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which the electric equilibrium is destroyed by friction; then the positive and negative electricities are called into action or separated; the positive is impelled in one direction, and the negative in another. Electricities of the same kind repel, whereas those of different kinds attract each other. The attractive power is exactly equal to the repulsive power at equal distances, and when not opposed they coalesce with great rapidity and violence, producing the electric flash, explosion, and shock; then the equilibrium is restored. One kind of electricity cannot be evolved without the evolution of an equal quantity of the opposite kind. Thus when a glass rod is rubbed with a piece of silk, as much positive electricity is elicited in the glass as there is negative in the silk. The kind of electricity depends more upon the mechanical condition than on the nature of the surface; for when two plates of glass, one polished and the other rough, are rubbed against each other, the polished surface acquires positive and the rough negative electricity. The manner in which friction is performed also alters the kind of electricity. Equal lengths of black and white ribbon applied longitudinally to one another, and drawn between the finger and thumb so as to rub their surfaces together, become electric. When separated the white ribbon is found to have acquired positive electricity, and the black negative; but if the whole length of the black ribbon be drawn across the breadth of the white, the black will be positively and the white negatively electric when separated. The friction of the rubber on the glass plate of the electrifying machine produces abundance of static electricity. The friction of the steam on the valve of an insulated locomotive steamengine produces seven times the quantity of electricity that an electrifying machine would do with a plate three feet in diameter, worked at the rate of 70 revolutions in a minute. Pressure is a source of electricity which M. Becquerel has found to be common to all bodies; but it is necessary to separate them to prevent the reunion of the electricities. When two substances of

any kind whatever are insulated and pressed together they assume different electric states, but they only show contrary electricities when one of them is a good conductor. When both are good conductors they must be separated with extreme rapidity to prevent a return to equilibrium. When the separation is very sudden the tension of the two electricities may be

great enough to produce light. M. Becquerel attributes the light produced by the collision of icebergs to this cause. Iceland spar is made electric by the smallest pressure between the finger and the thumb, and retains it for a long time. All these circumstances are modified by the temperature of the substances, the state of their surfaces and that of the atmosphere. Several crystalline bodies become electric when heated, especially tourmaline, one end of which acquires positive, and the other negative electricity, while the intermediate part is neutral.. If the tourmaline be broken through the middle, each fragment is found to possess positive electricity at one end and negative at the other. Electricity is evolved by substances passing from a liquid to a solid state, and by chemical action during the production and condensation of vapour, which is a great source of atmospheric electricity. In short, it may be generally stated, that when any cause whatever tends to destroy molecular attraction there is a development of electricity; if, however, the substances be not immediately separated, there will be an instantaneous restoration of equilibrium.

Electricity may be transferred from one body to another in the same manner as heat is communicated, and like it too the body loses by the transmission.

Although no substance is altogether impervious to electricity, nor is there any that does not offer some resistance to its passage, yet it moves with more facility through a certain class of substances called conductors, such as metals, water, the human body, &c., than through atmospheric air, glass, silk, &c., which are therefore called non-conductors. The conducting power is affected both by temperature and moisture. The terrestrial globe is a conductor on account of its moisture, though dry earth is not. Though metals are the best conductors of electricity, it affects their molecular structure, for the heat which accompanies its passage acts as a transverse expansive force, which increases their breadth by diminishing their length, as may be seen by passing electricity through a platinum wire sufficiently thick to resist fusion. Through air the force is disruptive on account of its non-conducting quality, and it seems to act chemically on the oxygen, producing the substance known as ozone during its passage through the atmosphere. If a conductor be good and of sufficient size the electricity passes imperceptibly

but it is shivered to pieces in an instant if it be a bad conductor or too small to carry off the charge. In that case the physical change is generally a separation of the particles, or expansion from the heat, as in trees, where it turns the moisture into steam, but all these effects are in proportion to the obstacles opposed to the freedom of its course.

Bodies surrounded by non-conductors are said to be insulated, because when charged the electricity cannot escape. When that is not the case, the electricity is conveyed to the earth: consequently it is impossible to accumulate electricity in a conducting substance that is not insulated. There are a great many substances called non-electrics in which electricity is not, sensibly developed by friction unless they be insulated, because it is carried off by their conducting power as soon as elicited. Metals, for example, which are said to be non-electrics can be excited, but being conductors they cannot retain this state if in communication with the earth. It is probable that no bodies exist which are either perfect non-electrics or perfect non-conductors. But it is evident that electrics must be non-conductors to a certain degree, otherwise they could not retain their electric state.

A body charged with electricity, although perfectly insulated, so that all escape of electricity is prevented, tends to produce an electric state of the opposite kind in all bodies in its vicinity. Positive electricity tends to produce negative electricity in a body near to it, and vice versá, the effect being greater as the distance diminishes. This power which electricity possesses of causing an opposite electrical state in its vicinity is called induction. A Leyden jar, for example, or glass jar coated half way up both outside and in with tin foil, when charged with positive electricity, immediately induces negative electricity on the tin foil outside. Notwithstanding their strong mutual attraction they are prevented from coalescing by the glass, which is a non-conductor; but if the tin inside and out be connected by a conducting wire they instantly unite. When a body in either electric state is presented to a neutral one, its tendency in consequence of the law of induction is to disturb the condition of the neutral body by inducing electricity contrary to its own in the adjacent side, and therefore an electrical state similar to its own in the remote part. Hence the neutrality of the second body is destroyed by the action of the first, and the adjacent parts of the

two, having now opposite electricities, will attract each other. The attraction between electrified and unelectrified substances is a consequence of the altered state of their molecules. Induction depends upon the facility with which the equilibrium of the neutral body can be overcome, a facility which is proportional to its conducting power. Consequently the attraction exerted by an electrified substance upon another substance previously neutral will be much more energetic if the latter be a conductor than if it be a non-conductor.

It is clear that one body cannot act upon another at a distance without some means of communication. Dr. Faraday has proved that the intervening non-conducting substance or dielectric has a great influence upon induction. Thus the inductive force is greater when sulphur is interposed between the two bodies than when shellac is the dielectric, and greater when shellac is the dielectric than glass, &c. Professor Matteucci has proved by the following experiment that the intervening substance is itself polarized by induction. A number of plates of mica in contact were placed between two plates of metal, one of which was electrified, so that the whole was charged like a Leyden jar. On separating the plates with insulating handles, each plate of mica was electrified; one side of it was positive and the other negative, showing decidedly a polarization by induction. throughout the whole intervening non-conducting substance; and thus, although the interposed substance or dielectric is incapable of conducting the electrical force from one body to the other, it becomes by induction capable of transmitting it. In the atmosphere induction is transmitted by that of the intervening strata of air. It is true that induction takes place through the most perfect vacuum we can make, but there always remains some highly elastic air; and even if air could be altogether excluded, the ethereal medium cannot, and it must be capable of induction, since, however attenuated, it must consist of material atoms, otherwise it would be a nonentity.

The law of electrical attraction and repulsion has been determined by suspending a needle of gum-lac horizontally by a silk fibre, the needle carrying at one end a piece of electrified gold leaf. A globe in the same or opposite electrical state when presented to the gold leaf will repel or attract it, and will therefore cause the needle to vibrate more or less rapidly according

to the distance of the globe. A comparison of the number of oscillations performed in a given time at different distances will determine the law of the variation of the electrical intensity, in the same manner that the force of gravitation is measured by the oscillations of the pendulum. Coulomb invented an instrument which balances the forces in question by the force of the torsion of a thread, which consequently measures the intensity;. and Sir William Snow Harris has constructed an instrument with which he has measured the intensity of the electrical force in terms of the weight requisite to balance it. By these methods it has been found that the intensity of electrical attraction and repulsion varies inversely as the square of the distance. However, the law of repulsive force is liable to great disturbances from inductive action, which Sir William Snow Harris has found to exist not only between a charged and neutral body, but also between bodies similarly charged; and that, in the latter case, the inductive process may be indefinitely modified by the various circumstances of the quantity and intensity of the electricity and the distance between the charged bodies.

The quantity of electricity bodies are capable of receiving does not follow the proportion of their bulk, but depends principally upon the form and extent of their surface. It appears from the experiments of Sir W. S. Harris that a given quantity of electricity, divided between two perfectly equal and similar bodies, exerts upon external bodies only one fourth of the attractive force apparent when disposed upon one of them; and if it be distributed among three equal and similar bodies, the force is one ninth of that apparent when it is disposed on one of them. Hence, if the quantity of electricity be the same, the force varies inversely as the square of the surface on which it is disposed; and if the surface be the same, the force varies directly as the square of the quantity of electricity. These laws however do not hold when the form of the surface is changed. A given quantity of electricity disposed on a given surface has the greatest intensity when the surface has a circular form, and the least intensity when the surface is expanded into an indefinite straight line. The decrease of intensity seems to arise from some peculiar arrangement of the electricity depending on the extension of the surface. It is

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