346 Sir W. Snow Harris on a General Law of 



they discover to us any specific quality of the electrical agency 

 calculated to modify its effects when discharged under the form 

 of a momentary current. All we learn from these instruments 

 is, the relative state of actinty of the whole, or a portion of the 

 charge in a certain direction taken in terms of a given statical 

 force, either attractive or repulsive, and by which we may occa- 

 sionally, and under certain conditions, measui'e the quantity of 

 electricity accumulated. Now whatever may be the extent of 

 the battery upon which we suppose the charge to be expanded, 

 or whatever may be the distance of discharge as determined by 

 Lane's discharger, in any case at the moment of discharge, when 

 the statical indications often termed " intensity " vanish, and the 

 whole accumulation becomes, as it were, precipitated upon the 

 met.allic wire, the force of the momentary current through the 

 circuit is px'ecisely the same, as may be demonstrated by incon- 

 trovertible experiments to be presently referred to. The heating 

 effect, therefore, of the discharge must necessarily be independent 

 of any variable indication of an electrometer attached to the bat- 

 tery, and which may be caused at pleasui'c to indicate with the 

 same quantity of electricity any " density " we please. 



9. In my paper of 1830, printed in the Transactions of the 

 Plymouth Institution, as well as in my several communications to 

 the Royal Society, I have shown that the electrometer indica- 

 tions are proportional to the square of the charge divided by the 

 square of the surface or extent of coated glass upon which the 

 accumulated electricity becomes expanded, all other things being 



unchanged* ; so that we have F = ^. If, therefore, the heating 



effect of the discharge be dependent on the extent of the battery, 



as insisted on by M . Riess, we should have T=-^^, and not T = — , 



OS 



as given in his expression. But both these expressions are evi- 

 dently inapplicable to the heating effect of the discharge, which 

 is altogether independent of S or s taken to represent the extent 

 of the battery; nevertheless I do not doubt the truth of the 



expression T= — when correctly interpreted. I will therefore 



now endeavour to show in what the difiFerence between my expe- 

 riments and i\I. Ricss's interpretation of the phfcnomena consists. 



10. It is to be here observed, that when we discharge a given 

 quantity of electricity through a metallic wire, the heating effect 

 will be reciprocally proportional to the resistance in the circuit, 

 that is generally to the extent of the circuit ; so that in put- 

 ting C = the extent of the circuit = the resistance, we have 



* Phil. Tiiins. for 1834. p. 221. 



