When a body has more of the electric fluid than its natural quantity, it is said to be electrified positively, when less, negatively; but neither of these cases can occur in a conductor, unless the communication between it and the earth be cut off by the intervention of an electric or non-conductor. When this happens, the conductor is said to be insulated.
It may not be amiss here to mention, that the terms electric or an electric per se, and non-electric, were at first made use of from an erroneous idea that only those called electrics, contained the electric matter in their substance, which was capable of being excited by friction, and communicated by them to those called non-electrics, and supposed to be destitute of it: for glass and other electrics, being rubbed, discovered signs of having it, by snapping on the approach of a finger or other conductor, and by attracting and repelling light bodies; while other substances could not be made to produce any such effect. It has however since been proved by experiments, that both electrics and non-electrics contain this matter in their substance; but that non-electrics cannot be excited, owing to the fluid diffusing itself through them as soon as collected. These terms are therefore improper, and as the only difference is that some bodies will conduct electricity and others will not, the terms non-conductor and conductor are those which might generally be used with the most propriety in speaking on this subject; though, in conformity with custom, we shall often use non-conductor and electric as synonymous.
CHAP. II.
Electric substances; with some of the phenomena attending their excitation.
Those substances by which electrical phenomena may be produced, form the subject which next demands our attention; but these are so numerous that it would be vain to attempt to specify them all. Perhaps it may be doubted, whether every material substance, with the exception only of metals, water, and charcoal, may not be considered as an electric.
Some however exhibit particular phenomena more obviously than others; and hence a number of catalogues have been formed, for shewing the effects which arise when electrics are excited with different rubbers. The specification which we esteem the most complete, was formed by the ingenious Mr. Cavallo, and we shall give it in his own words.
“In the following table (says he) may be seen what electricity will be excited in different bodies, when rubbed with different substances. Smooth glass, for instance, will be found by this table to acquire a positive electricity, when rubbed with any substance hitherto tried, except the back of a cat: (by which I mean the skin of a cat while on the animal alive:) rough glass, (viz. glass, the polish of which has been destroyed by emery or otherwise) will be found to acquire the positive electricity, when rubbed with dry oiled silk, sulphur &c. and the negative when rubbed with woolen cloth, the hand &c. and so of the rest.”
| Electrics. | Qualities. | Rubbers. |
|---|---|---|
| “The back of a cat | Positive | Every substance with which it has hitherto been tried. |
| Smooth Glass | Positive | Every substance hitherto tried, except the back of a cat. |
| Rough Glass | Positive | Dry oiled silk, sulphur, metals. |
| Negative | Woolen cloth, quills, wood, paper, sealing wax, white wax, the human hand. | |
| Tourmaline | Positive | Amber, or air blown upon it. |
| Negative | Diamond and the human hand. | |
| White silk | Positive | Black silk, metals and black cloth. |
| Negative | Paper, hand, hare’s & weasel’s skin. | |
| Black silk | Positive | Sealing wax. |
| Negative | Hare’s, weasel’s and ferret’s skin, load-stone, brass, iron, silver, hand. | |
| Weasel’s skin | Positive | Metals, silk, load-stone, leather, hand, paper, baked wood. |
| Negative | Other fine furs. | |
| Sealing wax | Positive | Metals. |
| Negative | Hare’s, ferret’s and weasel’s skin, hand, leather, woolen cloth, paper. | |
| Baked wood | Positive | Silk. |
| Negative | Flannel.” |
From the above table it appears, that the powers of electric substances vary prodigiously from one another; and that, according to the different rubbers made use of, we may sometimes produce one phenomenon and sometimes another. Hence we have a foundation for classing electric substances according to the various powers they occasionally exhibit; which may be done in the following manner.
First. Those which exhibit a strong and permanent attractive and repulsive power, of which the most remarkable is silk.
Second. Those which exhibit the electric light, attraction, repulsion, and all the other phenomena of electricity in a very vigorous, though not durable manner; of these glass is eminently preferable to all others.