=Polar Surface.= The surface of a magnetic substance through which the magnetic flux passes in or out. =Pole-changer.= An automatic, oscillating switch or contact-breaker which reverses the direction of the current. =Pole, Negative.= The S pole in a magnet or compass needle. =Pole, Positive.= (_See_ Positive Pole.) =Pole-switch, Single.= A switch designed to open or close one lead only. =Poles.= The terminals of an open electric circuit at which there necessarily exists a potential difference. The terminals of an open magnetic circuit, or the ends of a magnetized mass of iron. =Porcelain.= A fine variety of earthenware, valuable for insulators and insulating purposes. =Porosity.= The state or property of having small interstices or holes. The opposite of density. =Porous Cup or Cell.= A cup or cell made of pipe-clay or of unglazed earthenware through which a current of electricity can pass when wet or in a liquid. Porous cups are used in cells and batteries to keep two liquids apart, and yet permit electrolysis and electrolytic conduction. =Positive Currents.= Currents which deflect the needle to the left. =Positive Electricity.= The current that flows from the active element, the zinc in a battery, to the carbon. The negative electricity flows from the carbon to the zinc. =Positive Electrode.= The electrode which is connected with the positive pole of a source of electric energy. =Positive Feeders.= The lead or wire in a set of feeders which is connected to the positive terminal of the generator. =Positive Plate.= In a voltaic cell, the plate which is acted upon and corroded. The current from the positive plate is negative electricity. =Positive Pole.= The N pole in a magnet or magnetic needle. So called because it seeks the north or negative pole of the earth. =Positive Wire, or Conductor.= The wire, or conductor, connected with the positive pole of any apparatus which produces electro-motive force. =Potential, Electric.= The power to perform electric work. =Potential Energy.= Capacity for doing work. Potential energy when liberated becomes actual energy for the performance of work. =Power-generator.= Any source from which power is generated. =Power-house.= A station in which the plant of an electric power system is operated and the current distributed to local or long-distance points. Power-houses are either primary or secondary stations. In the primary station the current is generated directly by the aid of mechanical power, either the steam-engine or the steam-turbine. The secondary station, or sub-station, is located at a distance from the main power-house, and has no mechanical means of generating current. The current, usually of high alternating voltage, is supplied to the sub-station from the main power-house; and by means of transformers and converters, the high-voltage current is transformed into one of lower E-M-F and higher amperage, for distribution over local lines.