DERIVATION OF ELECTRICITY—SEALING-WAX EXPERIMENT—THE ELECTROPHORUS—LEYDEN JAR—POSITIVE AND NEGATIVE—THE ELECTROSCOPE—ELECTRIC MACHINES.

We have now briefly and of course imperfectly reviewed the phenomena of Vibration, as exemplified in what we term Heat, Light, and Sound. We now come to a most mysterious servant of mankind, as mysterious as any Djinn of romance; viz., Electricity.

The term Electricity is derived from the Greek word electron, meaning “amber”; because from amber the properties of what we call “Electricity” were first discovered. Six hundred years before the Christian Era, Thales wrote concerning the attraction which amber, when rubbed, possessed for light and dry bodies. But it is to an Englishman named Gilbert that we owe the word “Electricity,” which he derived from the Greek, and in his works (about 1600 A.D.) he discusses the force of the so-called “fluid.” Otto von Guerike, of “air-pump” celebrity, and many other philosophers after him, continued the investigation of the subject. At the beginning of the last century great attention was paid to the Electric Machine. The Leyden Jar was, as its name denotes, discovered by Muschenbrock, of Leyden, (though the honour was disputed). Franklin made the first lightning conductor in 1760. Volta and Galvani, to whose invention we owe “Voltaic Electricity” and “Galvanism,” and Faraday in more modern times gave a great impetus to electrical science. The great part that electricity has been playing in the domestic history of the world since Faraday’s lamented death, is probably known to the youngest of our readers. What the future of this agent may be we can only guess, but even now we may regard electricity as only in its infancy.

There are few scientific studies more attractive to the general reader than electricity, and few admit of more popular demonstration. The success of the late electrical exhibition in Paris, and its successor in London at the present time, are proofs of the interest taken in this great and mysterious agent whose origin we are in ignorance of, and of whose nature and powers we are daily discovering more and more, and finding there is still an immense field for its application.

Some fundamental facts regarding electricity may very easily be studied with the assistance of every-day objects at hand. Amber was the first substance to show attraction when rubbed, but Gilbert found out that glass and sealing-wax, etc., possessed like properties with amber.

If we rub a stick of sealing-wax with a piece of cloth, we shall see that it will attract some small fragments of paper placed near it. Nothing is easier than to construct a small pendulum to show with perfect clearness the phenomenon of electric attraction. A piece of iron is fixed on a wooden pedestal, and supports a thread of silk, to the end of which is fastened a little ball cut out of a piece of cork. The stick of sealing-wax after being rubbed with the cloth will attract the ball as shown in fig. 200.

By means of a piece of paper we can produce a spark. I take a large, strong sheet of drawing paper, heat it very thoroughly, and lay it on a wooden table. I rub it with a perfectly dry hand, or with a piece of woollen material until it adheres to the table. That done, I place a bunch of keys in the centre of the sheet of paper, which I raise, lifting it by two corners. If at this moment any one touches the bunch of keys with his finger, a bright spark will be elicited. The metal is charged with the electricity developed on the paper; if the weather is dry, and the paper thoroughly heated several times, the spark may attain nearly an inch in length.

We can easily construct other electrical apparatus. For instance, an “Electrophorus,” or instrument for obtaining electricity by means of induction, or a Leyden jar, can both be made at home. Let us proceed to construct the former, of which we give an illustration (fig. 201).

We take a lacquered tea-tray about a foot long, and cut out a sheet of thick wrapping paper, so that it will lie over all the level portion of the tray. At each side of this sheet of paper we fix two bands of paper, as in the illustration (fig. 201), so as to serve as handles. The tea-tray should be placed upon two tumblers to support it and to insulate it, glass being a “non-conductor.” (We will speak of conductors and non-conductors presently.) We have now our Electrophorus made ready for action; let us proceed to see how it will act.

First, rub the thick packing paper over a hot fire or a stove, and the friction must be continued for some time, until the paper has become thoroughly dry, and as hot as possible without charring. When this has been accomplished, place it quickly upon a wooden table, and rub it rapidly and energetically with a clothes’ brush, dry and hard as can be obtained. Place the paper upon the tray; touch the tray with the knuckle, and draw away the paper by the handles fixed to it (see fig. 201); a spark will result. Then if the paper be replaced upon the tray, and the hand again presented, the same result will follow. This experiment may be repeated five or six times, at least, with success.

We have in this tea-tray and its paper covering a real electric machine. How can we manage to provide a Leyden jar to contain our electricity? Nothing is more easy. Let us take a tumbler and partly fill it with shot; insert into the glass a tea-spoon, and if all the articles are quite dry we shall possess a Leyden jar.

To charge the jar we have thus provided we must work the Electrophorus we have already described. While one person lifts off the paper as directed, another must hold the glass filled with shot close to the edge of the tray, and touch the corner with the tea-spoon; the spark will then enter the “jar” or tumbler. We can thus charge the jar as we please, and by presenting the finger as in the illustration (fig. 202), we shall obtain a discharge from it.

Mr. Louis Figuier, in his “Merveilles de la Science,” relates that Wollaston, meeting one of his friends one evening in the streets of London, drew from his pocket a copper thimble, and proceeded to turn it into a microscopic pile.13

In order to do this he removed the bottom of the thimble, flattened it with a stone, so as to bring the two internal surfaces about on a line with each other, then placed between the copper surfaces a small strip of zinc, which was not in contact with the copper, owing to the interposition of a little sealing-wax. He then placed it in a glass cup, previously filled with the contents of a small phial of water, acidulated with sulphuric acid. He next wound round the strip of zinc and its copper covering a piece of platinum wire, the wire becoming red through the electricity developed in the pile. The dimensions of this platinum wire were extremely small; it was only 30/1000 of an inch in diameter, and 1/30 of an inch in length. By reason of its small dimensions it could not only be reddened, but fused by the little battery.

Thus Wollaston’s friend, who was a witness of the experiment, was able to light a tinder at the red wire. In this little battery of Wollaston’s the copper enveloped the strip of zinc in every part; that is to say, the negative element was on a higher surface than the positive metal.

After considering Electricity, it is not impossible to approach the study of Magnetism, and even to construct a mariner’s compass. We shall find the method of doing so by borrowing an interesting passage from the “Magasin Pittoresque.” Let us take a small cork and pass through it an ordinary knitting-needle (fig. 203), which we have already magnetized by placing it N.S., rubbing it gently, and always in the same direction, with one of those little iron magnets with which children amuse themselves. After the needle has been passed through the cork, we also fix into it a sewing-needle, or rather a pin, the point of which rests in one of the little holes in the upper part of the thimble. In order to balance the magnetised needle, we thrust a match into both sides of the cork, as shown in the illustration, and fasten to the ends of each a ball of wax. Thus the needle, the balls, and the pin are all balanced at once, so that the contrivance has the appearance of the illustration.

As it is very important that with such a sensitive instrument any agitation of the air should be avoided, the thimble must be placed at the bottom of a common earthen pan, B D T, which should be covered over with a piece of glass, V. To graduate the compass a circle is described on a piece of paper. On this dial we trace the divisions sufficiently close only at the north extremity of the needle, and the paper is fixed underneath, as in fig. 203. Then we fix a piece of wax at the end of the match pointing N., opposite the northern extremity of the needle inside the basin. In this way we have a very useful and inexpensive compass.

We may also magnetize a fine sewing-needle, and grease it by rubbing it with a little suet. It is then capable of floating on the surface of water running in the direction of the north pole. We might go on multiplying indefinitely examples of physical experiments without apparatus, but we have probably already given a sufficient number to aid our readers in imagining others.

We have now in a simple manner shown how we can easily produce electricity. We may understand that electrical phenomena are produced—(1) friction between different bodies; (2) by placing bodies which differ in contact; (3) by the transition of bodies from one condition to another; (4) by chemical changes; (5) by animals. The two first, and the fourth, are the most usual causes.

We know that certain substances when rubbed with silk or wool acquire the property of attracting other substances. But in the case of a rod of glass or stick of wax, the attraction will only be perceived when the rubbing has been applied. But metal will behave differently. Any part of the metal rod will continue to attract. So metals are Conductors of electricity; while glass, wax, silk, amber, sulphur, etc., are bad, or Non-Conductors. Metals are the best conductors we have, but trees, plants, liquids, and the bodies of animals, including men, are all good conductors of electricity. Dry air is a bad conductor.

There are two kinds of electricity, known as positive and negative (plus or minus), vitreous or resinous. We saw in fig. 200 that we can attract a small ball of pith or cork by a piece of sealing-wax rubbed with flannel. If we then present a glass rod rubbed with silk to it, it will be equally attracted, but will be at once repelled; and after being so repelled, if we put the wax to it, it will be attracted to the sealing-wax again. So wax at first attracts then repels the ball, and so does glass, but either will attract the ball if presented alternately (fig. 204). The reason for this is as follows :—

When we have rubbed the glass with silk, we charge it with positive electricity, and when the rod touches the ball, the latter imbibes that electricity, and flies away from the glass rod. The sealing-wax imparts negative electricity in the same way.

The law is, that bodies charged with the same kind of electricity repel each other, and those containing the opposite kinds attract each other. Positive repels positive; negative repels negative. But positive attracts negative, and negative attracts positive. Opposite electricities unite, and so neutralize each other that no effect is perceived; but it must be borne in mind that all bodies possess both electricities in some quantity, greater or less. By rubbing we separate these electricities, the rubber becoming negative, the rubbee positive. The friction of glass supplies positive electricity, and sealing-wax supplies negative electricity, or we can obtain the same effect by rubbing either with certain material.

The manner in which a body is electrified depends upon its nature and condition; but we may accept as a general axiom,—but by no means as a law,—that when two bodies are rubbed together, that which gets the hotter in the process takes the negative kind of electricity. In the following list the substances have been so arranged that each is negatively electrified by those preceding, and positively by those succeeding it. 1, cat’s skin; 2, glass; 3, woollen stuffs; 4, feathers; 5, wood; 6, paper; 7, silk; 8, shellac; 9, rough glass. We append a list of conducting and non-conducting bodies in their order:—

It should be observed that the degree of value as a conductor or non-conductor depends somewhat upon the atmosphere. For instance, glass is an excellent insulator, or non-conductor, when dry, but when wet it changes to a conductor. So insulators are at times covered with a solution of shell-lac, or fat, to keep away moisture. We may reasonably conclude that bodies which are good and bad conductors are good and bad conductors of electricity. Water is a good conductor, air is a bad one; were it otherwise, electricity would escape from the ground into the air; as it is, the air manages in some degree to retain the electricity at the surface of bodies, for it is on the surface that we find the electric “fluid.”

We have mentioned electrical induction in a former experiment with the tea-tray. We will now explain it more fully, as a consideration of it will bring us to the electric spark, or lightning, with the account of the discovery of the Conductor and the Electrical Machine.

Let us look at the illustration next below. A B is a cylinder supported on a glass rod, and at each extremity is a small pith ball, a and b. The cylinder is in a neutral condition, as is evidenced at first by the pellets being in a vertical position. But suppose we bring a ball, C, towards the cylinder. C is charged with positive electricity, which attracts the negative to itself, and so repels the positive away at the opposite side. So the pellet at one side will be attracted to C, and the other will fly in an opposite direction.

Let us take another illustration. Here we have a horizontal metal rod, cc', insulated on a glass stand. Two balls of cork are attached at both ends of the rod by metallic wires. Hold a rod of resin, r, which has been made negatively electrical, and apply it to one pair of the cork balls. The positive electricity will be attracted at c' and the negative repulsed, and fly away at c. If we remove the resin the equilibrium will be again established, and the balls will fall to a vertical position.

We can also by drawing off the negative electricity by the finger at c, while the resin rod is still held to the other side, c', fill the whole of the metal rod with the positive electricity when the finger and the resin have been removed respectively first and last. The balls will then fly in opposite directions again, in consequence of the repulsion exercised by the positive poles.

The “Electrophore,” or “Electrophorus,” we have already learned to make for ourselves, as also the Leyden Jar. But we give cuts of them. The former is very simple, and can be made by mixing two parts of shell-lac and one of turpentine, and pouring the mixture upon a metal plate. If this be rubbed with a cat’s skin when dry, and a metal cover with a glass handle be placed upon it, it will be found that the positive and negative electricity are collected on the lower and upper surfaces of the plate respectively, and can be drawn away with a spark as before, and made use of.

The Leyden Jar requires a little more detailed description, as it is to it we are indebted for our Battery. It is a common glass bottle or jar, coated both inside and out with tinfoil nearly as high as the shoulder, a a. The mouth should be firmly closed with a bung of wood, g g; a hole should be bored in the bung, through which a brass rod is tightly pushed. The rod, too, is topped by a brass knob, and a brass chain is attached to the other extremity. The interior of the tinfoil receives positive electricity, and the exterior negative when the jar is charged from the “Electrophorus.” To discharge the jar and create a shock it is necessary to put one hand on the outside, and the other on the knob of the jar. A brilliant spark and a severe shock will result if the jar has been fully charged. It is as well to be cautious when trying this experiment. The effect of the shock may be felt by any number of persons joining hands, if one at one end of the row, and one at the other end, touch the knob and the outside of the jar simultaneously.

This electric discharge is lightning in miniature, and it is to Benjamin Franklin that the world is indebted for the discovery. The philosopher was greatly interested in the science of Electricity and, having retired from business, he devoted himself to the consideration of thunderstorms. He wrote a treatise to show that points drew off electricity, and that electricity and lightning were similar. He urged that metallic rods might be attached to ships and buildings, so that during thunderstorms, or at other times, the electricity might be harmlessly carried into the ground. This suggestion he made without being able to explain why points did carry off electricity without a spark. The reason is because there is no place to store it; it runs away at once, without having time to collect, as in a ball.

Franklin made one or two experiments before his renowned kite-flying arrangement, which convinced him that electricity was by no means an agent to be played with. He endeavoured to kill a turkey by electricity, but by incautious handling of the jars in which the “fluid” was stored, he discharged them, and describes the result: “The flash was very great, and the crack was as loud as a pistol; yet my senses being instantly gone, I neither saw the one nor heard the other, nor did I feel the stroke on my hand, though I afterwards found it raised a round swelling where the fire entered as big as half a pistol bullet.” On a subsequent occasion he was again struck senseless while endeavouring to administer a shock to a paralytic patient.

It was not until June 1752 that Franklin made the experiment with the kite, which resulted in such great discoveries. He made his kite of a silk pocket-handkerchief, and he fixed a pointed rod upon the upright portion of the frame at the top; the string ended in a foot or so of silk, which was held by the philosopher, and to the end of the hempen portion of the string a large key was tied. For some time, notwithstanding the approach of most unmistakable thunder-clouds, his patience was tried. But at last the strands of the hempen string began to bristle up, and soon after, when Franklin applied his knuckle to the key, a spark was obtained. The great discovery was made. Franklin subsequently obtained lightning in his own house, and performed several experiments with it.

The Electroscope (fig. 211) is an instrument by which we can ascertain whether electricity is present or not, and the nature of it. If we bring an object unelectrified close to the ball or knob on the top of the glass shade, the two needles, or strips of gold-leaf, which are often used, will remain still. But if the body has been electrified it will communicate the electricity to the rod inside, and attract to itself the fluid of opposite quality; the same kind of electricity then is in action in the gold-leaf or needles, and they fly apart—repel each other. Supposing that positive electricity were first communicated, we can cause the contraction of the leaves or wires by applying a negative kind, which, meeting the positive, neutralizes it, and the wires collapse.

If the electricity with which the instrument is charged be positive, by approaching the baton to the ball, A, we shall see the wires diverge more than before, and they will finally be discharged by the knobs within. If the electricity be contrary to that in the baton, the wires will approach each other, but by gradually withdrawing the baton they will again separate, and even to a greater distance than before.

The Electric Machine is shown in the illustration (fig. 212). It consists of a large plate of glass fixed upon a glass stand, between wooden supports. The handle is of glass; two pairs of rubbers are fastened above and below; the plate is turned between them, and becomes “positively” electrified. The rubbers are covered with leather and stuffed with horsehair, DD, and press very tightly against the glass, so that the friction is constant. The rubbers are covered with an amalgam made of mercury, zinc, and tin, two parts of the first to one each of the others. A chain (of metal) connects the machine with the ground. The conductors, PP, are united by a cross-piece, Q, and sustained upon glass supports. At the end of the conductors are two curved rods, CC, which are provided with points to take the electricity from the plate, but do not touch it.

The electricity is thus stored in the insulated conductors as the machine is turned. The negative portion is carried into the ground by the chain from the rubbers, while the positive electricity is retained. The longer we turn the more we shall obtain, and the quantity is measured by an electric pendulum on one of the conductors, which flies out by degrees as the charge increases, and indicates its power by means of a needle it works upon an ivory index.

It is not difficult to make an electric machine out of a glass bottle. This will furnish the glass cylinder. If a stick be run through it (for which purpose a hole must be drilled in the bottom of the bottle), a handle can be fixed, and the bottle mounted on a stand. A wash-leather cushion, stuffed, can be so arranged that it will press against the bottle as it is turned; a piece of silk should be permitted to hang from the cushion frame over the glass. A conductor may be made from a piece of wood neatly rounded and smoothed, and coated with tinfoil. The ends should be rounded like “knobs.” Stick pins in to collect the electricity (and it will be readily obtained). The cushion should of course be well smeared with amalgam. From this, as well as from the glass-plate machine, the “positive” electricity can be drawn off and stored in a Leyden jar, and then discharged by the “discharging rod,” which is represented on the cut. It may have one or two handles, and one knob is placed outside the jar, the other near the ball surmounting it. The glass being a non-conductor saves the operator, and some long sparks and loud reports may be obtained.

The Electric Machine is always assumed to give off positive electricity.

Sir William Armstrong’s Electric Machine is a mode of obtaining electricity by moist steam. The design is Armstrong’s, and Professor Faraday subsequently went into all the conditions to produce the “fluid” by the friction of steam. The machine was something like a small boiler supported on glass legs. A row of nozzles was fixed upon the escape pipe so as to create a great velocity and friction in the escaping steam. Round the nozzles was a box of cold water, for that fluid was found necessary for the production of electricity as demonstrated by Professor Faraday. The steam rushed against a row of points attached to the prime conductor of an electric machine, and the electricity of the steam was thus given off to the conductor. There are many other forms of electric machines, but it will serve no purpose to detail them.

The Electric Battery (see fig. 206) is formed by a collection of Leyden jars. The inside and outside coatings are connected in a box divided into partitions lined with tinfoil. The rods of the jars are also connected, as in the illustration, by brass rods, and when this battery is charged people should be careful how they handle it, for a shock may be produced which would cause serious injury, if not death. The battery can be charged from the machine by a chain fastened to the central ball, while a second chain connects the exterior of the box and all the outside of the jars, by means of the handle, to the ground.

When electricity is at rest it is termed “static electricity,” and when in motion “dynamic” electricity. The latter treats of electric currents which can be sent through wires or chains. We can keep this current moving by means of a machine, and the battery called a Voltaic battery, from Volta. We will describe it presently. Electric currents can be measured, for they may be of different strengths according to the battery, and they are measured by the Galvanometer. Electricity can therefore be transferred and carried by the conducting substances, and much heat will be engendered as the “electric fluid” passes along a wire. Lightning frequently fuses bell-wires as it passes, and when we touch upon Galvanism or Dynamical Electricity we shall hear more about it.

By the Electric Machine we can obtain some very powerful currents of electricity; we can produce many pleasing effects, and perform a number of experiments, such as making balls or figures of pith dance, and several other easy and entertaining tricks, which will be found in books more specially devoted to the entertainment of young people.

We have now given some explanation of the manner in which electrical phenomena can be produced,—viz., by the Electric Machine and by the Leyden Jar,—but we must not expect to find any electricity inside any charged body. It has been proved that all the electricity is upon the surface of bodies, even if in varying quantity, and that equal quantities of electricity are always produced when bodies are excited by friction, but the kinds are different. The rubbing body is of one kind, the body rubbed another, and consequently the forces neutralize each other. The two forces or kinds of electricity we have seen repel or attract each other, and we can imagine the farther they are apart the less will be the force, and the rate of diminution of force, according to distance, is ascertained by an ingenious apparatus called a “Torsion” Electromoter, which was constructed by Coulomb, and was frequently used by Faraday.

Perhaps some people may not be aware of the term “torsion.” It means twisting, and “the torsion of a thread suspended vertically is the force tending to twist the lower extremity when the upper end is turned through an angle.” This instrument is really an Electromoter, and is not considered suited to beginners, and it is scarcely accurate in its workings. We need not therefore describe it in detail. There are some excellent Electromoters, the Elliott being, we believe, the best for use. A full and detailed description of the Quadrant Electromoter will be found in Mr. Gordon’s treatise on Electricity.

Recapitulation of foregoing Chapter. So far, we have seen there is electricity in everything, although some bodies are termed conductors and others non-conductors; though, as in applying the terms heat and cold, we must remember that no body is entirely devoid of electricity, and no body is therefore an absolute non-conductor any more than any object is absolutely devoid of heat. Faraday, indeed, was of opinion that “conduction and insulation are only extreme degrees of one common condition”; they are identical both in principle and action, except that in conduction an effect common to both is raised to the highest degree, and in the case of insulation it occurs in an almost insensible quantity.

We have also read of positive and negative electricities, and we must not fancy there is any particular reason for this distinction. It was Du Fay, whom we have mentioned, who gave the names “vitreous” and “resinous” to the two kinds, as one was developed by rubbing glass, and the other by rubbing resin. But, as shown by our experiments, either kind of electricity can be excited in glass or sealing-wax, and both kinds are produced at once. You cannot get “positive” without negative electricity. “Positive” is the term applied to the kind produced by rubbing glass with silk or wool; “negative” is the term applied to the kind developed by rubbing sealing-wax, but the kind developed by friction depends on the rubbing substance and certain conditions.

Bodies charged with the same electricity repel; if charged with different kinds they attract each other. The more readily displaced particles when bodies are rubbed become negatively electrified as a rule.

Similar electricities repel each other with a force inversely proportional to the squares of the distance between their centres, as established by Coulomb. So if the space between any two similarly electrified bodies be reduced by say one-half, the force of the repulsion will be increased four times. The rule for attraction is similar; so when two bodies are charged with opposite electricities, and the distance between them is increased, the attractive force is diminished in proportion as the square of the distance between them. Many confirmations of this theory were made by the late friend of our boyhood, Sir W. Snow Harris, and published in the Philosophical Transactions.

The following full list of conductors and non-conductors (copied from Professor Noad’s Text-book of Electricity, and compared with De La Rive’s Treatise) may be useful:—

The following may be added to the Insulators, viz.:—

There are, as we know, two kinds of electricity, the static and dynamic; and when the latter state is instantaneous, it is referred to as the “electric discharge,” which occurs when opposite electricities seek each other, and the bodies return to a state of equilibrium or neutralization. “These bodies, if insulated, obtain no more electricity after the spark has passed; but if there be a constant source of negative electricity supplying one, and a constant source of negative electricity supplying the other, there will be a succession of sparks; and if they communicate by a conductor there will be, through this conductor, an uninterrupted neutralization of a continual reunion of the two electricities, and this is what is termed the continuous dynamic state or electric current” (De la Rive).