100 



HEAT. 



Experiments of Wiedemann and Franz. Relation between Heat 



Conductivity and Electric Conductivity* Wiedemann and Franz experi- 

 mented on the relative conductivities of metal bars working by the same 

 general method as Despretz, but using a thermopile instead of thermo- 

 meters to give the temperatures along the bar, the one junction of the 

 pile being brought by a suitable arrangement in contact with any desired 

 point of the bar. They found afterwards that the same results were 

 given by using thermometers. The bars were silver-plated, and polished 

 so as to have the same emissivity, and the chamber in which they were 

 placed could be exhausted. The results obtained were comparative. 

 They appeared to show that there is some connection between conducting- 

 power for heat and for electricity. For the metals were found not only 

 to follow the same order for the two conductivities, but in many cases 

 the numbers bore nearly the same ratio to each other. 



More recent work has confirmed this supposition. The following 

 are some of the values for metals and alloys of the ratio thermal con- 

 ductivity/electrical conductivity or k/c at 18 C. as determined by Jaeger 

 and Diesselhorst (Phys. Tech. Reichsanstalt Wiss., Abh. 3, 1900), together 

 with the temperature co-efficient of the ratio. 



Copper, pure (1) 

 (2) 



Silver 

 Gold 

 Zinc 



Cadmium 

 Lead 

 Tin 



The electron theory of conduction for heat and for electricity (J. J. 

 Thomson, Corjwscular Theory of Matter) gives an explanation of the 

 connection between the two quantities. According to that theory the 

 ratio should be proportional to the absolute temperature, i.e. should have 

 a temperature co-efficient 0-00367, and at 0. its value should be 

 6 '3 x 10 10 . The table shows that for many metals the values are not 

 very different from those given by the theory. With alloys considera- 

 tions of thermo-electric effects probably come in to add to the effective 

 resistance, and so to diminish the conductivity and increase the ratio. 



Kundt's Experiments on the Relation between the Velocity of Light in 

 Metals and their Electric and Heat Conductivities. Kundtt determined 

 the refraction of light by exceedingly thin prisms of various metals and 

 taking the velocity in each metal as being inversely as the refractive 

 index, he found for red light 



Silver 



Gold 



Copper 



100 

 71 

 60 



Platinum 

 Iron 



15-3 

 14-9 



Nickel 

 Bismuth 



12-4 

 10-3 



values not very different from the relative values for heat or electrical 

 conductivity. The actual refractive index from air into silver was 

 0'27, and there was in this case but little dispersion. In other 



Pogg. Ann., Ixxxix., 1853, p. 497. f Phil. Mag., xxvL, 1888, p. 1. 



