It is not necessary however in this form of the machine to have two plates, the second being added only to increase the power. The plate must be firmly fastened by its centre to an axis—so as to turn vertically between two uprights of baked wood, as in the construction of the cylindric machines; but in this case the uprights must be so close together, as barely to leave room for a rubber on each side of the plate. The rubbers may be made of the same form with that in the cylindric machine—except that they must have a projection at the back, to fit a niche cut in the uprights which support the plate. The power of the machine will be increased by having four rubbers; two above and two below the axis of the plate. The prime conductor is placed opposite one of the ends of the axis, and is divided at the end towards the electric into two branches or arms, which extend horizontally to the circumference of the plate, each of which is furnished with points to collect the electricity. As plates are not always to be procured, a good substitute may be found in a thick pane of glass or a piece of an old looking-glass. Mark with a diamond or file a circle on the glass, of the size you intend for your plate. Then putting the plate into warm water, after some time cut the glass with a diamond in tangents. The more numerous the cuts, the nearer the plate will be to a circle. A hole may be made in the centre for the axis, by scratching with a diamond, and grinding with a rod of iron (held between the hands) and emery. CHAP. V. _Of communicated electricity._ Having described the electrical machine, we are now to consider some of the phenomena attending its operation. When the prime conductor receives electricity from the cylinder, it is said to be _electrified by communication_, and it then acts in every respect like the cylinder itself, except that the latter, when touched by a conductor communicating with the earth, gives a considerable number of sparks before it is discharged; whereas the conductor discharges itself by a single spark. The cause of this difference is that the cylinder, being an electric, cannot convey the electricity of all its surface to that part, to which the conducting substance is applied; but the fluid accumulated in the whole conductor, passing easily through its substance, is transmitted at once to the point from which the discharge is made. Hence it appears that the electricity discharged from an electrified conductor is more powerful than that discharged from an electric—the conductor acquiring a large quantity of electricity from an electric, by receiving it gradually, spark after spark, and afterwards, when touched, discharging it all at once.