404 COMPARISON OF RESISTANCES. 



a plane. The temperature being supposed uniform thoughout 

 the whole extent of the medium, the entire surface of the plane 

 is raised at a given moment to a different temperature, which is 

 kept constant, and the course of the internal temperature is 

 noted. If U is the excess of the temperature of the plane over 

 the initial temperature, and if u is the excess at the time / 

 at a point at distance r, we have 



(55) 



The medium was represented by a cube 15 cm. in the side, 

 the front face of which was fitted in a zinc plate; the temperature 

 being uniform, and equal to that of the external medium, a jet 

 of water of a fixed temperature, which was a few degrees higher 

 or lower then the surrounding temperature, was directed against 

 the face, and kept playing against it for the duration of the 

 experiment Thermoelectrical probes gave the variation of tem- 

 peratures at different distances from the plane. 



In order to obtain the electrical conductivity, the cube was 

 then divided into prisms with a square base 5 mm. in the side, 

 and length equal to the edge of the cube, and the method (981) 

 was used. Unfortunately prisms from the same cube often showed, 

 differences amounting to 10 or sometimes 25 per cent, which 

 greatly detracts from the definiteness of the conclusions which can 

 be drawn from these experiments. 



They tend to show that the ratio is virtually constant for 

 the metals tried that is to say, for lead, tin, zinc, and copper 

 except for iron. On the contrary, they do not confirm the law 

 propounded by Weber. 



1002. Lorenz* investigated the thermal and the electrical 

 conductivity of bars 30 cm. in length, and 1*5 cm. in diameter. 



Let us suppose the bar divided in n equal parts of length /, 

 and let U Q , u^ ---- u n be the excess of temperature at the points 

 of division o, i, 2.... n. If q is the section, the quantity of 

 heat which in unit time traverses the section A at an equal 

 distance from the points and i, is 



* LORENZ. Wiedemann's Annalen, Vol. xni., pp. 422 and 582. 1881. 



