Experiments 4-5

Experiment IV. The Electrophorus of Volta.

4. This instrument is very convenient for electrical experiments and is much more compact than any other electrifying apparatus.

It consists of two disks, and an insulating handle which can be screwed to the back of either plate. One of these disks consists of resin or of ebonite in front supported by a metal back.

In the centre of the disk is a metal pin∗ , which is in connection with the metal back, and just reaches to the level of the surface of the ebonite. The surface of the ebonite is electrified by striking it with a piece of flannel or cat’s fur. The other disk, which is entirely of metal, is then brought near the ebonite disk by means of the insulating handle.

When it comes within a certain distance of the metal pin, a spark passes, and if the disks are now separated the metal disk is found to be charged positively, and the disk of ebonite and metal to be charged negatively.

In using the instrument one of the disks is kept in connection with one conductor while the other is applied alternately to the first disk and to the other conductor. By this process the two conductors will become charged with equal quantities of electricity, that to which the ebonite disk was ap- plied becoming negative, while that to which the plain metal disk was applied becomes positive.

[This was introduced by Professor Phillips to obviate the necessity of touching the carrier plate while in contact with the ebonite.]

5.] Definition.—Whatever produces or tends to produce a transfer of Electrification is called Electromotive Force.

Thus when 2 electrified conductors are connected by a wire, and when electrification is transferred along the wire from one body to the other, the tendency to this transfer, which existed before the introduction of the wire, and which, when the wire is introduced, produces this transfer, is called the Electromotive Force from the one body to the other along the path marked out by the wire.

To define completely the electromotive force from one point to another, it is necessary, in general, to specify a particular path from the one point to the other along which the electromotive force is to be reckoned.

In many cases, some of which will be described when we come to electrolytic, thermoelec- tric, and electromagnetic phenomena, the electromotive force from one point to another may be different along different paths. If we restrict our attention, however, as we must do in this part of our subject, to the theory of the equi- librium of electricity at rest, we shall find that the electromotive force from one point to another is the same for all paths drawn in air from the one point to the other.

POTENTIAL OF A CONDUCTOR

6. The electromotive force from any point, along a path drawn in air, to a certain point chosen as a point of reference, is called the Electric Potential at that point.

Since electrical phenomena depend only on differences of potential, it is of no consequence what point of reference we assume for the zero of potential, provided that we do not change it during the same series of measurements. In mathematical treatises, the point of reference is taken at an infinite distance from the electrified system under consideration.

The advantage of this is that the mathematical expression for the potential due to a small electrified body is thus reduced to its simplest form. In experimental work it is more convenient to assume as a point of reference some object in metallic connection with the earth, such as any part of the system of metal pipes conveying the gas or water of a town.

It is often convenient to assume that the walls, floor and ceiling of the room in which the experiments are carried on has conducting power sufficient to reduce the whole inner surface of the room to the same potential. This potential may then be taken for zero. When an instrument is enclosed in a metallic case the potential of the case may be assumed to be zero. Potential of a Conductor.

7.] If the potentials at different points of a uniform conductor are different there will be an electric current from the places of high to the places of low potential. The theory of such currents will be explained afterwards (Chap. ix). At present we are dealing with cases of electric equilibrium in which there are no currents. Hence in the cases with which we have now to do the potential at every point of the conductor must be the same. This potential is called the potential of the conductor.

The potential of a conductor is usually defined as the potential of any point in the air infinitely close to the surface of the conductor.

Within a nearly closed cavity in the conductor the potential at any point in the air is the same, and by making the experimental determination of the potential within such a cavity we get rid of the difficulty of dealing with points infinitely close to the surface.

8. When two different metals are in contact and in electric equilibrium their potentials as thus defined are in general different.

Thus, if a copper cylinder and a zinc cylinder are held in contact with one another, and if first the copper and then the zinc cylinder is made to surround the flame of a spirit lamp, the lamp being in connection with an electrometer, the lamp rapidly acquires the potential of the air within the cylinder, and the electrometer shews that the potential of the air at any point within the zinc cylinder is higher than the potential of the air within the copper cylinder.

We shall return to this subject again, but at present, to avoid ambiguity, we shall suppose that the conductors with which we have to do consist all of the same metal at the same temperature, and that the dielectric medium is air.

9. The region of space in which the potential is higher than a certain value is divided from the region in which it is lower than this value by a surface called an equipotential surface, at every point of which the potential has the given value.

We may conceive a series of equipotential surfaces to be described, corresponding to a series of potentials in arithmetical order. The potential of any point will then be indicated by its position in the series of equipotential surfaces.

No two different equipotential surfaces can cut one another, for no point can have two different potentials.

10.] The idea of electric potential may be illustrated by comparing it with pressure in the theory of fluids and with temperature in the theory of heat.

If two vessels containing the same or different fluids are put into communication by means of a pipe, fluid will flow from the vessel in which the pressure is greater into that in which it is less till the pressure is equalized. This however will not be the case if one vessel is higher than the other, for gravity has a tendency to make the fluid pass from the higher to the lower vessel.

Similarly when two electrified bodies are put into electric communication by means of a wire, electrification will be transferred from the body of higher potential to the body of lower potential, unless there is an electromotive force tending to urge electricity from one of these bodies to the other, as in the case of zinc and copper above mentioned.

Again, if two bodies at different temperatures are placed in thermal com- munication either by actual contact or by radiation, heat will be transferred from the body at the higher temperature to the body at the lower temperature till the temperature of the two bodies becomes equalized. The analogy between temperature and potential must not be assumed to extend to all parts of the phenomena of heat and electricity. Indeed this anal- ogy breaks down altogether when it is applied to those cases in which heat is generated or destroyed.

We must also remember that temperature corresponds to a real physical state, whereas potential is a mere mathematical quantity, the value of which depends on the point of reference which we may choose. To raise a body to a high temperature may melt or volatilize it. To raise a body, together with the vessel which surrounds it, to a high potential produces no physical effect whatever on the body. Hence the only part of the phenomena of electricity and heat which we may regard as analogous is the condition of the transfer of heat or of electricity, according as the temperature or the potential is higher in one body or in the other.

With respect to the other analogy—that between potential and fluid pressure—we must remember that the only respect in which electricity resembles a fluid is that it is capable of flowing along conductors as a fluid flows in a pipe. And here we may introduce once for all the common phrase The Electric Fluid for the purpose of warning our readers against it.

It is one of those phrases, which, having been at one time used to denote an observed fact, was immediately taken up by the public to connote a whole system of imaginary knowledge. As long as we do not know whether positive electricity, or negative, or both, should be called a substance or the absence of a substance, and as long as we do not know whether the velocity of an electric current is to be measured by hundreds of thousands of miles in a second or by an hundredth of an inch in an hour, or even whether the current flows from positive to negative or in the reverse direction, we must avoid speaking of the electric fluid.