sulated conductors: we must protect ourselves from it as much as possible, and, with this view, be careful frequently to wipe the conductors and the insulating supports with clean cloths.

In the electrical machine, as we have been describing it, the cushions communicate with the ground by means of the wooden supports, and of the table, also of wood, upon which these supports are placed; sometimes a metal chain is added to facilitate this communication: thus the negative electricity which they acquire by friction is lost in proportion as it is liberated; if we wish to collect it, the cushions must be insulated by glass supports, which require a slightly different construction. By thus insulating the cushions, we succeed indeed in obtaining negative electricity, but the quantity of positive electricity collected upon the insulated conductors is reduced to one half: this is a consequence of the laws of the liberation of electricity by friction, as we shall see further on. Hence it is better, when we wish to have positive electricity alone, to make the cushions communicate with the ground by means of a metal chain. If we wish to have negative electricity alone, we must, for the same reason, put the conductors, upon which the positive is liberated, in communication with the ground, by means of a metal chain. If we want to have both electricities at the same time, we must then make neither the conductors nor the cushions to communicate with the ground.

Formerly, in the construction of electrical machines, glass muffs or cylinders were employed, instead of circular plates. There are still some powerful machines constructed after this fashion. This gives rise to a few differences, which are easy to be perceived, in the details of the construction. We shall not dwell upon them. The machine Fig. 6., the cushions of which are insulated, is a cylinder machine.

An important circumstance is the quality of the glass of which the plate or cylinder is made. It is impossible to point out any rule in this respect; experiment alone can decide in each particular case. In general, the best glasses are those into the composition of which the least amount of alkali enters,

and the glasses with the potash base; also tolerably ancient glass, especially that of Bohemia, is preferable to modern

Fig. 6.

glass. It is affirmed that exposure to the sun for a certain time, during the whole of a fine summer's day for example, gives to a glass plate a great electrical power.

Different Apparatus that accompany the Electrical Machine.

The electrical machine is always accompanied by certain apparatus, with which we must become equally acquainted, on account of the frequent use that is made of them. They are, in particular, the quadrant electrometer, the discharging rod, and the insulating stool.

The Quadrant Electrometer (Fig. 7.) consists of a metal stem, which is fixed vertically on a foot, or upon the conductor of an electrical machine; the stem carries, in its upper part, a small wooden or ivory needle, terminated by a ball of elder pith, and movable in a vertical plane. It naturally acquires a vertical position, consequently parallel to that of the stem, with which the pith ball is in contact. But the electricity that is communicated to the instrument obliges the ball to recede from the fixed stem, and consequently makes the needle that carries it describe a larger or a smaller angle, which is measured upon a circular division.

Fig. 7.

The Discharging Rod (Fig. 8.) is a metal conductor composed of two similar stems, united by

a joint, which permits of their being separated to various distances, and each terminated by a ball; one, or more frequently two glass handles permit the experimenter to hold the metal stems with his hands, so as to use them for passing the electricity from one body to another, which, but for this contrivance, would escape into the ground. The Insulating Stool (Fig. 9.) is a wooden table, varying in size, supported by glass legs: all bodies that are placed upon it are thus in a state of insulation.

The feet are commonly of sufficient strength to support the weight of a man. If the latter, when placed upon an insulating stool, touches with his hand the conductor of an electrical machine in action, sparks

Fig. 8.

Fig. 9.

may be drawn from all parts of his body, similar to those that could be drawn from the conductor itself.

Some Experiments with the Electrical Machine.

Among the numerous experiments that may be made with the electrical machine, we shall confine ourselves, for the present, to mentioning the following:

If the hand is brought near to the conductor of the machine, a vivid and instantaneous light is manifested between the conductor and the hand, which is accompanied with a certain degree of painful sensation. It is the electric spark which we have already seen to be produced in the neutralization of the two electricities, and which here shows itself, when one alone of the two electricities, no matter which, is escaping from a conductor upon which it is accumulated, to pass through the air into another conductor.

Any conductor whatever, if substituted for the hand, produces the same effect.

We can compel the electric spark to traverse bodies of small conducting power, or of none at all, such as ether and alcohol, and it instantly inflames them.

It also inflames combustible gases, such as hydrogen: thus, if we bring a candle that has just been extinguished, near to the electrised conductor of the machine, so that the spark passes from the conductor to the candle, through the smoke, which is nothing more than carburetted hydrogen gas, the candle is relighted.

For inflaming hydrogen we often use Volta's pistol (Fig. 10.), which is a small sheet iron vessel, in the interior of which is situated the rounded end of a metal rod, insulated by means of a glass tube and wax, and the other extremity of which appears outside the glass. A little hydrogen, which becomes mixed with the air, is placed inside the vessel; Fig. 10. then, by means of a cork stopper, all communication with the exterior air is intercepted. The electric spark is made to pass into the interior, by means of the insulated conducting rod. There arises a strong detonation, produced from the combination of the hydrogen with the oxygen of the air, and the cork is projected afar.

The employment of the electrical machine and of the insulating stool, enables us to demonstrate in an elegant manner the differences of conductibility of divers substances. A person places himself upon the stool; he takes successively into one of his hands rods of glass, of wood, of metal, with which he touches the conductor of a machine, the plate of which is made to revolve; he brings the other hand to a conductor communicating with the ground. According to the insulating or conducting property of the rod, there is or there is not a spark, and according to its degree of conductibility, there is a longer or a shorter spark. We may even show the presence of the spark by employing it, although it comes out from the hand, to set fire to an inflammable substance, such as ether or hydrogen gas. In the same manner is demonstrated the insulating or conducting property of threads of various substances, such as caoutchouc, gutta

percha, dry or moist silk, &c., by making them communicate by one of their extremities with the conductor of the machine, and holding them with one hand by the other extremity.

Electroscopes.

Electroscopes or electrometers are apparatus designed for detecting the presence of electricity, for indicating its nature, and for measuring, if possible, its intensity. We have already spoken of simple electroscopes, and of the quadrant electrometer. The others are in general founded upon the principle that two very light bodies, when freely suspended, and very near to each other, are mutually repulsive, when they are charged with the same electricity; and this to a distance greater in proportion as the electricity is stronger. The most simple is formed of a glass rod, fixed vertically on a foot, and surmounted by a small horizontal metal cross-piece, terminated at the one end by a knob, and at the other by a hook, to which is suspended a linen thread, whose two ends are stretched vertically by two balls of elder pith fixed at each of the two extremities (Fig. 11.). The metal crosspiece is to be touched with the electrised body, and the electricity is immediately communicated through the linen thread, which is a conductor, to the two balls, which recede from each other. To know the nature of the electricity, we touch the metal cross-piece, whilst the electroscope is charged, with a stick of electrised wax: we thus give to the instrument negative electricity. If the two balls approach, we conclude that the electricity with which the instrument was charged was positive; if they separate further, we conclude that it was negative.

Fig. 11.

To render the instrument more sensitive and more accurate, the two light bodies are inclosed in a bell-glass, whence there arises a metal rod that sustains them, and which is to be touched with the electrised substance. The light bodies themselves are then either the two pith balls (Fig. 12.), or two blades of straw, as in Volta's electrometer (Fig. 13.),