/2K( 



404 Prof. Thomson on Transient Electric Currents. 

 Hence the first of equations (11) gives 



''""'y (19). 



Now Weber has not only determined T, in certain cases alluded 

 to above, but has shown how K may be determined for any linear 

 conductor. Again, the value of C for a Leyden phial is, accord- 

 ing to Green and Faraday, expressed by the equation 



C = l4j? • (2«)' 



where S denotes the area of one side of the coated glass, t the 

 thickness of the glass, and I the specific inductive capacity of its 

 substance*. Thus, either by using a Leyden phial or some other 

 conductor of which the electro- statical capacity can be found, 

 and by determining the "duration'' of a discharge from it 

 through a linear conductor, of which the resistance in absolute 

 electro- magnetic measure has been determined, we have every- 

 thing that is required for calculating the value of a by means of 

 equation (19). The determination of this quantity enables us 

 to compare the electro-statical and electro-magnetic measures of 

 electromotive force. For if V denote a constant diff'erence of 

 potentials kept up between the two extremities of a linear con- 

 ductor, and if 7 denote the strength of the uniform current that 

 results, we have, according to the conclusions drawn above 

 from (16), 



V 



But if F denote the electromotive force between the ends of the 

 linear conductor in electro-magnetic measure, we have 



1 _F__JP_ 



and therefore 



F=o-V (21). 



Many different ways of determining the value of this important 

 element, o-, besides that suggested above, might probably be put 

 in practice. Perhaps the most accurate would be to take a mul- 

 tiple galvanic battery of constant and known electromotive force 

 (consisting, for instance, of a hundred or more cells of DanielPs), 

 and measure the force of attraction between two plane conduct- 



* The value of I for flint-glass is, according to Faraday (Experimental 

 Researches, Series XL), greater than 1*76, for shell-lac about 2, for sulphur 

 rather more than 22. See a paper " On the Elementary Laws of Statical 

 Electricity," § 8. (Cambridge and Dublin Mathematical Journal, vol. i. 

 Nov. 1845.) 



