Oldalképek
PDF
ePub

slender rod of varnished glass. P is a conductor insulated by a glass stalk, and charged. M is a similar insulated conductor not charged. By

touching P with « and then touching M with it we are ahle to bear off a great portion of the electricity of the charged body P, and deposit it on M so as to charge it. Repeated contacts or transfers are, however, needed before this is accomplished, as the disc transfers only a ittle each time. The disc or sphere of metal employed is called a carrier. 6. The phenomena of attraction and repulsion of bodies by electricity, are described in §§ 3, 4 and 5, pp. 136-7, "Fifth Reader," and do not require repetition. The illustrations will recall them to your memory. In Fig. 47 the two rods in the two hands are of glass, about 3 feet long

[graphic]
[ocr errors]
[graphic][graphic][merged small][ocr errors]

and an inch in diameter. The objects in the air near them are two feathers, suspended by a light thread from the ceiling. If the rod, being perfectly dry and clean, be rubbed briskly with a warm silk handkerchief, it will soon be excited, and on holding it within a few feet of the feather the feather will fly towards it and adhere to it. On withdrawing the rod, however, and again bringing it near the feather it will no longer attract but will repel it. How this comes about is stated in the " Fifth Reader," to which I must refer you.

In the other illustration (Fig. 48) m is a pith ball, insulated by being hung at the end of a thread of unspun silk. It thus forms an insulated electroscope. T is a glass tube furnished with a metal conducting rod and ball, previously electrified by friction. When the metal ball d is brought near the pith ball m it will immediately attract it towards itself, as in the cut, but, if it be allowed to touch the metal ball, m will instantly rebound to »n', and will not again come near d until touched by the finger or other freely conducting substance which makes it once more non-electrified.

If a metal ball, on a rod of metal, so as to form a conductor, be substituted for the finger, the pith ball will swing first towards the excited glass rod, then, after touching the conductor, will fly back to the glass rod, and this will be repeated according to the amount of electricity in operation.

Other details will be found in the " Fifth Reader."

7. Another class of phenomena connected with electricity is known by the name of Induction. It is that property by which bodies duly excited develope powers similar to their own on other bodies, without coming in contact with them, by some electrical emanation or sympathetic action, operating between the two bodies even at considerable distances.

If some conductor, Q, (Fig. 49), such as a light hollow cylinder of gilded wood, about 5 inches in length, and from 3 ?f P

to 4 inches in diameter, duly insulated, on glass, &c., be put directly in a line with another similar insulated conductor, P, charged either positively or negatively — say positively* — the two will forthwith exhibit a sympathetic action, which will change their electrical state. Q, which has not been charged, will show a state of excitement, without any contact with P, which has been charged, and will

[graphic]

Fig. 49.

[ocr errors]

attract the pith ball of the electroscope e, placed at p\ at the one end of ft, towards ft. On the other hand, a similar pith ball on the electroscope f will first be repelled from the far end of P, and will then fall back towards P, in a greater or less degree, showing a lessening of the repellent force of P at the end opposite Q. The changes effected are peculiar. The face « of ft, next the charged body P, acquires an opposite electricity to that of P, whilst its far away face p' acquires the same electricity as P has. Thus, if P be charged with positive, or vitreous electricity, the face n of ft will be negative, and the face p', be positive. Still further, Q reflects an influence again on P, which increases the force of P at its face p directly opposite the face n of ft, and lessens it at its face q, which is at the farther end from the face ». Thus p' is positive, n, negative, p is positive again, and q less positive.

8. Electrical Machines, to produce and accumulate positive and negative electricity in great quantity, and by an easy means, date back to the beginning of the seventeenth century. They were very simple at first, but have gradually become more and more perfect and complicated.

There are two kinds, the cylindrical and the plate machines, the electric to be excited being a hollow cylinder of glass in the one, and a glass plate in the other.

9. The cylindrical machine is represented in Fig 50. In this illustration

A, m, B is a hollow glass cylinder. It is blown with a short open projecting neck, A 6, at each end, and these open necks are closed air-tight with fine cork, the air within the cylinder being previously made perfectly dry, and the glass within freed from all moisture. The necks are covered with neat brass caps, or caps of varnished mahogany, cemented over them. The caps have short axles, or axes, A B, of hard wood, which are fitted on two mahogany or glass pillars, C D, and the cylinder is made to turn round on them by a winch, w, or a wheel and band, against a fixed rubier or pad, E E'. This rubier, with its rubbing cushion, is made to press against the cylinder by an insulated conductor, N, termed the negative conductor, insulated on the glass pillar, N n. The rubber has a flap of oiled

[graphic]
[ocr errors]

silk, m, attached to it and lying over a third of the circumference of the cylinder. The silk is oiled on one side only, and has a rough side towards the glass.

The positive electricity generated hy the friction of the cylinder against the rubber and the silk is received upon a row of metallic points, q q'. These are connected with an insular conductor, P, which has a projection ending in a brass ball through which passes a sliding rod, s a'. On the opposite side is a similar conductor, N.

When the cylinder is made to revolve freely, the conductor N being connected with the ground, a surprising stream of positive electricity may be obtained by the friction against the cushion and silk flap, from the conductor P, in a current of bright sparks. If the conductor P be connected with the ground, and the cushion insulated, negative electricity is obtained as freely from the conductor N.

10. The plate machine is the same in principle, so that it is not necessary to occupy space by explanations of the letters on the illustration (Fig. 51). The details may be readily understood by comparison with the cylindrical machine.

The Leycen Jar.

1. Electricity which is thus produced by the electric machine is accumulated by means of the jar which gets its name, the Leyden Jar, from the Dutch town in which its properties were first discovered.

2. The principle of this valuable instrument will be understood from (Fig. 52), which represents a pane of glass, A B, with a square of tinfoil on each side c d ef, in the centre, the glass being left bare between the tinfoil and the edges of the pane. This plate being supported on a stand made of some good conductor, such as metal, we lave two conducting surfaces of tinfoil, the upper one insulated by the pane of glass; the lower communicating by the metal stand with the earth. If a charge of electricity be communicated to the insulated surface, c d,

[graphic]
[ocr errors]

an equal charge of electricity of the opposito kind will immediately he produced on the opposite conducting surface e f—that is on the under coating of tinfoil. The upper surface, or insulated conductor, e d,

[graphic]

Kg. 52.

will be positive; the under surface, ef, which is not insulated, will be negative. The ball of an electrometer will be repelled by the uncondensed electricity on the charged plate.

3. These two opposite electricities, thus kept apart by the glass, have so great a tendency to come together, so as to produce the neutral state, or state of equilibrium, that in many cases the glass is fractured by it. But if a discharging rod be brought into contact with the two opposite coatings, the two electricities at once unite, emitting a brilliant spark and causing an explosion as they do so. Indeed, so strong is the tendency to come together that this effect is produced if one ball of the discharging rod, usually the negative, first touch the negative coating, while the other only comes near the positive coating.

4. The Leyden Jar (Fig. 53) is an application of this principle. Let me describe it from the illustration.

m A is a jar of clear glass, of almost any size or shape. Its diameter, a d should be about half its height, narrowing from the shoulder, S, to about half as broad at its mouth, m. The jar is coated outside and within with tinfoil for the shoulder, S. The neck, r q, which separates the two metal coatings, is varnished with a solution of shell-lac dissolved in naphtha.

5. Electricity is communicated to the jar from the electric machine by a light

metallic tube, E, called the charging rod, which passes through its

[graphic]
[ocr errors]
« ElőzőTovább »