medium of induction, suggests that the molecules of air and other dielectrics are conducting, but that they are insulated from each other. We have already seen that by induction, part of the electricity of an insulated body can be in effect transferred to a surface at some distance from it, without any loss experienced by the exciting body. If, now, we could imagine a series of insulated cylinders diverging in all directions from the glass tube, we have reason to expect that the whole of the electricity of the tube would be in effect transferred to their outer extremities without loss of electricity to the tube. To prove that such would be the case, Faraday took a pewter ice-pail, 10 in. high, and 7 in. in diameter, and insulated it, placing the outside of it in conducting connection with the knob of a gold-leaf electroscope. An insulated ball, charged with electricity, was then introduced into it without touching. The pail was thus subjected to polarization, the electricity being on the inner, and the electricity on the outer surface. The divergence of the leaves increased as the ball was lowered, until it sunk 3 in. below the opening, when they remained steadily at the same points. The ball was lowered till it touched the bottom and communicated its charge to the pail, when the leaves remained in the same state as before, showing that the + electricity developed by induction on the outer surface was exactly the same in amount as that of the ball itself. He then altered the experiment so as to have four insulated pails inside each other, and the effect on the outmost pail was in no way altered. Here the action of the air between the pails was in effect the same as that of the pails themselves, and if the molecules of air were insulate conductors like these, they would have acted in no way different from what they did The action of the molecules of air, in certain circumstances, appears to favor the ide that they are individually conducting. The discharge of electricity by spark through the air, shows that they can be forced to act as conductors; and the currents which proceed from points highly charged with electricity, appear to indicate that they can be attracted and repelled like the pith-balls of our first experiment.

Conductors, according to this theory, are bodies whose molecules have the power of communicating their electricities to each other with great ease, whilst non-conductors are those whose molecules only acquire this power under great force. Wheatstone has shown, as we shall afterwards see, that facility of discharge is not perfect even in the best conductors, as time is needed for its propagation, and it has been found that the terminal lamina of non-conductors between two charged plates become penetrated with opposite electricities, which indicates the slow progress of conduction. The molecules of conductors and non-conductors, therefore, have the same power of mutual discharge, but in very different degrees, so that a good non-conductor may be regarded as an excessively slow conductor.

Potential, Density, Tension, Capacity.—Some idea of the meaning of the word potential may be got from the following comparison. Suppose we have a supply of water with a certain head, to fill an elastic bag when the water is admitted, the bag will swell till the elasticity of the bag is equal to the head of water, and then the flow will cease. The potential is the head of water or elasticity of the bag, so many feet high, or so many pounds per square inch. The capacity of the bag is usually the amount it holds, but capacity in an elastic bag is a shifting quantity, and we must use the term in this way if we wish to compare the capacity of two elastic bags-viz., the ratio of the water it holds to the head that filled it. Thus, a bag holding 10 galls., with a head 1 ft., would have a 10 times greater capacity than a bag holding 10 gails. with a head 10 ft.; for if the first were pressed by a head of 10 ft., it would hold 100 galls., the resistance of the bag being supposed to increase with its contents. Now, let us take a somewhat similar electric problem. An insulated ball is connected with a magazine of energy, ready to make electricity flow when occasion offers, such as a galvanic battery. Let the+pole of a gigantic battery be connected with the ball, and the other pole with the ground, electricity will flow to the ball till the air between the ball and the ground presents an electric reaction equal to the potential of the battery. The charge of the ball taken with reference to this potential gives the capacity of the ball. So much, then, for a popular view of these two words. The potential of a body, or any point in the field, is defined thus-viz., the amount of work that would be expended in bringing a small quantity, a unit of elec tricity, from an infinite distance to the body or point. If the body is positive, the work would be expended; if negative, the work would be done on the body and the potential -. The said unit of electricity will always move from a point where the potential is high to one where it is lower; in other words, electricity will always flow between two points where there is a difference of potential, and will cease to flow when that difference ceases. If E be charge, V the potential, C the capacity, then C E ÷ V. From the definition of potential just given, what we have called the potential of the battery in the preceding illustration is in reality its electro-motive force, or the difference of potentials of it's poles. As these are alike in power, but different in sign, and as the difference of two quantities of unlike sign is their sum, the electro-motive force is twice the potential of one pole. If the charging line be withdrawn, the ball will be in all respects as if charged by an electric machine. The battery having, so long as it acts, an unlimited supply of electricity, its electro-motive force remains the same; but when balls charge one another, the potential falls just as when a limited supply of water has its head reduced when made to run into another vessel. Potential, then, must be estimated by the resistance of the field, or the work value of the unit of charge. The charge being the same, the potential

rises with the smadness of the body, or the thickness of the dielectric. Density is the quantity of electricity on a unit of surface, and tension is the strain which Faraday supposes to exist in the molecules of a dielectric when charged. Tension is commonly used in Gt. Britain and abroad for potential, though our best writers never use it now in this

sense.

Distribution of Electricity.-We might take it almost as a self-evident truth, that the greater the surface over which electricity is diffused, the less is its electric potential at any particular point, and so we are taught by experiment. When two equal balls are insulated, and a charge is given to one of them, and then communicated to the other by contact with the first, it is found that both equally divide the charge, but that the potential of the electricity of each is one half of that of the originally charged ball. When a watch guard-chain is charged and laid on the plate of an electroscope by means of a glass rod, the gold leaves diverge most when the chain lies in a heap on the plate; and as it is lifted up, the leaves approach each other, showing that as the exposed surface of the chain increases, the electric potential of each part diminishes. The reason of this is obvious. Let us begin with one ball with a certain charge, then take another equal ball and impart half the charge to it by making the two touch. A spark will be seen at the charge of the second ball. The quantity in both is still the same, but energy has been lost by the spark, and the heat generated by the spark is the measure of the loss. If we continue to add ball after ball until we have a very large surface, the quantity is the same as at first, but energy has been squandered in the sparks of each additional ball, and so the potential is lowered.

Experiment teaches us, that electricity is exhibited only on the surfaces of conductors. A brass ball is suspended by a silk thread and covered with two hemispheres, which can be held by insulating handles, and which exactly fit it. A charge is then communicated to the ball so compounded. When the hemispheres are withdrawn, they are found to take away all the electricity with them, not the slightest charge being left in the ball. The same fact is exhibited by a hollow ball placed on a glass pillar, with a hole in the top large enough to admit a proof plane to the inside. When charged, not the faintest evidence of electricity is found on the inner surface, however thin the material of the ball may be. The thinnest metal plate, when under induction, shows opposite electricities on its two faces. We learn from these and numerous other experiments, that electricity is only found on the outer surfaces of conductors in an envelope of inappreciable thickness. This fact is quite in keeping with Faraday's theory of the action of dielectrics. Within a conducting body we cannot expect electricity, for the moment it appears in it, the particles communicate their electricities to each other, and the electric state ceases. In a dielectric they cannot communicate, and the charge remains. Hence the charge at the conductor only appears at the junction of a conductor and dielectric.

We are also taught by experiment that the distribution of electricity on the surface of insulated conductors is influenced materially by their form. An electrified ball, for example, exhibits the same density all round, for the resistance is sensibly the same on all sides of it. When, however, a conducting body is made to approach near enough to it, the density of the electricity is found to be greater on the side on which the approach is made. This is proved by the aid of a proof plane and an electrometer. When work is done in drawing away the proof planes from the charged body, its potential, as tested by the electrometer, is proportional to the density of the charge at the point where it touched. The reason of this unequal distribution is obvious, from the fact that the potential of the ball must be the same at every point. If, therefore, the resistance at one side be less than at another, the density there must be greater to maintain equality of potential. The disturbance of equal distribution here spoken of holds true only for short distances; the disturbing body, for instance, in the case under consideration, has to be brought very near before any inequality in the distribution of electricity on the ball becomes manifest. It is to this concentration of electricity on the side of the approaching conductor that we owe the electric spark; and it is as we near the striking or sparking distance that this disturbance is revealed. The concentration or fixing of electricity on the side of the thinnest and best dielectric, is particularly illustrated in the condenser (q.v.) and Leyden jar, whose action depends upon it; but in these the dielectric must be very thin to secure decided effect. When a conductor somewhat in the form of a prolate spheroid is charged, and the electric density of the several parts tested by the proof plane, it is found to be least at the thickest part, and to increase.towards either end; and the difference is found to be all the greater as each end becomes more and more pointed. It is found likewise that the electric density on a point is so great with a considerable charge as to destroy the dielectric condition of the air, the particles of which become electrified and carry by convection the charge of the point to surrounding conductors. We therefore learn that electricity concentrates on points and projections. A similar reasoning with regard to the relations of potential resistance and consequent density bears here as in the previous case. It may be here remarked that the density of charge at any point regulates the amount of tension at that point on the molecules of the dielectric. The constraint which they experience in being charged, and which Faraday calls tension, can only be carried to a certain limit. When that is reached, the molecules are forced to be conducting, and the tension ceases.

Static electricity for experimental purposes is now obtained from machines which ap

ply friction to suitable substances, such as glass, rubber, etc. In a tube of glass and silk
rubber we have the embryo of the electric machine, viz., a body which, when rubbed, is
positively electrified, and its rubber negatively. The first requisite we should expect in
a machine of this nature is a large surface, to give a great amount of electricity. But
there is another already casually referred to; glass being a non-conductor, the electricity
formed on its surface has not a combined action, so that some arrangement is necessary
to collect it, and render it available-to act, in fact,
as its conducting reservoir.

The

The

工具

FIG. 1. Electric Machine.

They

This portion of the machine is denominated the prime conductor. The rubbed surface of the electric machines is either a cylinder or plate of glass; hence we distinguish them into cylinder machines and plate machines. The former, from their more compact form, are the more manageable; and the latter, from both sides of the glass plate being rubbed, are the more powerful forms of the instrument. The description of Winter's plate machine (fig. 1) will be quite sufficient to show the general requirements and construction of electric machines. It is one of the best existing forms of the machine. The glass plate is turned on the axis ab by means of the handle c. The longer end of this axis, consisting of a glass rod, moves in the wooden pillar d, and the other rests in the wooden head of the glass pillar e. plate is thus completely insulated, and little loss of electricity takes place through its supports. two rubbers are triangular pieces of wood, covered with a padding of one or two layers of flannel enclosed in leather, and they present a flat, hard surface to the glass, so that friction between it and them takes place in every part. are placed in a wooden frame on each side of the plate, and the pressure is regulated by metal springs fixed to the outside between them and the frame. Before use they are covered with an amalgam of mercury, zinc, and tin, which is made to adhere with the aid of a little grease, and which increases immensely the production of electricity. The surfaces of the rubbers are therefore conducting, and are made to communicate by strips of tin-foil with the negative conductor, f (fig. 1). To limit the electric field in the neighborhood of the negative conductor, or, which is the same thing, to keep the potential of the glass from rising too high, so as to cause a discharge back into the rubbers, each rub ber has a non-conducting wing fastened to it, which is made of several sheets of oiled silk, kept together by shellac varnish, beginning at the rubber with several, and ending with one or two sheets. When the machine is in action, electrical attraction makes them adhere to the plate; but when it is out of action, they may be kept up by a split pin g. As the plate turns, the rubbers are kept in the frame by their ledges h. The whole framework of the rubbers and negative conductor is supported by the short glass pillar i, so that it can be insulated when required. The prime conductor, k, is a brass ball insulated on the long glass pillar, l, and to prevent the edges of the ball at the junction dissipating the electricity, the pillar enters the ball by a trumpet-shaped opening. The collection of the electricity from the glass is made by a row of points placed in the grooves, inside of two wooden rings, m, m, which are attached on each side of the plate to a piece of brass projecting horizontally from the ball of the conductor. The grooves are covered with tin-foil, which conveys the collected electricity to the ball, and the points are kept out of grooves. the way of injury by not projecting beyond the

Electrometers and Electroscopes.-These words are generally taken as synonymous; electroscopes, however, should be applied to the instruments which give evidence of electrical potential without giving the exact measure of it; and electrometers to such as show both. Of late years, immense progress has been made in the construction of delicate electrometers, chiefly to meet the demands for such in the working or testing of submarine cables. Sir William Thomson's quadrant electrometer and his absolute electrometer, in point of exactness and delicacy, are a hundredfold in advance of previous instruments. We shall here, meanwhile, describe the common forms of electric indicators. The quadrant electrometer consists of a conducting-rod, generally of box-wood or brass, with a graduated semicircle attached above, in the center of which is a pivot for the rotation of a straw carrying a pith-ball at its outer end. It is used for a charge of high potential, such as that of the electric machine. When placed on the prime conductor of the machine, the whole becomes charged with + electricity; the ball is repelled first by the electricity of the rod, then by that of the prime conductor, the height to which it rises being seen on the semicircle. This is not an electrometer in the strict sense of the word, for although it tells us, by the straw rising and falling, when one potential is greater or less than another, it does not tell us by how much, the conditions of its repulsion being too complicated for simple mathematical expression. It can show us, however, by the indicator standing at the same point, when the electric potential of the machine is the same at one time as another.

Electric Lighting.

342

Leyden Jar.-This is a glass jar, with a coating of tin-foil pasted carefully inside and out, extending to within a few inches of the mouth. This last is generally closed by a wooden stopper, through which passes the stalk of a brass knob or ball surmounting the whole. The connection between the inside coating and the ball is completed by a chain extending from the stalk to the bottom of the jar. If this jar be put on an insulating stool, so that sparks can pass from the prime conductor of a machine to the knob, when the jar is thus insulated, one or two sparks pass, and then the charge seems complete, for no more sparks will follow, though the action of the machine is continued; or, if they do, they are immediately dissipated from the knob in a brush discharge. If, then, however, the knuckle of the experimenter be brought near the outer coating, sparks begin again to pass freely; and for every spark of electricity that passes between the machine and the knob, a corresponding spark of the same name passes between the knuckle and outer coating. This continues for some time, and then the jar appears to be again saturated. It is now said to be fully charged. The outside of the jar can, in this state, be handled freely, and if it be still on the insulating stool, so may also the knob, although, when the hand first approaches, it receives a slight spark. But if, when the experimenter has one hand on the outer coating, he bring the other hand to the knob, before it can reach it, a straight, highly brilliant spark passes between the knob and his hand, and he experiences a shock of great violence. If he try the same thing again, a feeble spark and shock again ensue, and the jar is now thoroughly discharged. As it is highly inconvenient, if not dangerous, to discharge the jar through the body, discharging tongs are used for that purpose, which consist of two brass arms ending in balls, and moved on a hinge by glass handles.

The velocity of electrical discharge is found to be about 192,000 m. per second.

ELECTRICITY, STORAGE Of. an electric current. A storage of energy which is able to directly produce so-called storage battery or accumulator when acted upon by a current undergoes a cerThe term is a misnomer, as electricity itself cannot be stored. A tain chemical change. The chemicals which are thus separated recombine again when the circuit of the battery is closed, and in uniting give off a current of electricity about equal to that by which they were decomposed. Lead is the metal most commonly used in accumulators, the positive plate having a coating of lead peroxide, PbO2, and the negative plate a surface of spongy lead.

The idea of electrical storage may be traced back to 1801, at which time Gautherot showed that platinum wires used in the electrolysis of saline solutions developed secondary currents. Later, Ritter constructed a secondary pile of copper discs separated by cloths moistened with a solution of sal ammoniac. By charging this a few moments with a powerful galvanic battery, the pile gave a strong shock. Volta, Becquerel and others discovered that other metals, as gold, silver, and platinum, gave secondary electric currents when subjected to electrolytic action in certain solutions. In 1842, Grove produced his celebrated gas battery, which gave a current by means of the difference in polarity of oxygen and hydrogen, the constituents of water. With a battery of 50 gas cells Grove produced arc lights, electrolysis, and many well-known effects. In Faraday's "Researches" he mentions the high conductivity of peroxide of lead and its power of giving up its oxygen. that peroxide of lead was produced at the positive, and spongy metallic lead at the negaHe also discovered, in electrolysing a solution of acetate of lead, tive pole. Gaston Planté was the first to apply this principle to a secondary cell, which was afterwards developed and modified by Faure and others.

A great deal of attention has been given to the nature of the chemical reactions in the lead-sulphuric acid cell, and it cannot be said that these chemical changes are well understood yet. In a general way, however, they are sufficiently well known to enable manufacturers to produce cells of any desired capacity and rate of discharge. Storage batteries may be divided into two general classes: Those in which the active material (peroxide of lead) is formed on the surface of the plates by chemical or electro-chemical action, and those in which some easily reducible salt of lead is applied mechanically. The former are known as the Planté type, and the latter as the "pasted" or Faure type. The Planté cell is the simplest form of storage battery. The earliest cells were formed of two lead plates immersed in a dilute solution of sulphuric acid in water. The solution should have a specific gravity of 1.17 before charging, and as the charge proceeds the specific gravity increases to 1.195 at full charge. At each successive charge the peroxide formed on the positive plate sinks deeper into the metal, and this action continues until the metal is covered to a sufficient thickness to protect the lead from electrolytic action. There is no difficulty in forming the positive plate in a Planté cell, but with the negative plate the action is very slow. The latter is the great difficulty with all Planté cells. The usual method of forming the spongy lead is to charge the cell, allow it to rest, then reverse the charge through the cell. At each reversal of current the peroxide is liberated at the surface, leaving metallic lead in a very finely divided state. The voltage of a lead-sulphuric acid cell is about 2 volts. The above description is applicable in a general way to all cells of the Planté type, of which there are a great many varieties. Most of the modifications introduced by different manufacturers are mechanical changes with a view to exposing more surface of lead to the action of the electrolyte. Many of these plates are corrugated, built up of very thin sheets slightly separated, grooved, etc., and are too numerous to describe in detail. The Planté accumulator is a very efficient

cell when once formed, but the great amount of time it requires for forming is its chief drawback.

Pasted cells of the Faure type are now the most important and generally used cells. To avoid the great loss of time used in forming the Planté cells, Faure devised the method of pasting a layer of chemically prepared oxide of lead to the surface of the plates. This was done by spreading the plates with minimum, or litharge, made into a thick paste by the addition of acidulated water. After drying, these plates were placed A piece of in a bath of dilute sulphuric acid, and then subjected to an electric current. felt was placed between the plates to prevent the lead salts from disintegrating. After charging, the salt on positive plate is reduced to peroxide of lead, while that on the negative plate is converted into porous lead. The chief fault of the early Faure cells was the disintegration of the active material, which would drop away from the plate. Many methods have been devised for holding the active material on the plates, the most common of which is to cast a grid, or plate with cells or perforations, into which the active material is pressed. All of the modern cells are made with perforated plates of this description. Besides these two types of accumulators, which are the most important ones, there are a number of others, in which the elements are composed of lead-zinc, copper, etc., none of which are in very extensive use.

Its

The use of the storage battery has grown considerably in recent years, and yet it is still very far from being a satisfactory article. Its efficiency varies from 60 to 75 per cent., and its depreciation may be very rapid if it is not operated with great care. use in electric light stations to take extra loads and light all-night loads, has been found advantageous, and most of the large central stations have auxiliary storage plants. It is very important not to discharge accumulators faster than the rate for which they are built, as it results in the speedy destruction of the plates. They are not well adapted to traction purposes, as the motion of the car jars out the active material from the plates, and in starting the car a high rate of discharge is required. The weight and bulk of accumulators is also against their use on cars.

as a

The first practical application of electricity was ELECTRIC LIGHTING. means of furnishing artificial light, and this has been the most extensively developed of its uses. The first electric light was that made by sir Humphry Davy in 1809, at the Royal institution in London, when on separating the ends of the wires leading from a battery of two thousand cells, which was the largest battery that had ever been conThis was due to the heating of the ends of the wire structed, a brilliant light was seen.

by the passing current.
were made to give light in this way, with
a simple apparatus illustrated in fig. 1,
but electricity was too expensive to allow
lights so produced to be used to any
extent until the invention of dynamo
generators of electricity, about 1870.
Then Jablochkoff introduced his electric
candle, which was seen by all the world

in Paris in 1870. And although it did not furnish a means of producing an economical light, it did a great deal to awaken public interest in the new means of illumination. The Jablochkoff candle was immediately followed by a number of other lamps of greater simplicity, and consequently of great commercial value. These improvements in the lamp, together with the improvements in dynamos, have advanced the art of electric lighting to a position of successful competition, with all other methods of artificial illumination. The early forms of arc lamps possessed very bad features of regulation, that is, the method of feeding the carbons so that the arc is maintained constant as the carbon burns away. Lamps are generally made with the lower carbon stationary and the upper one to feed downwards. The feeding is operated either by a train of wheel-work, electrical or mechanical motors, gravity, or the action of a solenoid. When the lamp is not in operation, the upper carbon falls and rests upon the lower one,