The principal parts of the machine are, the electric, the rubber, the moving engine, and the prime conductor. We shall take notice of each of these parts separately and then describe the whole machine together.
Formerly different kinds of electrics were used; at present smooth glass is preferred before all others, as most convenient, and because it will, by itself, answer the purposes of several others. For when the machine has an insulated rubber, which is easily prepared, the operator may produce positive or negative electricity[[11]] at pleasure, without changing the electric.
With respect to the forms of the glass, those commonly used are globes, cylinders and plates. The most convenient size for a globe is from ten to twelve inches in diameter. It should have two necks, centrally opposite, which must be cemented[[12]] to strong caps, in order to adapt them to a proper frame. Cylinders are also made with two necks. Their common size is from six to seven inches in diameter, and from ten to twelve inches in length; the glass generally used is the best flint.
It has long been questioned whether a coating[[13]] of some electric substance, has any effect in increasing the power of an electric; but now it seems pretty well determined, that if it does not increase the power of a good one, it at least considerably improves a bad one.
The next thing to be considered is the rubber which is to excite the electric. This, as it is now made, consists of a cushion of buckskin, stuffed with hair or flannel, and fastened to a piece of wood well rounded at the edges; to this is glued a flap of Persian black silk, which goes over nearly one half of the cylinder or globe. The rubber should be supported by a small iron or brass spring, placed inside of it, as is represented edgewise by R, figure 2, in the frontispiece. This acts in a much more uniform and parallel manner than if it were placed under the cylinder. It suits any inequalities that may be on the surface of the glass, and by means of a screw may be made to press against the cylinder as occasion requires. It should likewise be insulated in the most perfect manner by glass, or by baked wood well varnished. But when experiments are to be made which do not require or admit of insulation, a communication must be made between the rubber and the earth, by a chain or conductor.
To increase the effect of the rubber several substances have been used with success, particularly whiting and pulverised chalk. But the best of all is an amalgam of zinc and mercury.[[14]] This amalgam is to be used by first applying a moderate quantity to the cushion; and afterwards by spreading it on a separate piece of leather, and applying it occasionally to the under part of the cylinder while turning. In this method of using it, only a small quantity of amalgam is consumed, while the glass is very strongly excited; and by degrees the whole rubber contiguous to the cylinder is covered with amalgam, in the form of a concave cake. It is with such a rubber that the cylinder is most powerfully excited.
An ingenious friend has favoured us with the following explanation of the manner in which electrics are excited, which to us is more satisfactory than any other we have seen. “In order that electricity may be accumulated in greater quantity in one body than in the surrounding ones, it must be set in motion. This may be effected by the rubbing of electrics; the juxta-position of non-electrics of different conducting powers; and by the chemical action of many, if not all bodies on each other. The rubber will act on the first principle, and the more perfect the contact between it and the electric the greater will be the effect. The chalk, whiting, amalgam &c. while they will, if properly prepared, make the contact more perfect, will also be of service on the second principle; and the amalgam will besides be of use on the third. Mercury and zinc may be exposed separately to the air without any alteration; but when combined they readily unite with the oxygen of the atmosphere; especially when the surface of contact is frequently renewed, and the temperature increased by friction.
“The glass, acquiring a different state of electricity from the rubber, will, as each portion passes from under it, carry away and impart to the prime conductor the excess which it has obtained; and this the more certainly if the dissipation of the electricity be prevented, or the accumulation increased, by a piece of silk connected with the rubber.—The chain making the communication between the rubber and the adjoining non-electrics will enable this process to go on; and perhaps may also assist on the second principle.”
With respect to the engine which is to give motion to the electric, it has been customary, simply to turn the globe or cylinder with a winch; but this will not produce the greatest power of which the glass is capable. To effect this it should be made to turn six or seven times in a second, which is more than can conveniently be done with the winch only; and therefore multiplying wheels are used with advantage.
The prime or first conductor is an insulated non-electric substance, furnished with a number of points on the end towards the electric, in order to collect the electricity from it. It is usually made cylindrical, but whatever be its form it should always be perfectly free from points or sharp edges, except the points toward the electric already mentioned; and if holes are made in it, which on many accounts are very convenient, they should be well rounded and perfectly smooth.—The larger this conductor is, if not disproportionate to the cylinder or globe, the stronger and more dense will be the electric spark, which will proceed from it when touched by a blunt conductor. There must however always be a certain proportion between the cylinder or globe and the prime conductor, for if the former be small and the latter large, the electricity will not be collected fast enough, to preserve an accumulation of it in the prime conductor, because a portion is always taken off by the air, in proportion to the surface presented to it by the conductor.