402 COMPARISON OF RESISTANCES. 



This correlation seems to result from the experiments of Forbes,* 

 of Angstrom,! of Neumann,! and of Lenz, who, working by the 

 method of variable temperatures, endeavoured to determine, not 

 merely the ratios of the conductivities of different bodies, but also 

 their absolute conductivities. Nevertheless, the numbers obtained 

 present some uncertainty. They are, moreover, insufficient, the cal- 

 culations being made on the supposition that the two coefficients 

 of internal and external thermal conductivity are independent of 

 the temperature. 



Professor Tait|| considers the two coefficients as linear functions 

 of the temperature. The method which he uses is that of Forbes. 

 It requires two experiments one statical, which consists in de- 

 termining the distribution of stationary temperatures in a bar heated 

 at one end; the other dynamical, in which the cooling of a very 

 short piece of the same bar is investigated. This second experiment 

 gives the quantity of heat lost by the outer surface at all tem- 

 peratures. Tait's experiments were on iron, pure copper, and 

 ordinary copper, lead, and argentan. It follows from the numbers 

 obtained that the direction of the variations is the same in the 

 two orders of phenomena, but that these two variations are far 

 from being proportional. 



1000. The physical properties of the metals vary so greatly 

 from one specimen to another, that the question could only be 

 strictly solved by studying the two phenomena in the same specimen. 

 The experiments of F. Weber, of Kirchhoff and Hansemann, and 

 of Lorenz, have been made under these conditions. 



F. Weber II takes the metal to be studied in the shape of an 

 anchor ring. In order to determine the thermal conductivity he 

 heats one of the sections until the distribution of the temperatures 

 is stationary; he then studies the cooling, observing the successive 

 temperatures at two points, one at 45 and the other at 225, from 

 the section originally heated. Theory shows that if the dimensions 

 of the ring are conveniently chosen, the temperature may, without 

 appreciable error, be considered constant throughout an entire 

 section, and the propagation as parallel to the mean circumference 



* FORBES. Phil. Trans. Roy. Soc. Edin. Feb., 1860, and Feb., 1864. 



t ANGSTROM. Pogg. Ann., Vol. cxiv., p. 513, 1861 ; Vol. cxvin., p. 423, 1863. 



I NEUMANN. Ann. de Chim. et de Phys. [3], Vol. LXVI., p. 185. 1863. 



LENZ. Bull, de FAcad. de St.-Petersb., Vol. xv., p. 54. 1870. 



|| TAIT. Phil. Trans. Roy. Soc. Edin. Vol. XXVIII., p. 717. 1877. 



IT F. WEBER. Bibl. Univ. de Geneve [3], Vol. iv., p. 107. 1880. 



