HEAT. 



violently renewed, had the same temperature as that further off round the 

 thermometer. But though, no doubt, * 2 wa s small, it is to be re- 



f 



membered that with metals -j might be great. Thus for copper and 



water, we now know that it is about 700 ; so that a layer of water which 

 remained against the plate on each side of it, and TT Vff of its thickness 

 would make the result about double the true value. That P^clet failed to 

 remove such a layer is shown by the fact that for copper he obtained 

 a value about five times that now accepted. In fact the layer of water 

 of sloping temperature was practically 2/700 of the thickness of the 



plate on each side. If - 1 were not large the layer might be still so 



thick, and yet the results would not be far wrong. Accordingly, Pellet's 

 work for low conductors is fairly confirmed by later work. 



Bar Methods of Finding the Conductivity of Metals. There are 



FIG. 66. Despretz's Bar Experiment. 



two types of experiment in which a long metal bar is heated at one end, 

 and the conductivity is deduced from temperature observations along the 

 bar. In one the steady-flow method one end is kept at a constant 

 high temperature until the temperature at each point has come to a 

 steady value. As no part of the bar is now gaining or losing heat, 

 the heat conducted per second through any cross-section must equal that 

 emitted per second from the surface beyond that section, and if the rate 

 of emission can be determined, an observation of the slope of tempera- 

 ture at the cross-section will give the conductivity necessary to supply 

 heat at the rate at which it is emitted beyond. In the other type, one 

 end of the bar is subjected to periodic variations of temperature, and 

 consequently waves of varying temperature travel down the bar. The 

 conductivity is calculated from the march of these waves. 



To the first method belong the researches of Despretz, Forbes, 

 Wiedemann and Franz, and, more recently, Tait and others. 



Despretz's Experiments. Despretz used bars of different metals 

 of the same dimensions, and with the surfaces varnished in the same 

 way, so that the loss of heat to the surroundings should be the same for 

 the same excess of temperature. Thermometers 1, 2, 3, 4, 5, 6, 7 (Fig. 66) 

 were placed in holes in the bar filled with mercury. 



Fourier had calculated the flow of heat along such a bar, assuming that 



