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An Elementary Treatise on Electricity
By James Clerk Maxwell, William Garnett Dover Publications, Inc.
Copyright © 2005 Dover Publications, Inc.
All rights reserved.
ISBN: 978-0-486-17463-1
CHAPTER 1
EXPERIMENT I.
Electrification by Friction.
1.] TAKE a stick of sealing-wax, rub it on woollen cloth or flannel, and then bring it near to some shreds of paper strewed on the table. The shreds of paper will move, the lighter ones will raise themselves on one end, and some of them will leap up to the sealing-wax. Those which leap up to the sealing-wax sometimes stick to it for awhile, and then fly away from it suddenly. It appears therefore that in the space between the sealing-wax and the table is a region in which small bodies, such as shreds of paper, are acted on by certain forces which cause them to assume particular positions and to move sometimes from the table to the sealing-wax, and sometimes from the sealing-wax to the table.
These phenomena, with others related to them, are called electric phenomena, the bodies between which the forces are manifested are said to be electrified, and the region in which the phenomena take place is called the electric field.
Other substances may be used instead of the sealing-wax. A piece of ebonite, gutta-percha, resin or shellac will do as well, and so will amber, the substance in which these phenomena were first noticed, and from the Greek name of which the word electric is derived.
The substance on which these bodies are rubbed may also be varied, and it is found that the fur of a cat's skin excites them better than flannel.
It is found that in this experiment only those parts of the surface of the sealing-wax which were rubbed exhibit these phenomena, and that some parts of the rubbed surface are apparently more active than others. In fact, the distribution of the electrification over the surface depends on the previous history of the sealing-wax, and this in a manner so complicated that it would be very difficult to investigate it. There are other bodies, however, which may be electrified, and over which the electrification is always distributed in a definite manner. We prefer, therefore, in our experiments, to make use of such bodies.
The fact that certain bodies after being rubbed appear to attract other bodies was known to the ancients. In modern times many other phenomena have been observed, which have been found to be related to these phenomena of attraction. They have been classed under the name of electric phenomena, amber, [TEXT NOT REPRODUCIBLE IN ASCII], having been the substance in which they were first described.
Other bodies, particularly the loadstone and pieces of iron and steel which have been subjected to certain processes, have also been long known to exhibit phenomena of action at a distance. These phenomena, with others related to them, were found to differ from the electric phenomena, and have been classed under the name of magnetic phenomena, the loadstone, [TEXT NOT REPRODUCIBLE IN ASCII], being found in Magnesia.
These two classes of phenomena have since been found to be related to each other, and the relations between the various phenomena of both classes, so far as they are known, constitute the science of Electromagnetism.
EXPERIMENT II.
Electrification of a Conductor.
2.] Take a metal plate of any kind (a tea-tray, turned upside down, is convenient for this purpose) and support it on three dry wine glasses. Now place on the table a plate of ebonite, a sheet of thin gutta-percha, or a well-dried sheet of brown paper. Rub it lightly with fur or flannel, lift it up from the table by its edges and place it on the inverted tea-tray, taking care not to touch the tray with your fingers.
It will be found that the tray is now electrified. Shreds of paper or gold-leaf placed below it will fly up to it, and if the knuckle is brought near the edge of the tray a spark will pass between the tray and the knuckle, a peculiar sensation will be felt, and the tray will no longer exhibit electrical phenomena. It is then said to be dischargerd. If a metal rod, held in the hand, be brought near the tray the phenomena will be nearly the same. The spark will be seen and the tray will be discharged, but the sensation will be slightly different.
If, however, instead of a metal rod or wire, a glass rod, or stick of sealing-wax, or a piece of gutta-percha, be held in the hand and brought up to the tray there will be no spark, no sensation, and no discharge. The discharge, therefore, takes place through metals and through the human body, but not through glass, sealing-wax, or gutta-percha. Bodies may therefore be divided into two classes: conductors, or those which transmit the discharge, and nonconductors, through which the discharge does not take place.
In electrical experiments, those conductors, the charge of which we wish to maintain constant, must be supported by non-conducting materials. In the present experiment the tray was supported on wine glasses in order to prevent it from becoming discharged. Pillars of glass, ebonite, or gutta-percha may be used as supports, or the conductor may be suspended by a white silk thread. Solid nonconductors, when employed for this purpose, are called insulators. Copper wires are sometimes lapped with silk, and sometimes enclosed in a sheath of gutta-percha, in order to prevent them from being in electric communication with other bodies. They are then said to be insulated.
The metals are good conductors; air, glass, resins, gutta-percha, caout-chouc, ebonite, paraffin, &c., are good insulators; but, as we shall find afterwards, all substances resist the passage of electricity, and all substances allow it to pass though in exceedingly different degrees. For the present we shall consider only two classes of bodies, good conductors, and good insulators.
EXPERIMENT III.
Positive and Negative Electrification.
3.] Take another tray and insulate it as before, then after discharging the first tray remove the electrified sheet from it and place it on the second tray. It will be found that both trays are now electrified. If a small ball of elder pith suspended by a white silk thread be made to touch the first tray, it will be immediately repelled from it but attracted towards the second. If it is now allowed to touch the second tray it will be repelled from it but attracted towards the first. The electrifications of the two trays are therefore of opposite kinds, since each attracts what the other repels. If a metal wire, attached to an ebonite rod, be made to touch both trays at once, both trays will be completely discharged. If two pith balls be used, then if both have been made to touch the same tray and then hung up near each other they are found to repel each other, but if they have been made to touch different trays they attract each other. Hence bodies when electrified in the same way are repelled from each other, but when they are electrified in opposite ways they are attracted to each other.
If we distinguish one kind of electrification by calling it positive, we must call the other kind of electrification negative. We have no physical reason for assigning the name of positive to one kind of electrification rather than to the other. All scientific men, however, are in the habit of calling that kind of electrification positive which the surface of polished glass exhibits after having been rubbed with zinc amalgam spread on leather. This is a matter of mere convention, but the convention is a useful one, provided we remember that it is a convention as arbitrary as that adopted in the diagrams of analytical geometry of calling horizontal distances positive or negative according as they are measured towards the right or towards the left of the point of reference.
In our experiment with a sheet of gutta-percha excited by flannel, the electrification of the sheet and of the tray on which it is placed is negative; that of the flannel and of the tray from which the gutta-percha has been removed is positive.
In whatever way electrification is produced it is one or other of these two kinds.
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 applied becoming negative, while that to which the plain metal disk was applied becomes positive.
ELECTROMOTIVE FORCE.
5.] Definition.—Whatever produces or tends to produce a transfer of Electrification is called Electromotive Force.
Thus when two 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, thermoelectric, 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 equilibrium 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.
ELECTRIC POTENTIAL.
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.] It is found that 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 communication 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 analogy 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.
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Excerpted from An Elementary Treatise on Electricity by James Clerk Maxwell, William Garnett. Copyright © 2005 Dover Publications, Inc.. Excerpted by permission of Dover Publications, Inc..
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