340 Mr. M. Donovan on the supposed Identity of the Agent 



tensity, the deflecting force upon the magnetic needle is the 

 same." 



It is very important to recall here, that in Colladon's experi- 

 ments it was clearly proved, although the inference was not 

 drawn by him, that the highest deflections were produced by the 

 highest intensities. With a Nairne's electrical machine, be- 

 tween the conductors of which a galvanometer was placed, he 

 could only obtain a deflection of 3° or 4° ; but when the charge 

 of a large Leyden battery was silently passed through the galva- 

 nometer by means of a point, the deflection amounted to 40°. 

 When he used a galvanometer capable of sustaining a high in- 

 tensity, and consisting of 500 turns, a Nairne's machine required 

 to be worked at the prodigious rate of three revolutions in a 

 second in order to produce a deflection of 35° ; and without this 

 velocity, the necessary intensity not being produced, that amount 

 of deflection could not be obtained. I do not see how these 

 facts are reconcileable with the law in question. 



The case appears to stand thus. The absolute quantity is the 

 total charge contained in the Leyden battery, all of which the 

 hypothesis assumes to pass in a current of successive portions. 

 I think it will scarcely be denied, that no portion of electricity 

 can act except that which is present in the galvanometer coil at 

 any particular instant of time ; the portions still in the Leyden 

 battery can have no effect ; or in other words, the total quantity 

 is not the efficient cause of deflection. The portion in the coil 

 at any particular instant can therefore only act according to its 

 quantity, and the space occupied by it at that moment ; and in 

 point of fact, the first portion of electricity which enters it, as 

 has already been shown, is that which determines the swing of 

 the needle and its amount. 



Even if it were admitted that, in the experiments with the 

 three wet cords or threads, there were slight differences of inten- 

 sity in them, the needle could scarcely be unequally affected, at 

 least in any discoverable degree. We have only to admit, with 

 Colladon, that the coil can carry a certain intensity of electricity, 

 and no more ; that Faraday's coil was charged to saturation with 

 the current, which was conveyed into it by the thinnest of the 

 wet strings ; and that if any excess of electricity had been trans- 

 mitted by the thicker strings, it overflowed laterally from wire 

 to wire. The result of all these admissions would be, that the 

 deflection must be equal for all. It is much to be regretted 

 that we are not informed of the amount of these equal deflections. 



There are other objections to the manner in which this law 

 has been derived. It does not appear to correspond with common 

 experience of the character of the electric fluid, to suppose that 

 the discharge of a battery would pass through a wet thread 



