Degrees of Heat in Absolute Measure. 69 



the specific conductivities of silver and mercury. This ratio alters 

 materially with the physical condition of silver; even if, as is 

 most usual, the silver be used in the form of hard-drawn wire, 

 no great accuracy is attainable in the determination of the ratio. 

 In Wiedemann's Lehre von Galvanismus (Part I. page 181) we 

 find the numbers 1739 (E. Becquerel), 1-7 (Lamy), 1*63 (Mat- 

 thiessen) for the conductivity of mercury, that of silver being 

 taken as 100. Matthiesseu subsequently* gave the number 

 1*65 ; and Siemens's determinations f gave 1'72 and 1*78. These 

 numbers hold for 0° C. 



We may thus take the conductivity of mercury at 0° C. as 

 approximately equal to 1'72; so that, in accordance with the 

 above calculation, the resistance in absolute measure would be 



* = 0-00272A = M6xlQ 7 . 



Hence Siemens' s unit of resistance denoted by S is, in absolute 

 measure, 



S = 2/2 A= 1-16 xlO 10 . 



We will compare this result with the direct absolute measure- 

 ments of Siemens's unit of resistance, which have been made 

 partly with the aid of induced currents, and partly by measuring 

 the quantity of heat developed in a conductor by a constant 

 current. 



By the first method Weber J found 



S = l-0257xl0 10 , 



while the Committee of the British Association § found as a 

 mean 



S = 0-964 xlO 10 , 



A small correction introduced by Matthiessen, by which both 

 values are reduced 03 per cent., is in the present case without 

 significance. 



These determinations do not, therefore, materially deviate from 

 the values of S calculated from the thermal conductivity ; they 

 are both, however, somewhat lower. The reason for this differ- 

 ence might be sought in the less accurate determination of the 

 thermal conductivity as compared with the electrical conducti- 

 vity ; I think, however, that the reason for this discrepancy is to 

 be sought elsewhere. 



The great difference, considering the accuracy with "which the 

 experiments were made, between Weber's results and those of 



* Phil. Mag. S. 4. vol. xxii. p. 195. 



t Phil. Mag. S. 4. vol. xxi. p. 25. 



X Abh. d. k. Gesellsch. d. Wissensch. Gottingen, 1862. 



§ British Association Report, 1863, Jenkin. 



