ELECTRIC MACHINE.
An understanding of induction is essential to an appreciation of static electricity. Static in duction is the production of an electrical charge upon one body by another body that is statically charged. It might be termed electrical influence.
a — If the circle a becomes charged by friction, so that it is positively and electrostatically charged, as indicated by the sign -F, then on being brought near to another body b, the near end of b will become negatively charged, as indicated by the minus sign —, and the far end positively -F. By touching b with the finger the electricity may be drawn off, or °earthed" or °grounded," passing through the body of the toucher to the earth if he is not insulated. The electrophorus is the simplest device for illustrating this principle and is described under Ft rrnuc MACHINE BLit it should be under stood that the current producible by a static electric machine does not differ from that of a battery or a dynamo, except that it is naturally and usually much less in quantity, being a primi tive form of apparatus. From these early ex periments it was demonstrated that every body positively charged repelled another body of positive charge; and any negatively charged body repelled another negatively charged body, while attracting a positively charged body. It was soon apparent, however, that the forces of attraction and repulsion varied greatly when the bodies were exposed in some other medium than the ordinary atmospheric air. The word di electric was coined to express the material medium in which a body statically charged was located, and the term has been extended by common consent to mean any non-conductor, as glass or mica. A static machine was sometimes termed a dielectric machine, and the transmit ting of electric forces by induction instead of conduction was dielectric; while the power of a dielectric to resist stress caused by induction across it, as measured by difference of potential necessary to break through in a violent dis ruptive discharge, was denominated dielectric strength.
Referring again to the positively charged circle a, let it represent a sphere. The electric charges it has been determined by various ex periments. are on the outside surface of the sphere and not inside. This is apparent in the case of a cylinder open at the ends through which something may be passed; and also in the transferring of a charge on a globe by enclosing it with larger hemispheres. If an electroscope be covered by a wire cage and insulated below a charge may be brought near or in actual contact without any disturbance of the sensitive mechanism of the instrument. The surface being the thing affected, it becomes ap parent that if one wishes to increase the capacity of a conductor, as a trolley-wire, the simple way is to increase its surface, as by malcing it twice as thick. Capacity may also be increased by filling the space through which the lines of force tend to pass with some dielectric other than air, as with petroleum, mica or glass. Large quantities of mica are so used in elec trical machinery.
To measure the force of electricity obtained . in primitive electric experiments a delicate in strument was necessary, and Coulomb invented the torsion balance, using a needle suspended at the centre of its gravity by a very fine wire or silk thread, etc. A minute electrical force of repulsion allowed to operate against one end of the needle so as to twist the thread was therefore measurable as a mechanical force. This principle is used in the torsion electrometer and various other instruments. Experimenting with a torsion balance, Coulomb demonstrated that the transfer of statical electricity by in duction involved an expenditure of force, the force varying inversely as the square of the distance of the centre of the bodies affected See ELEcruc MACHINE—Static Electric Induc tion Machine and ELECTROTHERAPEUTICS, where various static machines are described and illustrated.