Chap. III The Franklinian Theory of Electricity
We are now to give Dr. Franklin’s theory of plus and minus, or positive and negative electricity, and adduce facts, to shew how far this theory will go to explain the different phenomena.
The Doctor supposed that all the operations in electricity, depended upon one fluid, sui generis, extremely subtile and elastic.—That there subsists a very strong repulsion between the particles of this fluid, in regard to one another, and as strong an attraction, with regard to other matter.—Thus one quantity of electric matter will repel another quantity of the same, but will attract, and be attracted by, any terrestrial matter that happens to be near it. The pores of all bodies are supposed to be full of this subtile fluid; and when its equilibrium is not disturbed, that is, when there is neither more nor less of it in a body than its natural share, or than it is capable of retaining by its own attraction, the fluid does not manifest itself to our senses. The action of the rubber upon an electric disturbs this equilibrium, occasioning a redundancy of the fluid in one place, and a deficiency of it in another. This equilibrium being forcibly disturbed, the mutual repulsion of the particles of the fluid is necessarily exerted to restore it. If two bodies be both of them over-charged, the electric atmospheres repel one another, and both the bodies recede from each other, to places where the fluid is less dense.—For as there is supposed to be a mutual attraction between all bodies and the electric matter, such bodies as are electrified must go along with their atmospheres. If both bodies are exhausted of their natural share of this fluid, they are both attracted by the denser fluid, existing either in the atmosphere contiguous to them, or in other neighbouring bodies; which occasions them still to recede from one another, as if they were over-charged.
Dr. Franklin’s theory has gained the greatest reputation, from the easy solution it affords of all the phenomena of the Leyden phial. The fluid is supposed to move with the greatest ease in bodies which are conductors; but with extreme difficulty in electrics per se; in so much that glass is absolutely impermeable to it. It is also supposed that all electrics, and particularly glass, on account of the smallness of their pores, do at all times contain an exceedingly great, and always an equal quantity of this fluid; so that no more can be thrown into any one part of any electric substance, except the same quantity go out at another, and the gain be exactly equal to the loss. These things being premised, the phenomena of charging and discharging a plate of glass, or a Leyden phial, may be easily solved. In the usual manner of electrifying by a smooth glass globe or cylinder, all the electric matter is supplied by the rubber, from all the bodies which communicate with it. If it be made to communicate with nothing but one of the coatings of a glass plate, while the prime-conductor is connected with the other, that side of the glass which communicates with the rubber, must necessarily be exhausted in order to supply the conductor, which must convey the whole of it to the coating with which it is connected. By this operation, therefore, the electric fluid becomes almost entirely exhausted from one side of the plate, while it is as much accumulated on the other; and the discharge is made by the electric fluid rushing, as soon as an opportunity is given it by means of proper conductors, from the side which was overloaded to that which was exhausted.
It is not however necessary to this theory, that the same individual particles of electric matter which were thrown upon one side of the plate, should make the whole circuit of the intervening conductors, especially in very great distances, so as actually to arrive at the exhausted side. It may be sufficient to suppose, that the additional quantity of fluid displaces and occupies the place of an equal portion of the natural quantity of fluid, belonging to those conductors in the circuit which lay contiguous to the charged side of the glass. This displaced fluid may drive forwards an equal quantity of the same matter in the next conductor; and thus the progress may continue, till the exhausted side of the glass is supplied by the fluid naturally existing in the conductors contiguous to it.
To account for the velocity with which electricity passes through good conductors, Dr. Franklin compares the electricity in the conductors, to a wire in the bore of a tube, which it exactly fills.—If one end of this wire be moved forward, every other part of it will move in the same direction, and at the same instant.
Dr. Priestley says, it may be thought a difficulty upon this hypothesis, that one of the sides of a glass plate cannot be exhausted, without the other receiving more than its natural share; particularly as the particles of this fluid are supposed to be repulsive of one another. But it must be considered, that the attraction of the glass is sufficient to retain even the large quantity of electric fluid which is natural to it, against all attempts to withdraw it, unless that eager attraction can be satisfied by the admission of an equal quantity from some other quarter. When this opportunity of a supply is given by connecting one of the coatings with the rubber and the other with the conductor, the two attempts, to introduce more of the fluid into one of the sides, and to subtract some from the other, are made, in a manner, at the same instant. The action of the rubber tends to disturb the equilibrium of the fluid in the glass; and no sooner has a spark quitted one of the sides to go to the rubber, than it is supplied by the conductor on the other; and the difficulty with which these additional particles move in the substance of the glass, effectually prevents its reaching the opposite exhausted side. It is not said, however, but that either side of the glass may give or receive a small quantity of the electric fluid, without altering the quantity on the opposite side. It is only a very considerable part of the charge that is meant, when one side is said to be filled while the other is exhausted.
The above is the substance of the theory most generally received. It depends upon the following principles.
1. All terrestrial substances, as well as the atmosphere which surrounds the earth, are full of electric matter.
2. Glass, and other electric substances, though they contain a great deal of electric matter, are nevertheless impermeable by it.
3. This electric matter violently repels itself, and attracts all other matter.
4. By the excitation of an electric, the equilibrium of the fluid contained in it is disturbed, and one part of it is overloaded with electricity, while the other contains too little.
5. Conducting substances are permeable to the electric matter through their whole substance, and do not conduct it merely over their surface.
6. Positive electricity is when a body has too much of the electric fluid, and negative electricity, when it has too little.
Of these positions we shall now adduce those proofs, drawn from different facts, which seem in the strongest manner to confirm them.
I. “All terrestrial substances, as well as the atmosphere which surrounds the earth, are full of electric matter.” The proofs of this are very easy. There is no place of the earth or sea where the electric fire may not be collected, by making a communication between it and the rubber of an electric machine. Therefore, considering that the whole earth is moist, and that moisture is a conductor of electricity, and that every part of the earth must thus communicate with another, it is certain that the electric matter must diffuse itself as far as the moisture of the earth reaches; and this may reasonably be supposed to be to the very centre.
The case is equally clear with regard to the atmosphere. The extract from Mr. Cavallo’s journal, given in the chapter upon atmospheric electricity, is a sufficient proof that the atmosphere is full of electric matter.
II. “Glass, and other electric substances, though they contain a great deal of electric matter, are nevertheless impermeable by it.” The principal arguments for the impermeability of glass by the electric fluid are drawn from the phenomena of the Leyden phial. It is very plain that there is, in charging this phial, an expulsion of fire from the outside, at the same time that it is thrown upon the inside. This appears from numberless experiments, but is most readily observable in the following. Let a coated phial be set upon an insulating stand, and the knob of another phial brought near its coating. As soon as sparks are discharged from the prime-conductor to the knob of the first, an equal number will be observed to proceed from its coating to the knob of the second. This is very remarkable, and an unphilosophical observer will scarce ever fail to conclude, that the fire runs directly through the substance of the glass. Dr. Franklin however concludes that it does not, because there is a very great accumulation of electricity on the inside of the glass, which discovers itself by a violent flash and explosion, when a communication is made between the outside and inside coatings. But it must be confessed, there is here no other reason for concluding the glass to be impermeable, than the probability that the electric matter is accumulated on one side of the glass and deficient on the other.
Another argument against the permeability of glass and other electrics is, that coated phials can receive but a very slight charge when their outside coating is insulated, and this can be effected only with a very powerful machine.
III. “The electric fluid violently repels itself, and attracts all other matter.” The proofs of this position have been so abundantly given in the course of this work, particularly in the chapter on electric attraction and repulsion, that we think it entirely superfluous to repeat them here.
IV. “By the excitation of an electric, the equilibrium of the fluid is disturbed, and one part of it is overloaded with electricity, while the other contains too little.” This position must be considered as entirely hypothetical, as the manner in which the electric fluid is collected by the excitation of glass, or any other electric substance, has not yet been satisfactorily explained.
V. “Conducting bodies are permeable by the electric fluid, through the whole of their substance, and do not conduct it merely over their surface.” Take a wire of any kind of metal, and cover part of it with some electric substance, as rosin, sealing-wax, &c. then discharge a jar through it, and it will be found that it conducts as well as without the electric coating. This, says Mr. Cavallo, proves that the electric matter passes through the substance of the metal, and not over the surface. A wire, adds he, continued through a vacuum is also a convincing proof of this assertion.
VI. “Positive electricity is an accumulation, or too great a quantity of the electric matter contained in a body; and negative electricity is when there is too little.” This position, like the fourth, must be considered as hypothetical—the peculiar nature of the electric fluid not admitting of experiments to prove, or to disprove it.