=Make and Break, Automatic.= An apparatus which enables the armature of a magnet to make and break its circuit automatically. =Make-and-break Current.= (_See_ Current, Make-and-break.) =Mercurial Air-pump.= An air-pump operated by mercury to obtain a high vacuum, and used extensively for exhausting incandescent-lamp bulbs. =Mercury Tube.= A glass tube sealed and containing mercury. It is so arranged as to give out fluorescent light when shaken or agitated by an electric current. For example, the Geissler tubes, the Cooper-Hewitt light, Crook’s tubes, etc. =Metallic Arc.= An arc which forms between metallic electrodes. =Metallic Circuit.= (_See_ Circuit, Metallic.) =Metallic Conductor.= A conductor composed of a metal. =Metallic Filament.= A metal wire used in an incandescent lamp--the filament. =Metallic Resistance.= (_See_ Resistance, Metallic.) =Metallurgy.= The art of working metals. Electro-metallurgy applies to the processes wherein electricity plays the most important part. =Mica.= A natural mineral of sheet form and translucent, used extensively as an insulator in electrical equipment and mechanism. =Mica, Moulded.= A composition composed of ground mica and shellac as a binder. When heated and pressed into various shapes and forms, it is a valuable insulator, and is employed for hooks, locks, tubes, sockets, and the like. =Micanite.= An insulating material made by cementing laminations of pure mica together and cementing them with shellac or other suitable non-conducting adhesives. =Molecular Adhesion.= The attraction of similar molecules for each other. =Molecular Attraction.= The attraction of molecules, or physical affinity. =Molecular Resistance.= The resistance which a mass or electrolyte offers when contained in an insulated vessel and a current of electricity is passed through it. =Molecule.= One of the invisible particles supposed to constitute matter of every kind; the smallest particle of matter that can exist independently. It is made up of atoms, but an atom cannot exist alone. =Morse Receiver.= The receiving instrument once universally used in the Morse system of telegraphy, but now superseded by the sounder. =Morse Recorder.= An apparatus which automatically records on a ribbon of paper the dots and dashes of the Morse telegraph alphabet. =Morse Sounder.= An electro-magnetic instrument designed to make a sharp, clicking sound when its armature lever is drawn down by the attraction of the magnets. =Morse System.= A telegraphic system invented by Prof. S. F. B. Morse, in which, by means of alternating makes and breaks of varying duration, the dots and dashes of the Morse alphabet are reproduced and received at a distance through the agency of wires and the electro-magnetic sounder.