Instead of four cushions, there are, as will be seen by the figure (Fig. 354), only two, which are in communication with a spherical conductor, supported on a glass pillar. This may be used to collect negative electricity, in the same way as the negative conductor in Nairne's machine. The chief or positive conductor consists of an insulated sphere, on the top of which is often another sphere of smaller size. The positive electricity is collected from the plate by

means of two rings opposite to each other, one on each side of the plate. On the side next the plate, they have a groove, which is lined with metal, and studded with points. They are supported by an arm which is inserted in the positive conductor. The size of the positive conductor is often increased by the addition of a very large ring (3 or 4 feet in diameter) which is supported on the top of the large sphere. The ring consists of very stout brass wire inclosed in wellpolished mahogany.

Winter's machine appears to give longer sparks than the ordinary machine under the same circumstances. This circumstance is owing, partly at least, to the considerable distance between the rubber and the positive conductor, which prevents the occurrence of discharges between them.

HYDRO-ELECTRIC MACHINE.

579

585. Hydro-electric Machine.-About the year 1840, Mr. (now Sir) W. Armstrong invented an electric machine, in which electricity was generated by the friction of steam against the sides of orifices, through which it is allowed to escape under high pressure. It consists of a

boiler with the fire inside, supported on four glass legs. The steam, before escaping, passes through a number of tubes which traverse a cooling-box containing water, into which dip meshes of cotton, which are led over the tubes, and passed round them. The cooling thus produced in the tubes, causes partial condensation of the steam. This has been found to be an indispensable condition, the friction of per

fectly dry steam being quite inoperative. Speaking strictly, it is the friction of the drops of water against the sides of the orifice, which generates the electricity, and the steam merely furnishes the means of applying the friction. The jet of steam is positively, and the boiler negatively electrified. The positive electricity is collected by directing the jet of steam upon a metal comb communicating with an insulated conductor.

The form of the outlet by which the steam escapes is shown in Fig. 356. The steam is checked in its course by a tongue of metal, round which it has to pass, before it can enter the wooden tube through which it escapes into the air. This machine, in order to work well, requires a pressure of several atmospheres. The water in the boiler should be distilled water. If a saline solution be intro

duced into the tube through which the steam escapes, all traces of electricity immediately disappear. The generation of electricity varies, both in sign and degree, according to the substance of which the escape-tube is composed, and according to the liquid whose particles are carried out by the steam. Thus, when a small quantity of oil of turpentine is introduced into the jet of steam, the boiler becomes positively, and the steam negatively electrified.

Fig. 356.-Outlet of Steam.

The hydro-electric machine is exceedingly powerful. At the Polytechnic Institution in London, there was one with a boiler 78 inches long and 42 in diameter, and with 46 jets. Sparks were obtained from the conductor at the distance of 22 inches. The machine is, however, very inconvenient to manage. A long time is required to get up the requisite pressure of steam. The boiler must be carefully washed with a solution of potash, after each occasion of its use; and, finally, the working of the machine is necessarily accompanied by the disengagement of an enormous quantity of steam, which, besides causing a deafening noise, has the mischievous effect of covering with moisture everything within reach. Accordingly, though very interesting in itself, it is by no means adapted to the general purposes of an electrical machine.

586. Electrophorus.—When electricity is required in comparatively 'small quantities, it is readily supplied by the simple apparatus called the electrophorus. This consists (Fig. 357) of a disc of resin, or some other material easily excited by friction, and of a polished metal

disc B with an insulating handle CD. The resin disc is electrified by striking or rubbing it with catskin or flannel, and the metal plate is then laid upon it. In these circumstances the upper plate does not receive a direct charge from the lower, but, if touched with the finger (to connect it with the earth), receives an opposite charge by induction. On lifting it away by its insulating handle, it is found to be charged, and will give a spark. It may then be replaced on the lower plate (touching it at the same time with the finger), and the process repeated an indefinite number of times, without any fresh excitation, if the weather is favourable.

Fig. 357.-Electrophorus.

The resinous plate has usually a base or sole of metal, which is in connection with the earth while the electrophorus is being worked. When the cover receives its positive charge on being connected with the earth, the sole at the same time receives from the earth a negative charge, and as the cover is gradually lifted this negative charge gradually returns to the earth.

The most convenient form of the electrophorus is that of Professor Phillips, in which the cover, when placed upon the resinous plate, comes into metallic connection with the metal plate below. That this arrangement is allowable is evident, when we reflect that, when the upper plate is touched with the

finger, it is in fact connected with the lower plate, since both are connected with the earth; and it effects a great saving of time when many sparks are required in quick succession, for the cover may be raised and lowered as fast as we please, coming alternately into contact with the resinous plate and the body which we wish to charge.

587. Bertsch's Electrical Machine.A machine which has been called a rotatory electrophorus was invented a few years ago by Bertsch, and is represented in Fig. 359. A circular plate of ebonite D can be made to revolve rapidly. A sector of the same material (Fig. 358), previously excited by friction, is

fixed opposite the lower portion of the plate; and on the other side, immediately opposite to this, is a metallic comb N forming the extremity of a conductor connected with the earth. At the upper part is another comb M connected with the conductor A. Under

the influence of the electrified sector, the conductor C discharges positive electricity on the plate through the comb N. In passing the comb M, a portion of this electricity is collected by the points, and charges the conductor A. The effect is increased by connecting A with another conductor E of very large dimensions.

588. Voss' Machine.-In Voss' machine, which is a modification of an earlier form invented by Holtz, the inducing charge may be indefinitely small at first, and is rapidly increased. It gives much more powerful effects than the friction machine, and is much easier to manage and keep in order. It is represented in Fig. 360.

There are two glass plates, a small distance asunder. The larger one is fixed, and the smaller one is made to revolve rapidly by means of a driving band passing over two grooved wheels, one of them much larger than the other, the larger one being turned by