THERMAL AND ELECTRICAL CONDUCTIVITY. 403 



of the ring. It is assumed further that the two coefficients of 

 conductivity, as well as the specific heat, are linear functions of 

 the temperature. 



It is sufficient to know the excess of temperature t^ and / 2 

 at a given instant, at the two points in question, to get from 

 it the two coefficients of conductivity. 



The electrical conductivity was determined by the method 

 of damping, the ring being placed vertically in the magnetic 

 meridian, and a small magnetic needle being made to oscillate 

 at the centre. The resistance R of the ring is deduced from 

 formula (20)" of 845, in which the term in L can be suppressed, 

 it being entirely negligable in the present case. If we suppose 

 the induced currents distributed uniformly throughout the entire 

 ring parallel to the mean circumference, which may, and even 

 must, differ appreciably from the reality, and if a is the radius of 

 the section, and b that of the mean circumference, the expression 

 for the coefficient c of conductivity is 



_ 

 ~ 



Weber's experiments do not favour the idea of a simple relation 

 , between the thermal and the electrical conductivity; but the 

 author thinks he can infer that the ratio of the thermal conductivity 

 to the electrical conductivity, is a linear function of the specific 

 heat y of unit volume of the metal, so that if a and b are two 

 constants, 



k ' 



- = a + ay. 



As Weber remarks, this equation would enable us to explain why 

 previous experiments seem to verify the proportionality of the 

 two conductivities, as the metals employed, such as copper, iron, 

 brass and argentan, have sensibly the same specific heat y. 



1001. Kirchhoff and Hansemann* have used a method of 

 measuring thermal conductivity in which the coefficient of external 

 conductivity only comes in as a term of correction of small 

 importance. They consider an unlimited medium bounded by 



* G. KIRCHHOFF and HANSEMANN. Wied. Ann., Vol. ix., p. i, 1880: 

 and Vol. xin., p. 406, 1881. KIRCHHOFF. Gesamm. Abh., p. 495. 



D D 2 



