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 gurrents is dealt with in Chapter III., and other later portions of this book. 9. Modes of representing Electrification. -- Several modes are used to represent the electrification of surfaces.

ABA B

B

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.

10. Conductors and Insulators. The term "conductors," 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

of light could be obtained from highly electrified bodies at the moment when they were discharged. Such sparks are usually accompanied by a snapping sound, suggesting on a small scale the thunder accompanying the lightning spark, as was remarked by Newton and other early observers. Pale flashes of light are also produced by the discharge of electricity through tubes partially exhausted of air by the air-pump. Other effects will be noticed in due course.

12. Other Sources of Electrification. The student must be reminded that friction is by no means the only source of electrification. The other sources, percussion, compression, heat, chemical action, physiological action, contact of metals, etc., will be treated of in Lesson VII. We will simply remark here that friction between two different substances always produces electrical separation, no matter what the substances may be. Symmer observed the production of electrification when a silk stocking was drawn over a woollen one, though woollen rubbed upon woollen, or silk rubbed upon silk, produces no electrical effect. If, however, a piece of rough glass be rubbed on a piece of smooth glass, electrification is observed; and indeed the conditions of the surface play a very important part in the production of electrification by friction. In general, of two bodies thus rubbed together, that one becomes negatively electrical whose particles are the more easily removed by friction. Differences of temperature also affect the electrical conditions of bodies, a warm body being usually negative when rubbed on a cold piece of the same substance. The quantity of electrification produced is, however, not proportional to the amount of the actual mechanical friction; hence it appears doubtful whether friction is truly the cause of the electrification. Something certainly happens when the surfaces of two different substances are brought into intimate contact, which has the result that when they are drawn apart they are found (provided at least

one of them is a non-conductor) to have acquired opposite charges of electrification; one surface having apparently taken some electricity from the other. But these opposite charges attract one another and cannot be drawn apart without there being mechanical work done upon the system. The work thus spent is stored up in the act of separating the charged surfaces; and as long as the charges remain separated they constitute a store of potential energy. The so-called frictional electric machines are therefore machines for bringing dissimilar substances into intimate contact, and then drawing apart the particles that have touched one another and become electrical.

If the two bodies that are rubbed together are both good conductors, they will not become strongly electrified, even if held on insulating handles. It is quite likely, however, that the heat produced by friction, as in the bearings of machinery, is due to electric currents generated where the surfaces meet and slip.

13. Electric Field. Whenever two oppositely charged surfaces are placed near one another they tend to move together, and the space between them is found to be thrown into a peculiar state of stress, as though the medium in between had been stretched. To explore the space between two bodies one of which has been positively and the other negaFig. 9. tively electrified, we may use a light pointer (Fig. 9) made of a small piece of very thin paper pierced with a hole through which passes a long thread of glass. It will be found that this pointer tends to point across from the positively electrified surface to the negatively electrified surface, along invisible lines of electric force. The space so filled with electric lines of force is called an electric field. In Fig. 8 A and B represent two bodies the surfaces of which have been electrified, the one positively, the other negatively. In