700

Conductors electrified only on the Surface.

ciency in another, the self-repulsion, where there is redundancy, causes the electricity to pass instantly from where it is in exceSS, through any conducting substance which offers, either to the earth, which is the general reservoir, or still more readily to a body in a negative state; and where no conducting medium is very near, the fluid may force its way or burst through a short length or thickness of a non-conducting substance, such as air, heating it by the violence of its passage.

937. By the use of the large machines other important facts were soon discovered. Thus, owing to the self-repellent nature of electricity, when it is thrown in excess on any conducting body, it diffuses itself not through the whole mass, but over the surface only. If the

B

Fig. 249.

body be a sphere or ball, A (fig. 249), suspended by a silken cord and highly electrified, and if there be two metallic cups, B and C, which exactly cover or fit the sphere, A, if the cups, having handles of glass, be applied closely to the ball, and then removed, the whole charge of electricity is found to have passed to the cups, leaving the ball in a perfectly neutral or normal state.

Faraday, with his beautiful experimental simplicity, showed the same fact by means of a conical muslin net attached to a metallic

938. Owing to the same self-repellent nature of the electricity, when a charge is thrown on any body, it does not spread or distri

Electrical Induction.

701 bute itself uniformly over the body unless the form be that of a perfect sphere or ball as a (fig. 251). If the body be oblong, as b, the repulsion of the fluid in the central parts increases the density of it towards the ends. Then, if one end be smaller than the other, as in c, the density is greatest at the small end. If the extremity be a point as in d, the density becomes there so great that the electricity is forced into the air, and gradually escapes. The flame of a candle held near an electrified point appears to be blown away

from it; this is the result of the outward rush of air-particles by electrification and repulsion at the point. For the same reason that positively electrified points quickly dissipate positive electricity, so do points in a negatively electrified body readily receive it. This is seen in the action of the points placed between the receiver or conductor of an electrical machine and the cylinder. The fact is turned to account by giving to common lightning-rods sharp points directed to the sky.

939. Electrical Induction.-The action of an electrified body upon a non-electrified conductor near it, but not in contact, is very curious, and of great importance. Its study reveals the nature of electric action generally, and forms one of the first analogies between the attractions and repulsions of electricity and those of magnetism. This action at a distance is known as induction, and we now proceed to explain it.

Let ab (fig. 252) be a conductor insulated by glass supports, and having pairs of pith-balls hanging by linen threads or fine wires from the ends, a b, and the middle, c. When not electrified, all the balls hang in contact, but if a positively electrified metal sphere, e, be brought near to the conductor, yet not touching it, the two balls at a and b will instantly repel and fly apart as here shown by the dotted lines, those at b becoming negatively electrified, owing to the natural electricity of the conductor being driven towards the end, a, by the repulsion of the positive electricity in the ball, e, and those at a becoming positively electrified, because the natural elecricity of the end, b, of the cylinder is driven to a, making a positive

702

Electrical Induction.

charge there. No effect is produced on balls at c, half way between the ends, because the quantity of electricity there remains the same. If the sphere, e, be withdrawn, all the repulsions described immediately cease.

If, during the last experiment, while the electrified body, e, is held near the conductor, a b, causing the disturbance of the electric pendulums above described, a finger be applied for a moment to

the conductor at a, allowing the positive electricity accumulated there to escape towards the earth, the pendulums immediately collapse. If after this the electrified ball, e, be taken away, the pitns, a and b, immediately diverge again, and the conductor remains permanently charged in the negative electrical state. If then a finger approach the conductor it gives a spark, which restores what was taken away when the charged ball was present. This operation may be repeated many times by bringing back the same charged ball, e, at every repetition. If the ball could be long preserved without losing its positive charge, it would give the power of producing at will for the whole time sparks of considerable force. This principle is referred to in the description of the electrophorus in Art. 942, the nature of which it explains.

It is a remarkable fact, that if a pane of glass be held between the ball, e, and the conductor, a b, or in the experiments described afterwards, between the glass tube or sealing-wax and the pithbails, the attractions and repulsions still occur as if the pane were not there. The glass, therefore, although, as a non-conductor, it prevents altogether the passage through it of electricity itself, does not lessen that influence at a distance, which is called its power of induction.

The Electroscope.

703

Fig. 253.

940. The Electroscope.-A very sensitive electroscope, or detector of electricity, which acts by induction, is represented in fig, 253, and may be described here. Its essential parts are the brass rod, a b, with a knob at the top, and two strips of gold leaf hanging from the bottom, b, within a glass shade to prevent disturbance from air-currents. When there is no free electricity near, the leaves hang in contact, but with the slightest electrical charge, whether positive or negative, they stand asunder towards cd, and more or less, according to the strength of the charge. If any electrified body approach the ball, a, it acts by induction and throws the ball, a, into the contrary electric state to its own, as explained in Art. 939, and the induced charge in a, produces the opposite charge in the leaves below, which are therefore mutually repelled. A piece of glass tube rubbed with silk, if made to approach a, causes the leaves to separate, because of its positive electricity, and when it is removed, the leaves collapse; but if the glass be allowed to touch the ball, the separation of the leaves remains after the tube is taken away, for a positive charge has been given. A stick of excited sealing-wax then brought near, causes, first, collapse of the leaves, but if allowed to touch, produces continued separation by negative electricity. The kind of electricity with which the leaves may be charged, is thus discovered at once by approaching to the ball either a rod of glass or one of sealing-wax, excited by rubbing.

941. Faraday made some experiments to ascertain the part played by the air in this phenomenon of electric induction ; and he came to the conclusion that the inductive action is not, like that of gravity, really an action at a distance, but that each little molecule of air is charged negatively on the side next the positively charged inducing body, and positively on the opposite face. These air particles are consequently the vehicle of the electric action, instead of playing the merely passive part which they were supposed to do according to former theories. Faraday strengthened this hypothesis by further experiments on the inductive power of other intervening insulators, such as glass, wax, resin, sulphur, and shellac; and he found remarkable differences. Glass, for instance, he found to operate almost twice as powerfully as air in inducing electricity between a charged and a neutral conductor: while shellac and sulphur are even more favourable to induction than glass.

704

G

The Electrophorus.

The inductive, or dielectric, power of a non-conducting medium such as glass, or resin, or vulcanite, may be very well shown in the following way :-If a source of positive electricity, such as the prime conductor, P (fig. 254), of a machine, be applied to one side of a pane of plate glass, G, each side of which is coated with a sheet of tin-foil, or other metal conductor, it will induce through the medium of the glass a negative charge on the face of the conductor, A, next to P, while the positive accumulation is on the side of B farthest from P. If, as in the case already described, the outside face of B be touched by the finger, its positive charge passes to the earth, and B, on the removal of both the hand and the prime conductor, remains permanently charged with negative electricity. On touching the plate, A, with one hand, and B with the other, we should then get a more or less powerful shock. Indeed this Franklin's pane, as it is termed, from the name of the illustrious philosopher who first used it, becomes a very powerful means of cumulating or storing electricity.

Fig. 254.

That the glass has most to do with the electric capacity which this arrangement possesses, is proved by the fact that if both metallic coatings be made movable they may be removed, discharged separately, put back into position, and give a spark almost as powerful as before.

942. The Electrophorus (electric carrier), as it is termed, is a simple arrangement on an analogous principle, by which a charge of electricity can be preserved for a long time (in dry weather for weeks or even months) in a condition which allows other small charges to be produced by it inductively.

α

It is formed in this way :—A mixture of shellac, resin, and turpentine, melted together, is poured into a mould of metal of the breadth of a dinner-plate, so as to form, when cooled, a solid cake about half an inch thick. Upon the surface of this cake, resting on a table, is placed on a metallic disc or plate, somewhat smaller than the cake, and having a glass handle, a (fig. 255), by which it can be lifted away from the cake. To prepare for use, the cake is struck briskly a few times with a catskin, or a piece of

Fig. 255.