state we wish to determine, cause the leaves to diverge still more, we conclude that it is resinously electrical; if, on the contrary, they collapse upon its approach, it is a proof that it is in the vitreous state (67).

There is another mode which is still preferable to this, as affording not only an indication of the kind of force which is required to be tested, but also an accurate measure of its intensity. It consists of a needle of shell-lac, suspended from a fine thread of spun glass. At one extremity of the needle is placed a small disc of paper, or other conducting substance; which, being thus insulated, is capable of receiving and retaining a charge. When another electrified body is brought near to this, the kind of electricity which has been communicated to it being known, it will indicate by its attraction or repulsion, the species of electricity thus brought within its sphere of action; and the intensity of the force may be measured upon a scale by the amount of torsion in the thread which may be required to bring the two repelling bodies into mutual contact (68).

$306. Experiment has demonstrated that the same substance is not always similarly electrified by friction; but that its electrical state is determined partly by the condition of its own surface and

(67) Bennet's gold-leaf electrometer is here represented; the leaves being in a state of divergence from a charge which has been communicated to them.

The improved mode of insulation introduced by Mr. Singer is shown

in the section a. The wire to which the gold leaves are attached passes through a glass tube packed with dry silk, and is covered by a brass cap, screwed to its upper end, by which it is protected from dust and damp.

(68) The following cut represents Coulomb's torsion-electrometer, or electrical balance. ab is a thread of silk or spun glass, from which the

partly by the body with which it is rubbed. Thus flint-glass with a smooth surface, when rubbed with woollen cloth, becomes vitreously electric; but if its surface be roughened, and it be rubbed with the same cloth, it will be resinously electrical.

Smooth glass, rubbed with every substance which has been tried, except the fur of an animal, as a cat-skin, becomes vitreous: with a cat-skin, resinous.

White silk rubbed with black silk, metals, or black cloth, is vitreous; rubbed with paper, or the hand, resinous.

Sealing-wax, rubbed with the metals, is vitreous; with cloth, paper, or the hand, resinous.

§ 307. It was first observed by Otto de Guericke, and Hawksbee, that the friction of glass and resinous substances not only produced the phenomena which we have just described, but, under favourable circumstances, was accompanied by a rustling or crackling noise; and, when the experiment was made in a dark room, by flashes and sparks of light upon their surfaces. When once the attention has been directed to the observation, most persons will find that such phenomena of electrical light are familiar occurrences, and often present themselves in suddenly drawing off from the person a silk stocking, or a flannel waistcoat, or in the friction of long hair, by combing. How small a degree of friction is sufficient to excite electricity in the human body, is shown in a striking way by placing a person upon an insulating stool with glass legs. If in such a position he place his finger upon a gold-leaf electrometer, and

needle of shell-lac, c, is suspended. It is attached to the screw, b, by which it can be twisted round its axis. The needle carries a gilt ball of pith, or a disc of paper, at one extremity, which is balanced by a counterpoise at the other; d is a metallic wire passing through the glass shade, and terminated by a metallic ball at each end. The ball of the needle and the interior brass ball of the wire are brought into contact by turning the screw, b, and the index then points to 0 on the scale, which is marked upon the circumference of the glass.

®

another person flip him lightly with a silk handkerchief, the leaves will immediately repel each other.

§ 308. We may now proceed to describe the means of producing these effects upon a large scale, and the different modes which have been devised of exciting, collecting, and measuring the electric fluids.

The electrical machine, as it is called, whatever form it may assume, resolves itself into a more or less perfect way of applying to glass the friction of silk, or some other dissimilar substance, as in our fundamental experiment, and of drawing off the electricity as it is excited, and collecting it in a good insulated conductor. In machines of the more perfect construction, not only is the vitreous electricity collected in what is called the prime conductor, but the resinous also is accumulated by insulating the rubber, and connecting it with a metallic cylinder (69).

(69) This figure represents the cylinder electrical machine; A is a glass cylinder, properly mounted, to turn upon two stout glass pillars,

It is made to revolve rapidly,

by means of the multiplying wheels, L and K. CC are two brass cylinders,

Experience has shown that the most effective exciter of glass is a metallic substance; and when a soft amalgam of mercury, tin, and zinc, is spread upon the rubber, it forms a flexible metallic surface which is perfectly adapted to the purpose and greatly heightens the effect. The best proportions of the metals have been found to be one part of tin and two of zinc, fused together, and mixed whilst fluid with six parts of mercury. The mixture must then be triturated in a mortar, with a little lard, to the consistence of a thick paste. When large cylinders, or plates of glass, are made to rotate by convenient mechanical means, pressing against cushions covered with this metallic coating, they are capable of producing all the phenomena of attraction and repulsion in a high degree, and their state of intense excitement is accompanied by flashes and sparks of brilliant light, and a crackling sound (70).

When the prime conductor is placed in its proper position it receives from the glass, without contact with it, a charge in the form of a stream of fire, which, whatever may be the size of its surface, rises to nearly the same intensity as that of the original source; and which being thus acumulated in quantity will wholly pass off at once to any uninsulated conductor, or will instantaneously divide itself with an insulated one, by means of a dense spark. Similar phenomena may be obtained with the conductor attached to the rubber; but to obtain the highest effect from either, it is necessary to make a good conducting communication from the other to the ground. The reason of this is, that when the two electricities are in presence of each other they counteract and limit each others intensity: by connecting either conductor with the earth, its charge is spread over an indefinitely large surface, and virtually annihilated. That the two conductors are in opposite electrical states is easily proved by suspending from each some light substances which will strongly attract each other when charged: and that the two charges are

supported upon the two glass pillars, DD, in a position parallel to the glass cylinder; to one of these a cushion, F, with a silk flap, G, is attached, which is made to press against the glass by a spring, its distance being capable of adjustment by means of the screw, H. The opposite brass cylinder has a row of points attached to it, to which the silk flap, G, extends. The latter goes by the name of the prime conductor, or the positive conductor; the former is distinguished as the negative conductor.

(70) The Plate Electrical Machine is figured in the next page. A is a circular glass plate, mounted upon a brass axis, and turning in a stout

exactly equal is shown by making a good metallic communication between the two, when all signs of excitement will cease in both.

§ 309. And now, it may be asked, where does the charge reside in a good insulated conductor? does it diffuse itself, like heat, throughout the whole of its substance? or is it confined merely to its surface? That it is merely superficial is susceptible of proof in several ways. If we take a solid globe of metal, and electrify it to a certain amount, and then bring it in contact for a moment with a globe of similar dimensions, made of the thinnest shell of the same metal, we shall find that the charge will distribute itself equally between the two. If we take a hollow sphere, with an aperture at its upper part, and having given it a charge of electricity, touch its interior with a small insulated ball, upon bringing this proof-ball in contact with an electroscope we shall find that it will afford no signs of having received a charge; but, on the

wooden frame, by means of the handle F; as it revolves it passes between two pair of cushions, BB, which are pressed lightly against it, by means of screws; c is the brass prime conductor, supported upon the stout glass

arm, D; it is armed at its two extremities, which are opposed to the plate, with two rows of points which meet the ends of the two silk flaps, which are attached to the cushions, BB.

E E,