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tube, or whatever is capable of being thus excited, is called an electric.
Charles. Are pot all sorts of solid substances capable of being excited ?
Tutor. You may rub this poker, or the round ruler for ever, without obtaining an electric spark from them.
James. But you said one might get a spark from the mahogany table if it had more than its share.
Tutor. So I say you may have sparks from the poker, or ruler if they possess more than their common share of the electric fluid.
Charles. How do you distinguish between bodies that can be, and those that caönot be, excited ?
Tutor. The former, as I have told you, are called electrics, as the glass tube; the latter, such as the poker, the ruler, your body, and a thousand other substances, are denominated conductors.
Charles. I should be glad to know the reason of the distinction, because I shall be more likely to remember it.
Tutor. That is right: when you held your knuckle to the glass tube, you had several sparks from the different parts of it: but if J, by any means, overcharged a conductor, as this poker, all the electricity will come away at a single spark, because the superabundant quantity flows instantaneously from every part to that point where it has 4 an opportunity of getting away. I will illustrate this by an experiment. But first of all let me tell you, that all electrics are called also non-conductors.
James. Do you call the glass tube a nonconductor because it does not suffer the electric fluid to pass from one part of it to another?
Tutor. I do :-silk, if dry, is a con-conductor. With this skein of sewing-silk, I hang the poker or other metal substance a 12 (Plate vii. Fig. 1.) to a hook in the ciel. ing, so as to be about twelve inches from it ; underneath, and near the extremity, are some small substances, as bits of paper, &c. I will excite the glass tube, and present it to the upper part of the poker.
Charles. They are all attracted but now you take away the glass they are quiet.
Tutor. It is evident that the electric fluid passed from one part of the tube through the poker, which is a conductor, to the paper, and attracted it :-if the glass be properly excited, you may take sparks from the poker.
James. Would not the same happen if another glass tube were placed in the stead of the poker?
Tutor. You shall try.--Now I have put the glass in the place of the poker, but let me excite the other tuts as much as I will, no effect can be producer on the paper : there are no signs of electrical attraction, which shows that the electric fluid will not pass through glass.
Charles. What would have happened if any conducting substance had been used, instead of silk, to suspend the iron poker?
Tutor. If I had suspended the poker with a moistened hempen string, the electric fluid would have all passed away through that, and there would have been no (or very trifling) appearances of electricity at. the end of the poker.
You may vary these experiments till you make yourselves perfect with regard to the distinction between electrics and conductors. Sealing-wax is an electric, and may be excited as well as a glass tube, and will produce similar effects. I will give you a list of electrics, and another of conductors, disposed according to the order of their perfection, beginning in each list with the most perfect of their class : thus glass is a better electric than amber, and gold a better conductor than silver :
Glass of all kinds.
the most transparent the
best. Amber. Sulphur. All resinous substances. Wax of all kinds. Silk and cotton. Dry extern.d substances,
as feathers, wool, and
hair. Paper ; loaf sugar. Air, when quite dry. Oils and metallic ox
ides.* Ashes of animal and vege
table substances. Most hard stones.
All the metals in the fol.
lowing order : Gold ; silver ; Copper; platina ; Brass ; iron ; Tin ; quicksilver ; Lead. The semi-metals. * Metallic ores.* Charcoal. The fluids of an animal
body. Water, especially salt wa
ter and other fluids, ex
* This, and other chemical terms, are esplained and familiarly illustrated in a work just published, by the author of the Scientific Dialogues, entitled “Dialogues in Chemistry," &c.