=Current, Periodic.= A current with periodically varying strength or direction. A current alternating periodically. =Current, Polarizing.= A current which causes polarization. =Current-reverser.= A switch or other contrivance for reversing the direction of a current in a conductor. =Current, Undulating.= A current whose direction is constant but whose strength is continuously varying. =Currents, Eddy.= Useless currents in an armature, in the pole pieces, and in the magnetic cores of dynamos and motors. They are created by the high speed of the armature in its rotation, or by other electric currents induced by the armature’s motion through magnetic fields. =Currents, Faradic.= Induced currents. They take their name from Michael Faraday, the original investigator of the phenomena of electro-magnetic induction. The secondary or induced electro-magnetic currents and their accompanying phenomena. A series of alternating electro-static discharges from influence machine, such as the Holtz and Wimshurst. The simple and commonly understood Faradic currents are those produced in the medical battery, and used in medical therapeutics. =Currents, Foucault.= A form of currents produced in revolving armature-cores; sometimes called eddy currents. They are useless. =Currents, Harmonic.= Currents which alternate periodically, and vary harmonically. Currents which vibrate at certain pitches, as, for instance, the currents in wireless telegraphy. Two instruments must be tuned to the same pitch in order to be responsive. Thus an instrument sending out waves of 70,000 vibrations cannot be recorded by one tuned much below or above the same number. Sound waves of sympathetic or harmonic vibrations. =Currents, Positive.= (_See_ Positive Currents.) =Cut-in.= To electrically connect a piece of mechanism or a conductor with a circuit. =Cut-out.= The reverse of the cut-in. To remove from a circuit any conducting device. The cut-out may be so arranged as to leave the circuit complete in some other way. An appliance for removing a piece of apparatus from a circuit so that no more current shall pass through the former. =Cut-out, Automatic.= A safety device for automatically cutting out a circuit to prevent accident or the burning-out of an apparatus, due to an overload of current. It is worked by an electro-magnet and spring. An overload of current causes a magnet of high resistance to draw an armature towards it, and this, in turn, releases the spring of the cut-out device. Sometimes a strip or wire of fusible metal is employed which is in circuit with a switch. The excess of current fuses the metal, and the broken circuit releases a spring-jack, which, in turn, breaks the circuit. =Cut-out, Safety.= A block of non-conducting material, such as marble, slate, or porcelain, carrying a safety-fuse or plugs. In these is enclosed a piece of fusible wire, which burns out or melts and breaks the circuit before the apparatus is damaged.