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Two kinds of Electricity.

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(z.) There is the same remarkable opposition, if, instead of using glass and silk to evolve the electricity, a stick of sealing-wax and silk or woollen cloth are employed. The electricity from the sealingwax is of the contrary nature to that from the glass, and of the same nature as that from the silk which has rubbed the glass.

931. The results of these and such like experiments may be summarized as follows:

1. If we rub with a dry silk or flannel cloth any of these substances — sealing-wax, shellac, glass, sulphur, ebonite, vulcanite, writing-paper or catskin, we develope in them an electric or attractive power which readily manifests its effects on light bodies such as pith-balls, chaff, or paper.

2. There are two distinct classes or kinds of the electric condition, which may be readily exhibited in this way. A stand, A B, is made of a glass rod or tube, bent as in the figure, with the heat of a gas or spirit flame; and a paper stirrup, C, is hung from the stand by a white silk thread. Having provided another similar insulating stand, with two small sticks of sealing-wax, and two A rods of glass, we find that, on rubbing the ends of two sticks of wax, and placing them in the stirrups and bringing them near, they mutually repel; also, if we substitute for these the two glass rods, after rubbing them briskly, they similarly repel each other; but if into one stirrup we put a rubbed stick of wax and into the other a rubbed stick of glass, they attract. Moreover, all substances capable of exhibiting these electric actions, either repel or attract the rubbed stick of wax, while they attract or repel the rubbed stick of glass.* These actions are in virtue of their electricities; hence

Fig. 246.

The fundamental law of electric action is-Like electricities repel each other (as we shall see like poles of a magnet repel), while unlike electricities attract each other.

Since all electricity generated by friction is thus either like that of the glass or of the wax ; the names vitreous and resinous used to

*These properties may be exemplified in another way. Place a portion of sheet gutta-percha, about eight inches long and two wide, on a folded silk handkerchief or on a layer of dry flannel. Gently rub it in one direction by the flat portion of the thumb. It soon becomes so electrical that it will be strongly attracted by the hand or anything brought near to it. If brought

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Positive and Negative Electricity.

be given to distinguish those two classes. The names positive electricity and negative electricity are the modern and less objectionable terms employed.

932. If we hang a small pith-ball instead of the paper stirrup at the end of our silk string (fig. 246), this forms a ready and sensitive test of the presence or absence of the electric power.

By means of it we discover that when we rub a stick of sealingwax with a silk handkerchief, it attracts the ball and then repels it; but the pith, electrified and repelled by the wax, will be attracted by the silk rubber. Thus the two kinds of electricity are always simultaneously produced; and this countenances the hypothesis of two electric fluids, which, like an acid and an alkali, neutralize each other when present in equal quantity, but which have a strong attraction or affinity when separated. The generation of electricity is but the separation of these two fluids, and friction is, as we shall see, only one of many means whereby this separation can be effected."

The other, and single-fluid theory, of electrical action regards the natural or unelectrified state of a body as merely a body having the same amount of the electric fluid as the surrounding surface of the earth; while electrification is, so to speak, the disturbance of this electrical equilibrium, a body being positively electrical when there is an accumulation or heaping up of the electric fluid on it, and negatively electrical when there is a withdrawal of the normal quantity.

near glass (a mirror) it will fly to it and adhere to it for many hours. It two layers of gutta-percha are thus separately treated and brought near to each other, they show all the properties of repulsion above described.

The thin transparent membrane left by the evaporation of collodion (gun-cotton dissolved in ether) is one of the most electrical substances ir. nature. The slightest friction, even through a layer of paper, renders it sc powerfully electrical that it can scarcely be removed from the paper without being torn.

Heat produces, or is converted into, electricity. Thus, if a flat iron, moderately heated, is passed over albumenized paper, placed on a surface of dry wood or silk, it becomes strongly electrical. Paper photographs thus ironed are powerfully attracted to the hands of the operator, and the sensation of a sort of aura, owing to the passage of electricity, is perceived. Owing to the well-known law of repulsion, these slips of paper or thin drawings strong.y repel each other; but are attracted to other drawings which have not been in contact with the flat iron.

Conductors and Insulators.

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The laws of the science are but little influenced whichever theory we choose to adopt.

933. There is a remarkable difference among substances in their power of conducting electricity. When a glass rod, or stick of wax, is rubbed it is electrified only at the spot where it was rubbed. If, however, we communicate its electricity to a metal rod, such as our poker (fig. 245), the manifestation appears at every part of the conductor at the same instant. Hence is explained the fact, that a metal rod treated like the glass rod shows no appearance of electricity. As fast as we produce it, the effect spreads over the whole rod and thence through the body to the earth, the universal recipient and reservoir of all our actions.

The following is a classified list of bodies, according to their power of transmitting electricity :

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934. The Electrical Machine.- When persons had become familiar with the simple experiments already described, they naturally concluded that with larger apparatus of a similar nature, the phenomena might be exhibited on a grander scale, and that probably new facts of importance would be discovered. The result of many devices and trials was the construction of what is called the frictional electric machine now to be described.

The essentials of any electrical machine are three :-the glass, or non-conductor, to be rubbed; the rubber; and the conducting reservoir to receive the electricity. In lieu of the simple tube in our elementary experiment, may be substituted a large glass cylinder as A B (fig. 247), about twenty inches long and half as wide, to be turned, like a grindstone or barrel-organ, by a winch-handle, w. It is insulated by being supported on glass pillars, H and I.

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The Electrical Machine.

Secondly, instead of the loose handkerchief for rubbing, there is used a flap or breadth of silk cloth laid on the upper part of the cylinder, of which flap half is here seen, C D E F, the other half being behind, with its horizontal edge or border fixed on a cross piece of wood, also supported on a glass pillar, K. This cross piece has a soft covering, rendering it a cushion which bears

W

K

B

M

Fig. 247.

gently against the cylinder, and becomes the solid part of the rubber. Thirdly, instead of the cork or pith balls to receive the electricity, there is the large metal cylinder with rounded ends, L M, also having a glass support. This cylinder is called the prime conductor, and when in its place, its extremity, L, with metallic points projecting to receive the electricity, is brought close to the loose border of the silk, E F.

As the cylinder is turned, electricity is evolved by the rubbing silk on the glass surface, and is carried forward to be delivered through the metallic points to the prime conductor, L M, where the silk terminates.* There is a metal chain connecting the cushion of the rubber with the ground at N; and by this means the rubber is kept at the same electrical level, as it is called, with the earth, while the electrical agitation, excitement, or affection is heightened, or so to speak, heaped up on the prime conductor.

935. The intensity or degree of this electrical accumulation on the prime conductor may be ascertained roughly by placing on some

*This is a hollow cylinder, as electricity is always accumulated over the surface of metals. See Art. 937.

Experiments with the Machine.

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part of it, as at M, a small pendulum electrometer, consisting of a pith-ball, b, hanging by a slender arm, a b, from the top

of an upright stalk, a c (fig. 248). The greater the elec- all trical accumulation, the farther is the pith-ball repelled from the stalk.

A similar electrometer placed on the rubber cushion behind, if the earth-chain, N, were removed, would show the degree of complementary electrical affections excited in both the prime conductor and the rubber, and would show the value of the earth-chain in allowing the Fig. 248. accumulation of positive electricity on the prime conductor; for if the earth-chain be removed the strength of electricity that can be accumulated is comparatively insignificant.

936. The Plate machine is another form of electrical machine in common use. It consists simply of a circular disc of plate-glass, turning on its axle, instead of the bottle or cylinder shown above; and the supports, rubbing silk, and conductor are formed and arranged to suit it. The advantages of the plate arrangement are that both sides of the glass are made available for friction, and the labour of turning is less than with the cylinder for the same extent of surface. On the other hand, the glass plate is very liable to crack when placed near the fire for drying purposes. An exceedingly good and suitable substitute for glass is vulcanite, or hard india rubber (caoutchouc durci, as the French term it), a compound of india-rubber and sulphur.

When a large electrical machine, like any one of those above described, is employed, instead of the diminutive apparatus sketched in Art. 929, there are the following differences :-The faint luminosity on the simple tube is converted into vivid sparks constituting almost a cascade of fire passing from the loose end of the rubbing silk to the prime conductor, and at short intervals a decided flash with loud crack darts from the conductor to any object placed near it. If a bystander approach his hand he receives the spark or flash with a painful prick and even with momentary numbness along the arm. A person insulated by standing on a stool with glass feet, if he touch the conductor, becomes so strongly electrified as to give out sparks from the fingers to a person wishing to shake hands with him. These sparks are strong enough to light a gas lamp or to explode a fulminating powder.

When the equable distribution of electricity in bodies is disturbed by friction or otherwise, causing redundancy in one body and defi.

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