Popular Science Monthly/Volume 9/June 1876/Lessons in Electricity III
|LESSONS IN ELECTRICITY.|
HOLIDAY LECTURES AT THE ROYAL INSTITUTION.
By Prof. TYNDALL, F. R. S.
SECTION 13. Electric Induction.—We have now to apply the theory of electric fluids to the important subject of electric induction.
It was noticed by early observers that contact was not necessary to electrical excitement. Otto von Guericke, as we have already seen, found that a body brought near his sulphur globe became electrical. By bringing his excited glass tube near one end of a conductor, Stephen Gray attracted light bodies at the other end. He also obtained attraction through the human body. From the human body, also, Du Fay, to his astonishment, obtained a spark. Canton, in 1753, suspended pith-balls by thread, and, holding an excited glass tube at a considerable distance, caused them to diverge. On removing the tube the balls fell together, no permanent charge being imparted to them. Such phenomena were further studied and developed by Wilcke and Æpiuus, Coulomb and Poisson.
These and all similar results are embraced by the law that, when an electrified body is brought near an unelectrified one, the neutral fluid of the latter is decomposed, one of its constituents being attracted, the other repelled. When the electrified body is withdrawn, the separated electricities flow again together and render the body unelectric.
This decomposition of the neutral fluid by the mere presence of an electrified body is called induction. It is also called electrification by influence.
If, while it is under the influence of the electrified body, the body influenced be touched, the free electricity (which is always of the same kind as that of the influencing body) passes away, the opposite electricity being held captive.
On removing the electrified body the captive electricity is set free, the conductor being charged with electricity opposite in kind to that of the body which electrified it.
You cannot do better here than repeat Stephen Gray's experiment. Support a small plank upon a warm tumbler, and bring under one of its ends and near it scraps of light paper or of gold-leaf. Excite your glass tube vigorously, and bring it over the other end of the plank, without touching it. The ends may be six or eight feet apart; the light bodies will be attracted. The experiment is easily made, and you are not to rest satisfied till you can make it with ease and certainty.
This is a fit place to say that you must keep a close eye upon the tumblers you employ for insulation. Some of them, made of common glass, are hardly to be accounted insulators at all. We shall prove this.
Our mastery over this subject of induction must be complete, for it underlies all our subsequent inquiries. Without reference to it nothing is to be explained; possessed of it you will enjoy, not only a wonderful power of explanation, but of prediction. We will attack it, therefore, with the determination to exhaust it.
And here a slight addition must be made to our apparatus. We must be in a condition to take samples of electricity, and to convey them, with the view of testing them, from place to place. For this purpose the little "carrier," shown in Fig. 10, will be found convenient. T is a bit of tin-foil, two or three inches square. A straw stem is stuck on to it by sealing-wax, the lower end of the stem being covered by sealing-wax. To make the insulation sure, the part between R and S' is wholly of sealing-wax. You can have stems of ebonite, which are stronger, for a few pence; but you can have this one for a fraction of a penny. The end M' is to be held in the hand; the electrified body is to be touched by T, and the electricity conveyed to an electroscope to be tested.
|Fig. 10.||Fig. 11.|
Touch your rubbed glass rod with T, and then touch your electroscope: the leaves diverge with positive electricity. Touch your rubbed gutta-percha or sealing-wax with T, and then touch your electroscope: the leaves diverge with negative electricity. If the electricity of any body augment the divergence produced by the glass, the electricity of that body is positive. If it augment the divergence produced by the gutta-percha, the electricity is negative. And now we are ready for further work.
Place an egg, E, Fig. 11, on its side upon a dry wineglass; bring your excited glass tube, G, within an inch or so of the end of the egg. What is the condition of the egg? Its electricity is decomposed; the negative covering the end a adjacent to the tube, the positive covering the other end b. Remove the glass tube: what occurs? The two electricities flow together and neutrality is restored. Prove this neutrality. Neither a carrier touching the egg, nor the egg itself, has any power to affect your electroscope, or to attract a lath balanced in the manner already described.
Again, bring the excited tube near the egg. Touch its distant part b with your carrier. The carrier now attracts the straw or the balanced lath. It also causes the leaves of your electroscope to diverge. What is the quality of the electricity? It repels and is repelled by rubbed glass; the electricity at b is, therefore, positive. Discharge the carrier by touching it, and bring it into contact with the end a of the egg nearest to the glass tube. The electricity you take away repels and is repelled by gutta-percha. It is, therefore, negative. Test the quality, also, by the electroscope.
While the tube G is near the egg touch the end b with your finger; now try to charge the carrier by touching b: you cannot do so—the positive electricity has disappeared. Has the negative disappeared also. No. Remove the glass tube, and once more touch the egg at b with the carrier. It is charged, not with positive, but with negative electricity. Clearly understand this experiment. The neutral electricity of the egg is first decomposed into negative and positive; the former attracted, the latter repelled by the excited glass. The repelled electricity is free to escape, and it has escaped on your
touching the egg with your finger. But the attracted electricity cannot escape as long as the influencing tube is held near. On removing the tube which holds the negative fluid in bondage, that fluid immediately diffuses itself over the whole egg. An apple, or a turnip, will answer for these experiments at least as well as an egg.
Discharge the egg by touching it. Reëxcite the glass tube and bring it again near. Touch the egg with a wire or with your finger at a. Is it the negative at a, into which you plunge your finger, that escapes? No such thing. The free positive fluid passes through the negative, and through your finger to the earth. Prove this, by removing first your finger and then the glass tube. The egg is charged negatively.
Again: place two eggs, E E, Fig. 12, lengthwise on two dry wineglasses, g g, and cause two of their ends to touch each other, as at C. Bring your rubbed glass rod near the end a, and while it is there separate the eggs by moving one glass away from the other. Withdraw the rod and test both eggs: a is negative, b is positive. The two charges neutralize each other in the electroscope. Again: bring the eggs together and restore the rubbed tube to its place near a. Touch a and then separate the eggs. Remove the glass rod and test the eggs: a is negative, b is neutral. Its electricity has escaped through the finger, though placed at a.
Push your experiments still farther, and, instead of bringing the eggs, T T', Fig. 13, together, place them six feet or so apart, and let a light chain, C, or wire stretch from one to the other. Two brass
balls or wooden balls covered with tin-foil, and supported by tall drinking-glasses, G G will be better than the eggs for this experiment, for they will bear better the strain of the chain; but you can make the experiment with the eggs, or very readily with two apples or two turnips. For the present we will suppose the straw-index I I' not to be there. Rub your glass tube R, and bring it near one of the balls; test both: the near one, T', is negative, the distant one, T, positive. Touch the near one, the positive electricity, which had been driven along the chain to the remotest part of the system, returns along the chain, passes through the negative which is held captive by the tube, and escapes to the earth. When the tube is removed, negative electricity overspreads both chain and balls.
In Fig. 6 you made the acquaintance of the plate N, and the straw-index I I', shown in Fig. 13. By its means you immediately see both the effect of the first induction and the consequence of touching any part of the system with the finger. The plate N rests over the ball or turnip T, the position of the straw-index being that shown by the dots. Bring the rubbed tube near T': the end N of the index immediately descends and the other end rises along the graduated scale. Remove the glass rod; the index I I' immediately falls. Practise this approach and withdrawal, and observe how promptly the index declares the induction and recomposition of the fluids.
While the tube is near T, and the end N of the index is attracted, let T' be touched by the finger. The end N is immediately liberated, for the electricity which pulled it down escapes along the chain and through the finger to the earth. Now remove your excited tube. The captive negative electricity diffuses itself over both balls, and the index is again attracted.
Instead of the chain you may interpose between the balls one hundred feet of wire supported by silk loops. This is done in Fig. 14, which shows the wire w supported by the silk strings S S S, and where, for the ball or turnip, the cylinder C, on a glass support G, is substituted. Every approach and withdrawal of the rubbed glass tube R is followed obediently by the corresponding motion of the index.
Or, substituting a carrot, a cucumber, or other elongated conductor for the ball T', Fig. 12, you cause your rubbed glass tube to act upon a greater extent of surface. You thus decompose more electricity and produce a greater attraction.
Repeat here an experiment, first made by a great electrician named Æpinus. I wish you to make these grand old experiments. Support an elongated metal conductor, or one formed of wood coated with tin-foil—even a carrot, cucumber, or parsnip, so that it will be insulated, will answer. Let a small weight suspended from a silk string rest on one end of the conductor, and hold your rubbed glass rod near the other end. You can predict beforehand what will occur when you remove the weight. It carries away with it electricity, which repels rubbed glass, and which attracts your balanced lath.
Stand on an insulating stool: make one, if necessary, by placing a board on four warm tumblers. Present the knuckles of your right hand to the end of the balanced lath, and stretch forth your left arm. There is no attraction. But let a friend or an assistant bring the rubbed glass tube over the left arm; the lath immediately follows the right hand.
While matters continue thus, touch the lath, which I suppose to be uninsulated; the "attractive virtue," as it was called by Gray, disappears. After this, as long as the excited tube is held over the arm there is no attraction. But when the tube is removed the attractive power of the hand is restored. Here, you will at once comprehend, the first attraction was by positive electricity driven to the right hand from the left, and the second attraction by negative electricity, liberated by the removal of the glass rod.
Stand on an insulating stool, and place your right hand on the electroscope: there is no action. Stretch forth the left arm and permit an assistant alternately to bring near, and to withdraw, an excited glass tube. The gold-leaves open and collapse in similar alternation. At every approach, positive electricity is driven over the gold-leaves; at every withdrawal, the equilibrium is restored.
I will now ask you to charge your Dutch gold electroscope positively by rubbed gutta-percha, and to charge it negatively by rubbed glass. A moment's reflection will enable you to do it. You bring your excited body near: the same electricity as that of the excited body is driven over the leaves, and they diverge by repulsion. Touch the electroscope, the leaves collapse. Withdraw your finger, and withdraw afterward the excited body: the leaves then diverge with the opposite electricity.
The simplest way of testing the quality of electricity is to charge the electroscope with electricity of a known kind. If, on the approach of the body to be tested, the leaves diverge still wider, the leaves and the body are similarly electrified. The reason is obvious.
The wealth of knowledge, and of interest, which these experiments involve, may be placed within any boy's reach by the wise expenditure of half a crown.
Once firmly possessed of the principle of induction and versed in its application, the difficulties of our subject will melt away before us. In fact, our subsequent work will consist mainly in unraveling phenomena by the aid of this principle.
Without a knowledge of this principle we could render no account of the attraction of neutral bodies by our excited tubes. In reality, the attracted bodies are not neutral: they are first electrified by influence, and it is because they are thus electrified that they are attracted.
This is the place to stamp upon your mind the following considerations: Neutral bodies, as just stated, are attracted, because they are really converted into electrified bodies by induction. Suppose a body to be feebly electrified positively, and that you bring your rubbed glass-rod to bear upon the body. You clearly see that the induced negative electricity may be strong enough to mask and overcome the weak positive charge possessed by the body. We should thus have two bodies electrified alike, attracting each other. This is the danger against which I promised to warn yon in Section 10, where the test of attraction was rejected.
We will now apply the principle to explain a very beautiful invention, made known by the celebrated Volta in 1775.
Sec. 14. The Electrophorus.—Cut a circle, T (Fig. 15), six inches in diameter, out of sheet-zinc, or out of common tin. Heat it at its
centre by the flame of a spirit-lamp or of a candle. Attach to it there a stick of sealing-wax, H, which, when the metal cools, is to serve as an insulating handle. You have now the lid of the electrophorus. A resinous surface, or what is simpler a sheet of vulcanized India-rubber, P, or even of hot brown paper, will answer for the plate of the electrophorus.
Rub your "plate" with flannel, or whisk it briskly with a fox's brush. It is thereby negatively electrified. Place the "lid" of your electrophorus on the excited surface: it touches it at a few points only. For the most part lid and plate are separated by a film of air.
The excited surface acts by induction across this film upon the lid, attracting its positive and repelling its negative electricity. You have in fact in the lid two layers of electricity, the lower one, which is "bound," positive; the upper one, which is "free," negative. Lift the lid: the electricities flow again together; neutrality is restored, and your lid fails to attract your balanced lath.
Once more place the lid. upon the excited surface: touch it with the finger. What occurs? You ought to know. The free electricity, which is negative, will escape through your body to the earth, leaving the chained positive behind.
Now lift the lid by the handle: what is its condition? Again I say you ought to know. It is covered with free positive electricity. If it be presented to the lath it will strongly attract it; if it be presented to the knuckle it will yield a spark.
A smooth half-crown or penny will answer for this experiment. Stick to the coin an inch of sealing-wax as an insulating handle; bring it down upon the excited India-rubber: touch it, lift it, and present it to your lath. The lath may be six or eight feet long, three inches wide, and half an inch thick; the little electrophorus-lid, formed by the half-crown, will pull it round and round. The experiment is a very impressive one.
Scrutinize your instrument still further. Let the end of a thin wire rest upon the lid of your electrophorus, under a little weight if necessary, and connect the other end of the wire with the electroscope. As you lower the lid down toward the excited plate of the electrophorus, what must occur? The power of prevision now belongs to you and you must exercise it. The repelled electricity will flow over the leaves of the electroscope, causing them to diverge. Lift the lid, they collapse. Lower and raise the lid several times, and observe the corresponding rhythmic action of the electroscope-leaves.
A little knob of sealing-wax, B, coated with tin-foil; or indeed any knob with a conducting surface, stuck into the lid of the electrophorus, will enable you to obtain a better spark. The reason of this will immediately appear.
Sec. 15. Action of Points and Flames.—The course of exposition proceeds naturally from the electrophorus to the electrical machine. But before we take up the machine we must make our minds clear regarding the manner in which electricity diffuses itself over conductors, and more especially over elongated and pointed conductors.
Rub your glass tube and draw it over an insulated sphere of metal—of wood covered with tin-foil, or indeed any other insulated spherical conductor. Repeat the process several times, so as to impart a good charge to the sphere. Touch the charged sphere with your carrier, and transfer the charge to the electroscope. Note the divergence of the leaves. Discharge the electroscope, and repeat the experiment, touching, however, some other point of the sphere. The electroscope shows the same amount of divergence. Even when the greatest exactness of the most practised experimenter is brought into play, the spherical conductor is found to be equally charged at all points of its surface. You may figure the electric fluid as a little ocean encompassing the sphere, and of the same depth every-where.
But supposing the conductor, instead of being a sphere, to be a cube, an elongated cylinder, a cone, or a disk. The depth, or as it is sometimes called the density of the electricity, will not be everywhere the same. The corners of the cube will impart a stronger charge to your carrier than the sides. The end of the cylinder will impart a stronger charge than its middle. The edge of the disk will impart a stinger charge than its flat surface. The apex or point of the cone will impart a stronger charge than its curved surface or its base.
You can satisfy yourself of the truth of all this in a rough but certain way, by charging, after the sphere, a turnip cut into the form of a cube; or a cigar-box coated with tin-foil; a metal cylinder, or a wooden one coated with tin-foil; a disk of tin or of sheet-zinc; a carrot or parsnip with its natural shape improved so as to make it a sharp cone. You will find the charge imparted to the carrier by the sharp corners and points, to be greater than that communicated by gently-rounded or flat surfaces. The difference may not be great, but it will be distinct. Indeed, the egg laid on its side, as we have already used it in our experiments on induction, yields a stronger charge from its ends than from its middle.
Let me place before you an example of this distribution, taken from the excellent work on "Frictional Electricity," by Prof. Riess, of Berlin, who is probably the greatest living exponent of the subject. Two cones, Fig. 16, are placed together base to base. Calling
the strength of the charge along the circular edge where the two bases join each other 100, the charge at the apex of the blunter cone is 133, and at the apex of the sharper one 202. The other numbers give the charges taken from the points where they are placed. Fig. 17, moreover, represents a cube with a cone placed upon it. The charge on the face of the cube being 1, the charges at the corners of the cube and at the apex of the cone are given by the other numbers; they are all far in excess of the electricity on the flat surface.
Riess found that he could deduce with great accuracy the sharpness of a point, from the charge which it imparted. He compared in this way the sharpness of various thorns with that of a fine English sewing-needle. The following is the result: Euphorbia-thorn was sharper than the needle; gooseberry-thorn of the same sharpness as the needle; while cactus, blackthorn, and rose, fell more and more behind the needle in sharpness. Calling, for example, the charge obtained from euphorbia 90, that obtained from the needle was 80, and from the rose only 53.
Considering that the electricity is self-repulsive, and that it heaps itself up upon a point in the manner here shown, you will have little difficulty in conceiving that, when the charge of a conductor carrying a point is sufficiently strong, the electricity will finally disperse itself by streaming from the point.
The following experiments are theoretically important: Attach a stick of sealing-wax to a small plate of tin, so that the stick may stand upright. Heat a needle and insert it into the top of the stick of wax; on this needle mount a carrot. You have thus an insulated conductor. Stick into your carrot at one of its ends a sewing-needle, and hold for an instant your rubbed glass rod in front of this needle without touching it. What occurs? The negative electricity of the carrot is discharged from the point against the glass rod. Remove the rod, test the carrot: it is positively electrified.
And now for another experiment, not so easily made, but still certain to succeed if you are careful. Excite your glass rod, turn your needle away from it, and bring the rod near the other end of the carrot. What occurs? The positive electricity is now repelled to the point, from which it will stream into the air. Remove the rod and test the carrot: it is negatively electrified.
Again, turn the point toward you, and place in front of it a plate of dry glass, wax, resin, shellac, paraffine, gutta-percha, or any other insulator. Pass your rubbed glass tube once downward or upward, the insulating plate being between the excited tube and the point. The point will discharge against the insulating plate, which on trial will be found negatively electrified. These experiments, I may say, were discussed, and differently interpreted by the two philosophers, during an important correspondence between Faraday and Prof. Riess.
Sec. 16. The Electrical Machine.—An electrical machine consists of two principal parts: the insulator which is excited by friction, and the "prime conductor."
The sulphur sphere of Otto von Guericke was, as already stated, the first electrical machine. The hand was the rubber, and indeed it long continued to be so. For the sulphur sphere Hauksbee and Winckler substituted globes of glass. Boze, of Wittenberg (1741), added the prime conductor, which was at first a tin tube supported by resin, or suspended by silk. Soon afterward Gordon substituted a glass cylinder for the globe. It was sometimes mounted vertically, sometimes horizontally. Gordon so intensified his discharges as to be able to kill small birds with them. In 1760 Planta introduced the plate machine now commonly in use.
Mr. Cottrell has constructed for these lessons the small cylinder machine shown in Fig. 18. The glass cylinder is about seven inches long and four inches in diameter; its cost is eighteen pence. Through the cylinder passes tightly, as an axis, a piece of lath, rendered secure
by sealing-wax where it enters and quits the cylinder. G is a glass rod supporting the conductor C, which is a piece of lath coated with tin-foil. Into the lath is driven the series of pin-points, P, P. The rubber, R, is seen at the farther side of the cylinder, supported by the upright lath, R', and caused to press against the glass. S' is a flap of silk. When the handle is turned sparks may be taken, or a Leyden-jar charged at the knob C. A plate machine is shown in Fig. 19. P is the plate; R and R', two rubbers which clasp the plate. A and A' are rows of points presented by the conductor, C. C C' is an insulating rod of glass, intended to cut off the connection between the conductor and the handle of the machine.
The prime conductor is thus charged: when the glass plate is turned, as it passes each rubber it is positively electrified. Facing the electrified glass is the row of points midway between the two rubbers. On these points the electrified glass acts by induction, attracting the negative and repelling the positive. In accordance with the principles already explained the negative electricity streams from the points against the excited glass, which passes on neutralized to the next rubber, where it is again excited. Thus the prime conductor is charged, not by the direct communication to it of positive electricity, but by depriving it of its negative.
If, when the prime conductor is charged, you bring the knuckle near it, the electricity passes from the conductor to the knuckle in the form of a spark.
Take this spark while the machine is being turned, and then try the effect of presenting the finger-ends, instead of the knuckle, to the conductor. The spark falls exceedingly in brilliancy. Substitute for the finger-ends a needle-point, you fail to get a spark at all. To obtain a good spark the electricity upon the prime conductor must reach a sufficient density (or tension, as it is sometimes called). To secure this, no points from which the electricity can stream must exist on the conductor, or be presented to it. All parts of the conductor are therefore carefully rounded off, sharp points and edges being avoided.
It is usual to attach to the conductor an electroscope, consisting of an upright metal stem, A C, Fig. 20, to which a straw with a pith-ball, B, at its free end, is attached. The straw turns loosely upon a pivot at C. The electricity passing from the conductor is diffused over the whole electroscope, and the straw and stem, being both positively electrified, repel each other. The straw, being the movable body, flies away. The amount of the divergence is measured upon a graduated arc.
If no point exist on the conductor, a single turn of the handle of the machine suffices to cause the straw to stand out nearly at right angles to the stem. If, on the contrary, a point be attached to the conductor, you cannot produce a large divergence. The reason is, that the electricity, as fast as it is generated, is dispersed by the point. The same effect is observed when you present a point to the
conductor. The conductor acts by induction upon the point, causing the negative electricity to stream from it against the conductor, which is thus neutralized almost as fast as it is charged. Flames and glowing embers act like points; they also rapidly discharge electricity.
The electricity escaping from a point or flame into the air renders the air self-repulsive. The consequence is that, when the hand is placed over a point mounted on the prime conductor of a machine in good action, a cold blast is distinctly felt. Dr. Watson noticed this blast from a flame placed on an electrified conductor, while Wilson noticed the blast from a point. Jallabert and the Abbé Nollet also observed and described the influence of points and flames. The blast is called the "electric wind." Wilson moved bodies by its action; Faraday caused it to depress the surface of a liquid; Hamilton employed the reaction of the electric wind to make pointed wires rotate. The "wind" was also found to promote evaporation.
Hamilton's apparatus is called the "electric mill." Make one for yourself thus: Place two straws S S, S' S', Fig. 21, about eight inches long, across each other at a right angle. Stick them together at their centres by a bit of sealing-wax. Pass a fine wire through each straw and bend it where it issues from the straw, so as to form a little pointed arm perpendicular to the straw, and from half an inch to three-quarters of an inch long. It is easy, by means of a bit of cork or sealing-wax, to fix the wire so that the little bent arms shall point not upward or downward, but sideways, when the cross is horizontal. The points of sewing-needles may also be employed for the bent arms. A little bit of straw is stuck into the cross at the centre, to form a cap. This slips over a sewing-needle, N, supported by a stick of sealing-wax, A. Connect the sewing-needle with the machine, and turn. A wind of a certain force is discharged from every point, and the cross is urged round with the same force in the opposite direction.
You might easily, of course, so arrange the points that the wind from some of them would neutralize the wind from others. But the little pointed arms are to be so bent that the reaction in every case shall not oppose, but add itself to, the others.
The following experiments will yield you important information regarding the action of points: Stand, as you have so often done before, upon a board supported by four warm tumblers. Hold a small sewing-needle, with its point defended by the forefinger of your right hand, toward your Dutch metal electroscope. Place your left hand on the prime conductor of your machine. Let the handle be turned by a friend or an assistant: the leaves of the electroscope open out a little. Uncover the needle-point by the removal of your finger: the leaves at once fly violently apart.
Mount a stout wire upright on the conductor of your machine; or support the wire by sealing-wax, gutta-percha, or glass, at a distance from the conductor. Connect both by a fine wire. Bend your stout wire into a hook, and hang from it a tassel composed of many strips of light paper. Work the machine. Electricity from the conductor flows over the tassel, and the strips diverge. Hold your closed fist toward the tassel, the strips of paper stretch toward it. Hold the needle, defended by the finger, toward the tassel: attraction also ensues. Uncover the needle without moving the hand; the strips retreat as if blown away by a wind.
And now repeat Du Fay's experiment which led to the discovery of two electricities. Excite your glass tube, and hold it in readiness, while a friend, or an assistant, liberates a real gold or silver leaf in the air. Bring the tube near the leaf: it plunges toward the tube, stops suddenly, and then flies away. You may chase it round the room for hours without permitting it to reach the ground. The leaf is first acted upon inductively by the tube. It is powerfully attracted for a moment, and rushes toward the tube. But from its thin edges and corners the negative electricity streams forth, leaving the leaf positively electrified. Repulsion then sets in, because tube and leaf are electrified alike. The retreat of the tassel in the last experiment is due to a similar cause.
There is also a discharge of positive electricity into the air from the more distant portions of the gold-leaf, to which that electricity is repelled. Both discharges are accompanied by an electric wind. It is possible to give the gold-leaf a shape which shall enable it to float securely in the air by the reaction of the two winds issuing from its opposite ends. This is Franklin's experiment of the Golden Fish. It was first made with the charged conductor of the electrical machine.
M. Srtsczek revived it in a more convenient form, using instead of the conductor the knob of a charged Leyden-jar. You may walk round a room with the jar in your hand; the "fish" will obediently follow in the air an inch or two, or even three inches, from the knob. (See A B, Fig. 22.) Even a hasty motion of the jar will not shake it away.
Well-pointed lightning-conductors, when acted on by a thunder-cloud, behave in the same way. The opposite electricity streams out from them against the cloud.
Franklin saw this with great clearness, and illustrated it with great ingenuity. The under-side of a thunder-cloud, when viewed horizontally, he observed to be ragged, composed of fragments one below the other, sometimes reaching near the earth. These he regarded as so many stepping-stones which assist in conducting the stroke of the cloud. To represent these by experiment, he took two or three locks of fine loose cotton, tied them in a row, and hung them from his prime conductor. When this was excited, the locks stretched downward toward the earth; but, by presenting a sharp point erect under the lowest bunch of cotton, it shrunk upward to that above it, nor did the shrinking cease till all the locks had retreated to the prime conductor itself. "May not," says Franklin, "the small electrified clouds, whose equilibrium with the earth is so soon restored by the point, rise up to the main body, and by that means occasion so large a vacancy that the grand cloud cannot strike in that place?"
- A course of six lectures, with simple experiments in frictional electricity, before juvenile audiences during the Christmas holidays.
- Philosophical Magazine," vol. xi., 1856.