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224.]

ELECTROSCOPES.

457

sealing-wax with a bit of dry flaunel, and bring it near the paper disk; the disk will at first be strongly attracted, and will then be as strongly driven away. Whilst it is in this condition of repulsion by the wax, bring towards it a warm glass tube that has been rubbed with a dry silk handkerchief; the disk will be immediately attracted, and in an instant afterwards it will again be repelled, but it will now be found to be attracted by the wax. It is therefore evident, that by the friction of the glass and of the wax, two similar but opposite powers are developed. A body which has been electrified or charged with electricity from the wax is repelled by the wax, but it is attracted by the excited glass, and vice versa. In order to distinguish these two opposite powers from each other, that power which is obtained from the glass has been termed vitreous or positive electricity: that from the wax resinous or negative electricity.

Let us suppose that the paper disk has been charged by means of the glass tube, so that it is repelled on attempting to bring the glass near it; this state will be retained by the disk for many minutes. This contrivance forms, in fact, an electroscope, for it furnishes a means of ascertaining whether a body be electrified or not, and even of indicating the kind of electricity. Suppose that a body suspected to be electrified is brought near the disk, which is in a state repulsive of the glass tube; if repulsion occur between the disk and the body which is being tested for electricity, it is at once obvious that the substance is electrified; and moreover, that it is vitreously electrified, since it produces an effect similar to that which would be exhibited by an excited glass tube.

The phenomena of attraction and repulsion may be further exemplified by the following experiments :-Suspend

two straws, separately, by a fibre of silk, each to a FIG. 169. glass rod (fig. 169); bring an excited stick of sealingwax towards each; each will be first attracted and then repelled; whilst thus repulsive to the wax, bring the one near to the other; they will recede from each other as they did from the wax. If both straws be excited by glass, they will in like manner repel each other; but if one be excited by the glass and the other by the wax, they will attract each other. Hence we learn, that bodies similarly electrified repel, those differently electrified attract each other.

Proceeding a step further, it will be found that whenever two bodies are rubbed together, both kinds of electricity are liberated,

but so long as the two bodies remain in contact, no sign of the presence of either electricity appears; on separating them, both are found to be electrified-one vitreously, the other resinously; for example, stretch a piece of dry silk over a brass plate, and rub it upon a glass plate; so long as the two bodies are in contact, the quantities of each kind of electricity set free are precisely sufficient to neutralize each other, and the combined plates will not affect the electroscope, but as soon as the glass plate and the silk are separated, the glass will repel the disk (fig. 168), while the silk will attract it.

(225) Insulators and Conductors.-Bodies that have been thus electrically excited, return to their neutral condition when touched by other substances, but with degrees of rapidity depending on the kind of body which touches them. A rod of sealing-wax or of shell-lac, for example, may be held in contact with any electrified body without sensibly lessening the charge; but the momentary touch of a metallic wire, or of the hand, is sufficient to remove all indications of electric excitement: it is therefore clear that there are some bodies which, like the wire or the hand, readily allow the passage of electricity, and these are termed conductors; whilst there are others which, like shell-lac, do not easily allow its passage, and these are called insulators. There is, however, no absolute line of distinction between these two classes of bodies; there is no such thing as either perfect insulation, or perfect conduction, for the two classes of bodies pass gradually one into the other.

In the following table each substance enumerated is superior in insulating power to all those which follow it. The nearer the substance is to the bottom of the table, the better, on the contrary, is its conductivity

Insulators.

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Dry Gases and Dry Steam.

Shell-Lac.

Sulphur.

Amber.

Resins.

Caoutchouc.

Gutta Percha.

precious stones.

Silk

Dry Fur.

Glass.

Spermaceti.

Turpentin and Volatile Oils.
Fixed Oils.

String and Vegetable Fibres.
Moist Animal Substances.
Water.

Saline Solutions.

Flame.

Diamond, and some other Melted Salts.

Plumbago.

Charcoal.

All the Metals.

Conductors.

Ice.

226.]

GOLD-LEAF ELECTROSCOPE-COULOMB'S ELECTROMETER.

459

Any object is spoken of as being electrically insulated when it is supported by means of some badly-conducting substance which prevents the free escape of the electricity. The presence of moisture deposited from the air upon the surface even of the best insulator converts it for the time into a conductor, and is one of the most annoying impediments to the success of electrical experiments, as the electricity is carried off as fast as it is accumulated. Glass is especially liable to this inconvenience, but by varnishing it when practicable, and keeping it thoroughly warm, the difficulty is diminished. By due precautions, instruments may be constructed which, in dry air, will preserve a charge for a considerable time, weeks or months.

The most perfect insulators still allow electric attraction to traverse them, although by a process different from conduction, and hence they are termed Dielectrics (230). For instance, if one side of a plate of glass be electrified by rubbing it with a piece of silk, the opposite face also acquires the power of attracting particles

of bran or other light objects.

FIG. 170.

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FIG. 171.

B

(226) Electroscopes.-Various instruments have been devised for detecting feeble charges of electricity. One of the most convenient of these is the goldleaf electroscope (fig. 170), which is sensible to extremely small charges. It consists of a pair of gold leaves suspended from the lower extremity of a metallic wire which terminates above in a brass plate. The wire is insulated by passing it through a varnished glass tube packed with silk, and the whole is surrounded and supported by a glass case. The approach of an excited body instantly causes the divergence of the leaves. If a glass tube be rubbed with a dry handkerchief, and touched with a small disk of paper insulated by attaching it to a rod of sealing-wax, as directed in preparing the electroscope (fig. 168), a small vitreous charge will be received by the paper, and if carried by it to the cap of the electroscope, the leaves will diverge permanently with vitreous electricity. The approach of the glass rod would cause the leaves to diverge further, whilst that of a stick of excited wax would cause them to collapse.

An instrument (fig. 171) called a torsion electrometer was devised by Coulomb for accurately measuring minute differences in the amount of electrical attraction or repulsion. The force which he opposed to that of electricity was the resistance to twisting which is offered by an elastic thread. fibre of silk, a fine silver wire, or a thread of glass,

A

has been used for the purpose of measuring the angle of torsion, this angle being exactly proportional to the force applied for small displacements.

By means of a long glass thread, fastened above to a pin, P (carrying an index which traverses the graduated plate, B), a needle of shell-lac is suspended freely in the glass case, A. This needle is terminated at one end by a gilt ball, b, at the other by a paper disk which serves to check its oscillations. In the glass cover of the instrument is a small aperture through which another gilt ball, a (the carrier), also suspended by shell-lac, can be introduced and withdrawn. In order to equalize the induction, two narrow strips of tinfoil, e and d, connected with the earth, and having a narrow interval between them, are pasted upon the inside of the glass cylinder, one a little above and the other a little below the level of the balls; a graduated circle is pasted on the glass for reading off the angular deviation of the needle. When the instrument is to be used, the carrier-ball is adjusted so that after it has been removed it can with certainty be replaced in the same position as at first; the ball upon the needle is adjusted by turning the pin until, without any twist upon the thread, it shall just touch the carrier, its centre being at the zero of the scale, and the position of the index on the upper graduated plate, B, is noted. The carrier-ball, a, is next made to touch the object the electricity of which is to be measured: it takes off a quantity proportional to the amount accumulated on the spot. The ball a is immediately replaced in the instrument; it divides its charge with the ball b on the needle, and repulsion ensues. The thread which supports the needle is then twisted until the centre of the ball b is, by the reaction of torsion, brought back towards the carrier, a, to some determinate angle (say 30°) marked on the graduation of the glass case; suppose the number of degrees through which it has been necessary to twist the thread to be 160°; 160 + 30, or 190, will represent the repulsion. To compare this amount with any other quantity, the balls must be discharged, and the experiment repeated under the new conditions, noting the number of degrees of torsion required to make the needle stand at 30° as before:

FIG. 172.

the repulsion is directly proportional to the torsion angle in the two cases. Suppose in a second experiment that the thread sustain a twist of 180° before the ball b is brought back to the angle of 30°; the repulsion will now be 180 + 30, or 210; and the relative electrical repulsions in the two experiments will be as 190: 210.

Another very convenient electrometer was devised by Peltier, in which the directive force exerted by the earth upon a small magnet is substituted for the torsion of a wire. Fig. 172 represents Peltier's electroscope: a b is a metallic wire terminating above in a brass knob, and cemented by means of shell-lac into an insulating foot of ebonite, c. At b is a brass ring, from which proceed two brass arms, d d. In the ring is supported a light metallic needle, e, which moves freely upon a pin like a compassneedle. This metallic needle carries a small magnetized steel wire, m. In order to use the instrument, it is placed so that

the needle, e, when the steel wire, m, is exactly in the magnetic meridian, is just made to touch the arms, d d. On communicating a charge of electricity to the

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226.]

ELECTROMETERS-EXCITEMENT OF ELECTRICITY.

461

ball, a, it spreads over the insulated wire and needle, e; the needle is immediately repelled by the fixed arms, d d, and the amount of its angular deviation gives the means of estimating the repulsion; this, however, is not directly proportionate to the number of degrees which represent the angle of deviation. values must be ascertained by direct experiment.

These

The most perfect form of instrument yet produced is Sir William Thomson's quadrant electrometer. (Report of Committee on Standards of Electrical Resistance. British Association Report for 1867, 490.) The needle consists of a thin aluminium plate, narrower in the centre than at the ends, and with the ends rounded off; through the centre of the plate a platinum wire is fixed, with a short cross-piece at its upper end, to which are attached two fibres of unspun silk, by which the wire and needle are suspended. The needle is placed within a flat circular box of brass, divided into four quarters, and with the centre cut away to permit the passage of the suspending wire and its prolongation. These quadrants are attached by insulating glass stems to the brass cover of a Leyden jar containing concentrated sulphuric acid as the inner coating, and coated externally with tinfoil. To the platinum wire below the needle is attached a piece of finer wire supporting a platinum weight, which hangs in the sulphuric acid, and the needle is thus kept charged with electricity when the apparatus is in use; a small concave mirror is fixed to the platinum wire above the needle, and by its means very small movements of the needle are rendered visible by the motion of the image of an illuminated slit or wire on a screen placed at some distance from the mirror. The quadrants are so mounted that their edges are not in contact, but the opposite quadrants are connected by metallic wires; two binding-screws are fixed to the lid of the instrument, by which a charge may be communicated to the quadrants. The silk fibres are placed in such a position that when the quadrants are discharged the needle stands over the spaces between the pairs of quadrants, and they bring the needle back into this position if displaced from it. In order to use the instrument, the Leyden jar is charged with positive electricity. It is essential that this charge should be constant, and the force of the charge is measured by the attraction experienced by a light aluminium plate placed at the top of the instrument, opposite a brass plate in connexion with the interior coating of the jar; the charge of the jar is increased or diminished by means of the replenisher, which raises the charge when a milled head is turned in one direction, and lowers it when turned in the other, but for a description of which the reader must be referred to the original paper. If now a feeble charge be communicated to one pair of the quadrants while the other pair is connected with the earth, or charged equally and oppositely, the needle will be attracted by one pair and repelled by the other, and will consequently rotate on its axis in a direction determined by the arrangement of the connexions of the quadrants. This instrument is so delicate that a single cell of Daniell will cause the spot of light on the scale to move over 60 divisions (of of an inch at a distance of 40 inches)—a very considerable effect if we consider the very low tension of the electricity at the poles of a voltaic couple (259).

It was long imagined that non-conductors only were capable of excitement by friction, and hence they were termed electrics ; all bodies, however, exhibit this phenomenon, if proper care be taken to insulate them. If, for example, a piece of brass tube insulated by a glass handle be rubbed upon fur, it receives a charge, as may be shown by bringing it near the disk of the electroscope (fig. 168). Even two dissimilar metals, after being brought into contact with each other, may, with proper precautions, be made to

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