The pole of a magnet which tends to point to the north, and whence lines of force are assumed to issue on their course to the other pole of the magnet. 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. P =Paper Cable.= A cable insulated with waxed or paraffined paper.