The remarkable property possessed by shell lac of transmitting the electric fluid with more difficulty than any other body, was discovered by Al. Coulomb, and has already been noticed in our History of Electricity.
When any of the substances in the preceding table aeccive the least accession of moisture, their non-con ducting power is diminished in proportion to the quan titv of moisture which they have absorbed.
The slightest examination of the table is sufficient to convince us, that there is no relation between any of the chemical and physical properties of the bodies which it contains, and their power of resisting the passage of the electric fluid. The early electricians imagined that metals and waters were the great conducting principles ; but this property is obviously possessed by bodies which, so far as we know, contain neither of these ingredients.
In our History of Electricity, p. 238, we have already given some details respecting the length to which elec tricity has been conducted along metallic wires, and the rapidity of its motion. These details are sufficient to shew, that no measurable interval of time elapses dur ing the passage of the electric fluid along a space of four miles.
IN1r, Singer, however, has expressed a doubt respect ing the accuracy of these experiments. " They were made," says he, " at a very early period, and have not, 1 believe, been repeated since the improved state of the science has afforded the means of effecting such experi ments with precision. Metals, although the most per fect conductors we have, oppose some resistance to the motion of electricity, and a charge will even prefer a short passage through air to a circuit of 20 or 30 feet through thin wire. It is therefore rather uncertain, that the charge of a small phial has ever passed through an interval of four miles." Elements of Electricity, p. 142.
note.
The subject of electric atmospheres belongs properly tf: the present Section. The term atmosphere is here THE attraction exhibited by electrified bodies was one of the first. phenomena that was observed by philoso phers ; and we have already had occasion to notice an example of repulsion, in the divergency of two pith balls.
These simple phenomena may be perceived with suffi cient distinctness, by employing merely a tube of glass excited by a piece of silk ; but, in order to exhibit the more interesting phenomena, of the approach and reces sion of light substances to an electrified body, we must be able to produce a higher degree of electricity than can be effected by a simple tube.
In order to obtain this, a cylinder of glass is mounted in a frame, so that it can be turned rapidly round its axis by means of a winch or handle. On one side of it is placed a small cushion covered with silk, against which the glass cylinder is 'rubbed during its rotatory motion ; and on the other side a brass or metal tube, resting upon a stand of glass, for the purpose of collect ing the electricity generated during the excitation of the cylinder. This apparatus is called an Electrical Ma chine, and the metallic tube is named the Prime Conduc tor. Several of these machines, fitted up in various ways, are represented in one of our Plates, of which we shall afterwards give a very particular description ; but it is enough for our present purpose to know, that a ve ry high degree of electricity can be excited by the sim ple apparatus which has been mentioned.
Exp. 1. If, upon a tube of glass highly excited by a piece of silk, or by the human hand, we let fall any light substance, such as a piece of gold leaf or thin paper, the substance will be attracted to the tube, and then quickly repelled from it. If the tube is made to follow the body, it will still escape from it, till it meets with some other substance which is a conductor, and is not electri fied. To this conductor it will impart all its electricity, and being again attracted by the tube, will experience another repulsion as before. This experiment was first made by Otto Guericke.
Exp. 2. Having electrified the prime conductor of the electrical machine by exciting the glass cylinder, bring near to the conductor a pith ball, or a small feather sus pended by a slender wire, the ball will be attracted by the conductor, and will continue to adhere to it till the electricity of the conductor is dissipated.