=Pith-balls.= Balls made from the pith of light wood, such as elder. They are used in the construction of electroscopes and for other experiments in static electricity. =Plant.= The apparatus for generating electric current, including engines, boilers, dynamos, mains, and subsidiary apparatus. =Plate, Condenser.= In a static apparatus, the condenser having a flat piece of glass for a dielectric. It is mounted on an axle so that it may be revolved. =Plate, Ground.= In a lightning-arrester, the plate connected to the earth or ground wire. =Plate, Negative.= In a voltaic battery, the plate which is unattacked by the fluid. It is made of carbon, platinum, or copper. =Plate, Positive.= (_See_ Positive Plate.) =Plating-bath.= A vessel of solution for the deposition of metal by electrolysis. Used in electro-plating. =Plating, Electro.= The process of depositing metal on surfaces of metals or other substances by the aid of an electrolyte and the electric current. =Platinum Fuse.= A slender wire of platinum roused to incandescence by current, and used to explode a charge of powder or other combustible substance. =Plug.= A piece of metal, with a handle, used to make electric connections by being inserted between two slightly separated plates or blocks of metal. A wedge of metal, slightly tapered, and used to thrust between two conductors to close or complete a circuit. =Plumbago.= Soft, lustrous graphite; a native form of carbon sometimes chemically purified. It is used chiefly in electrotyping for dusting the wax moulds to make the surface an electric conductor. =Plunge-battery.= (_See_ Battery, Plunge.) =Polar.= Pertaining to one of the poles of a magnet. =Polarity.= The disposition in a body to place its axis in a particular direction when influenced by magnetism. For example, the attraction and repulsion at the opposite ends of a magnet. The N and S seeking poles of a compass needle is the simplest example. =Polarity, Electric.= The disposition in a paramagnetic body to be influenced by electric waves and lines of force. The otherwise non-magnetic body or mass becomes magnetic to attract or repulse when influenced by electricity, but ceases to retain the phenomena after the electric influence is removed. A piece of soft iron wire, a nail, or a short rod of iron will become electro-polarized when a current of electricity is sent through a coil of insulated wire so wound that one end will be N the other S. So soon as the circuit is broken the polarity ceases. =Polarization.= The depriving of a voltaic cell of its proper electro-motive force. This may be brought about through the solution becoming spent, or in the event of the acid being saturated with zinc, and so failing to act on the metallic zinc. Counter electro-motive force due to the accumulation of hydrogen on the negative plate. =Polarizing-current.= (_See_ Current, Polarizing.)