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General Summary of Electricity, with Experiments.
TUTOR. You now understand what electricity is ?
Charles. Yes, it is a fluid which seems to pervade all substances, and when undisturbed, it remains in a state of equilibrium.
James. And that certain portion which every body is supposed to contain, is called its natural share.
Tutor. When a body is possessed of more, or retains less, than its natural share, it is said to be charged or electrified.
Charles. If it possess more than its natural share, it is said to be positively electri
fied; but if it contain less than its natural share, it is said to be negatively electrified.
Tutor. Does it not sometimes happen, that the same substance is both positively and negatively electrified at the same time?
James. Yes: the Leyden jar is a striking instance of this, in which, if the inside contain more than its natural share, the outside will contain less than its natural
Tutor. What is the distinction between conductors and non-conductors of electricity ?
Charles. The electric fluid passes freely through the former, but the latter oppose
Tutor. You know that electricity is excited in the greatest quantities, by the friction of conducting and non-conducting substances against each other.
Ex. Rub two pieces of sealing-wax, or two pieces of glass together, and only a very small portion of electricity can be obtained; therefore the rubber of a machine should be a conducting substance, and not insulated. VOL. III.
Every electrical machine, with an insulated rubber, will act in three different ways; the rubber will produce negative electricity: the conductor will give out positive electricity: and it will communicate both powers at once to a person or substance placed between two directors connected with them.
Fames. How does the rubber produce negative electricity ?
Tutor. If you stand on a stool with glass legs, or upon any other non-conducting substance, and lay hold of the rubber, or a chain that communicates with it, the working the machine will take away from you a quantity your natural electricity, therefore you will be negatively electrified.
Charles. Will this appear by the nature of the electric fluid, if I hold in my hand a steel point, as a needle ? Tutor. If
standing on a non-conducting substance, are connected with the the rubber, and your brother, in a similar situation, connected with the conductor, hold points in your hands, and I, while I stand
on the ground, first present a brass ball, or other substance, to the needle in your hand, and then to that in his hand, the appearance of the Auid will be different in both cases ; to the needle in your hand it will appear like a star, but to that in your brother's it will be rather in the form of a brush. What will happen if you bring two bodies near to one another that are both electrified?
James. If they are both positively or both negatively electrified, they will repel each other, but if one is negative and the other positive, they will attract one another till they touch, and the equilibrium is again restored.
Tutor. If a body containing only its natural share of electricity, be brought near to another that is electrified, what will be the consequence ?
Charles. A quantity of electricity will force itself through the air in the form of a spark.
Tutor. When two bodies approach each other, one electrified positively and the other negatively, the superabundant electri
city rushes violently from one to the other to restore the equilibrium. What will happen if your body, or any part of it, form part of the circuit.
James. It will produce an electric shock, and if, instead of one person alone, many join hands, and form a part of the circuit, they will all receive a shock at one and the same instant.
Tutor. If I throw a larger quantity of electricity than its natural share on one side of a piece of glass, what will happen to the other side ?
Charles. The other side will become negatively electrified: that is, it will have as much less than its natural share, as the other has more than its natural share.
Tutor. Does electricity, communicated to glass, spread over the whole surface ?
James. No; glass being an excellent nonconductor, the electric fluid will be confined to the part on which it is thrown: and for that reason, and in order to apply it to the whole surface, the glass is covered with tin foil, which is called a coating.