448 THE POPULAR SCIENCE MONTHLY 



Farther along he says : 



"We might consider this mutual contact of two different metals as the imme- 

 diate cause which puts the electric fluid in motion, instead of ascribing that 

 power to the contact of the two metals with the moist conductors. ... In both 

 suppositions the result, as may be easily seen, is the same. But though I have 

 reasons for adopting the first as true rather than the second, yet the latter repre- 

 sents the proposition with more simplicity, and it may be convenient to adhere 

 to it in the explanation, as it affords a readier view of it. 



In a postscript to this letter published the next year (1797 or 98) 

 Volta says: 



Some new facts, lately discovered, seem to shew that the immediate cause 

 which excites the electric fluid, and puts it in motion, whether it be an attraction 

 or a repulsive power, is to be ascribed much rather to the mutual contact of two 

 different metals, than to their contact with moist conductors. But though it 

 can not be denied that in the latter case there exists an action, it is proved that 

 it exerts itself in a far more considerable degree when two metals mutually 

 touch each other. There arises by the mutual contact, for example, of silver and 

 tin, an action or power by which the former communicates the electric fluid, and 

 the latter receives it; or the silver suffers it to escape and the tin attracts it. 

 This produces, when the circle is rendered complete by moist conductors, a 

 stream, or continual circulation of the fluid. When the circle is complete, there is 

 an accumulation in the tin at the expense of the silver; which indeed is very 

 small, and far under the point necessary to enable it to announce itself by the 

 most delicate electrometer. I have however been able, by the assistance of my 

 condenser, constructed on a new plan, and still better by Nicholson's doubler, to 

 render it very perceptible: I shall here communicate the result obtained by my 

 experiments, which I made some time ago with great satisfaction. 



Exper. I. The three plates of the doubler are of brass. I took two strong 

 wires, one of silver and the other of tin, and brought the former into contact 

 with the movable plate, and the other with one of the fixed plates; while they 

 both rested on the table, or, what is better, on moist pasteboard, or any other 

 moist conductor, so as to be in communication by the intervention of one or more 

 conductors of the second class. I suffered the apparatus to remain some hours in 

 this state, then removed the two wires and put the machine in motion. After 20, 

 30 or 40 revolutions (or more when the atmosphere was not dry, or the insulation 

 imperfect) I brought one of my straw electrometers into contact with the 

 movable plate, and observed indications of positive electricity (+ E) which 

 arose to 4, 6, 10 degrees, and more. If I suffered it to touch the fixed plates, I 

 had the corresponding indications of the opposite kind of electricity ( — E). 



The silver, therefore, poured the elastic fluid into the brass plate when it 

 had been some time in contact with it; and the tin attracted it from the other 

 plate, which was also of brass, while in contact with it. This was confirmed by 

 the following experiment, which is a real experimentum cj'ucis. 



II. I reversed the experiment, so that the silver was in contact with one of 

 the fixed plates, and the tin with the moveable one. The electricity which I 

 obtained from the latter, after the apparatus had remained a sufiicient time in 

 that position, was negative ( — E) ; while that of the fixed plato was positive 

 (+E). 



III. This is the reverse of the former. Tlie piece of tin was applied to 

 one of the fixed plates, and the moveable one was insulated from all metallic 



