermo-electric element, 

 which was caused to press against the heated bar ; the tempe- 

 rature being estimated from the deflection which it produced 

 upon a suitable galvanometer. A little reflection, however, wUl 

 suggest various expedients by which the accuracy of the mode 

 of observation introduced by Langberg may be increased ; and 

 this appears to have led to the recent resumption of the subject 

 by MM. Wiedemann and Franz, whose careful experiaicnts con- 

 stitute the matter of the present report. 



In an inquiry of this nature it is necessary that the exterior 

 conductivity, as it is called, should be the same in all the sub- 

 stances examined. Dcsj)retz secured this by coating all his bars 

 with the same description of varnish, thus imparting to them 

 the same radiative power. In the experiments of Wiedemann 



Phil. Mag. S. 4. Vol. 7. No. 42. .Tan. 1854. D 



