other kind; and the electricity excited in such substances as sealing-wax, resin, shellac, indiarubber, and amber, by rubbing them on wool or flannel, he termed resinous electricity. The kind of electricity produced is, however, found to depend not only on the thing rubbed but on the rubber also; for glass yields "resinous" electricity when rubbed with a cat's skin, and resin yields "vitreous" electricity if rubbed with a soft amalgam of tin and mercury spread on leather. Hence these names have been abandoned in favour of the more appropriate terms introduced by Franklin, who called the electricity excited upon glass by rubbing it with silk positive electricity, and that produced on resinous bodies by friction with wool or fur, negative electricity. The observations of Symmer and Du Fay may therefore be stated as follows: - Two positively electrified bodies apparently repel one another: two negatively electrified bodies apparently repel one another: but a positively electrified body and a negatively electrified body apparently attract one another. It is now known that these effects which appear like a repulsion and an attraction between bodies at a distance from one another are really due to actions going on in the medium between them. The positive charge does not really attract the negative charge that is near it; but both are urged toward one another by stresses in the medium in the intervening space.

The

6. Simultaneous Production of both Electrical States. Neither kind of electrification is produced alone; there is always an equal quantity of both kinds produced; one kind appearing on the thing rubbed and an equal amount of the other kind on the rubber. clearest proof that these amounts are equal can be given in some cases. For it is found that if both the-electricity of the rubber and the + electricity of the thing rubbed be imparted to a third body, that third body will show no electrification at all, the two equal and opposite electrifications having exactly neutralized each other. A simple

experiment consists in rubbing together a disk of sealingwax and one covered with flannel, both being held by insulating handles. To test them is required an insulated pot and an electroscope, as in Fig. 29. If either disk be inserted in the pot the leaves of the electroscope will diverge; but if both are inserted at the same time the leaves do not diverge, showing that the two charges on the disks are equal and of opposite sign.

In the following list the bodies are arranged in such an order that if any two be rubbed together the one which stands earlier in the series becomes positively electrified, and the one that stands later negatively electrified: Fur, wool, ivory, glass, silk, metals, sulphur, indiarubber, guttapercha, collodion, or celluloid.

7. Theories of Electricity. Several theories have been advanced to account for these phenomena, but all are more or less unsatisfactory. Symmer proposed a "two-fluid" theory, according to which there are two imponderable electric fluids of opposite kinds, which neutralize one another when they combine, and which exist combined in equal quantities in all bodies until their condition is disturbed by friction. A modification of this theory was made by Franklin, who proposed instead a “one-fluid" theory, according to which there is a single electric fluid distributed usually uniformly in all bodies, but which, when they are subjected to friction, distributes itself unequally between the rubber and the thing rubbed, one having more of the fluid, the other less, than the average. Hence the terms positive and negative, which are still retained; that body which is supposed to have an excess being said to be charged with positive electricity (usually denoted by the plus sign +), while that which is supposed to have less is said to be charged with negative electricity (and is denoted by the minus sign -). These terms are, however, purely arbitrary, for in the present state of science we do not know which of these two states really means more and

which means less. In many ways electricity behaves as a weightless substance as incompressible as any material liquid. It is, however, quite certain that electricity is not a material fluid, whatever else it may be. For while it resembles a fluid in its property of apparently flowing from one point to another, it differs from every known fluid in almost every other respect. It possesses no weight; it repels itself. It is, moreover, quite impossible to conceive of two fluids whose properties should in every respect be the precise opposites of one another. For these reasons it is clearly misleading to speak of an electric fluid or fluids, however convenient the term may seem to be. In metals and other good conductors electricity can apparently move and flow quite easily in currents. In transparent solids, such as glass and resin, and in many transparent liquids such as oils, and in gases such as the air (if still, and not rarefied) electricity apparently cannot flow. Even a vacuum appears to be a non-conductor. In the case of all non-conductors electricity can only be moved by an action known as displacement (see Art. 57).

It appears then that in metals electricity can easily pass from molecule to molecule; but in the case of nonconductors the electricity is in some way stuck to the molecules, or associated with them. Some electricians, notably Faraday, have propounded a molecular theory of electricity, according to which the electrical states are the result of certain peculiar conditions of the molecules of the surfaces that have been rubbed. Another view is to regard the state of electrification as related to the ether (the highly-attenuated medium which fills all space, and is the vehicle by which light is transmitted), which is known to be associated with the molecules of matter. Some indeed hold that the ether itself is electricity; and that the two states of positive and negative electrification are simply due to displacement of the ether at the surfaces of bodies. In these lessons we shall avoid as

far as possible all theories, and shall be content to use the term electricity.

8. Charge. The quantity of electrification of either kind produced by friction or other means upon the surface of a body is spoken of as a charge, and a body when electrified is said to be charged. It is clear that there may be charges of different values as well as of either kind. When the charge of electricity is removed from a charged body it is said to be discharged. Good conductors of electricity are instantaneously discharged if touched by the hand or by any conductor in contact with the ground, the charge thus finding a means of escaping to earth or to surrounding walls. A body that is not a good conductor may be readily discharged by passing it rapidly through the flame of a spirit-lamp or a candle; (for the hot gases instantly carry off the charge and dissipate it in the air.

Electricity may either reside upon the surface of bodies as a charge, or flow through their substance as a current. That branch of the science which treats of the laws of the charges, that is to say, of electricity at rest, upon the surface of bodies is termed electrostatics, and is dealt with in Chapter IV. The branch of the subject which treats of the flow of electricity in currents is dealt with in Chapter III., and other later portions of this book.

A B

A B A B

9. Modes of representing Electrification. Several modes are used to represent the electrification of surfaces. In Figs. 6, 7, and 8 are represented two disks- A covered with woollen cloth, B of some resinous body,which have been rubbed together so that A has become positively, B negatively electrified. In Fig. 6 the surfaces are marked with plus (+) and minus (−) signs. In Fig. 7 dotted lines are drawn just outside the posi

Fig. 6.

Fig. 7.

Fig. 8.

tively electrified surface and just within the negatively electrified surface, as though one had a surplus and the other a deficit of electricity. In Fig. 8 lines are drawn across the intervening space from the positively electrified surface to the opposite negative charge. The advantages of this last mode are explained in Art. 13.

con

10. Conductors and Insulators.. -The term 66 ductors," used above, is applied to those bodies which readily allow electricity to flow through them. Roughly speaking, bodies may be divided into two classes - those which conduct and those which do not; though very many substances are partial conductors, and cannot well be classed in either category. All the metals conduct well; the human body conducts, and so does water. On the other hand glass, sealing-wax, silk, shellac, guttapercha, indiarubber, resin, fatty substances generally, and the air, are non-conductors. On this account these substances are used to make supports and handles for electrical apparatus where it is important that the electricity should not leak away; hence they are sometimes called insulators or isolators. Faraday termed them dielectrics. We have remarked above that the name of nonelectrics was given to those substances which, like the metals, yield no sign of electrification when held in the hand and rubbed. We now know the reason why they show no electrification; for, being good conductors, the electrification flows away as fast as it is generated. The observation of Gilbert that electrical experiments fail in damp weather is also explained by the knowledge that water is a conductor, the film of moisture on the surface of damp bodies causing the electricity produced by friction to leak away as fast as it is generated.

11. Other Electrical Effects. The production of electricity by friction is attested by other effects than those of attraction and repulsion, which hitherto we have assumed to be the test of the presence of electricity. Otto von Guericke first observed that sparks and flashes