O
O. An abbreviation for Ohm.
Oersted’s Discovery. Oersted discovered, in 1820, that a magnetic needle tended to place itself at right angles to a current of electricity. This fundamental principle is the basis of the galvanometer, the dynamo, and the motor.
Ohm. The practical unit of resistance. A legal ohm is the resistance of a column of mercury one square millimetre in cross-sectional area and 106.24 centimetres in length.
Ohm, True. The true ohm is the resistance of a column of mercury 106.24 centimetres long and one square millimetre in cross-sectional area. An ohm may be measured by a No. 30 copper wire nine feet and nine inches long. If larger size wire is used the piece must be proportionately longer, since the resistance is less.
Ohmic Resistance. True resistance as distinguished from spurious resistance, or counter electro-motive force. (See also [Resistance, Ohmic].)
Ohm’s Law. The basic law which expresses the relations between current, electro-motive force, and resistance in active circuits. It is formulated as follows:
1. The current strength is equal to the electro-motive force divided by the resistance.
2. The electro-motive force is equal to the current strength multiplied by the resistance.
3. The resistance is equal to the electro-motive force divided by the current strength.
O. K. A telegraphic signal meaning yes, or all right. It is supposed to be a misspelled form of all correct, “Oll Kerrekt.”
Okonite. A form of insulation for wires and conductors; a trade name applied to insulations, and protected by copyright.
Open Arc. A voltaic arc not enclosed.
Open Circuit. (See [Circuit, Open].)
Oscillating Current. (See [Current, Oscillating].)
Outlet. That part of an electrolier or electric light fixture out of which the wires are led for attachment to incandescent light sockets.
Outside Wiring. The wiring for an electric circuit which is located outside a building or other structure.
Overhead Feeders. The same as overhead conductors.
Overhead Trolley. The system in which the current for the propulsion of trolley-cars is taken from overhead feeders or wires.
Overhead Trolley-wire. A naked, hard copper wire drawn at high tension, and suspended over or at the side of a car-track, and from which the trolley-wheel takes its current.
Overload. In an electric motor, an excess of mechanical load prevents economical working, causing the armature to revolve slowly and the wiring to heat. In this case heating implies waste of energy.
Overload Switch. A switch which operates automatically to open a circuit in line with a motor, and so save the motor from overheating or burning in the event of an overload.