=Unit.= The single standard of force, light, heat, magnetism, attraction, repulsion, resistance, etc. V =Vacuum.= A space empty or void of all matter; a space from which all gases have been exhausted. =Vacuum Tubes.= Tubes of glass through which electric discharges are passed after the gases have been partially removed; for example, the X-ray tube of Röntgen and the Crooke tubes. =Vibrator, Electro-magnetic.= The make-and-break mechanism used on induction-coils, or other similar apparatus, in which, through alternate attractions, an arm or spring is kept in motion. =Vitriol, Blue.= A trade name for copper sulphate. (Bluestone.) =Vitriol, Green.= A trade name for ferrous sulphate. (Copperas.) =Vitriol, White.= A trade name for zinc sulphate. (Salts of zinc.) =Volt.= The practical unit of electro-motive force; the volume and pressure of an electric current. =Voltage.= Electric-motive force expressed in volts--as, a voltage of 100 volts. =Voltaic.= A term derived from the name of the Italian scientist Volta, and used in many ways as applied to electrical current and devices. Formerly the term galvanic was commonly employed. =Voltaic Electricity.= (_See_ Electricity, Voltaic.) =Voltimeter.= An instrument for measuring the voltage of a current. =Vulcanite.= Vulcanized rubber. Valuable for its insulating properties and inductive capability. W =Watt.= The practical unit of electrical activity; the rate of work or rate of energy. It is a unit of energy or of work represented by a current of one ampere urged on by one volt of electro-motive force. The volt-ampere. The standard of electrical energy corresponding to horse-power in mechanics. =Watt-hour.= A unit of electric energy or work; one watt exerted or expended through one hour. =Waves, Electro-magnetic.= Ether waves caused by electro-magnetic disturbances affecting the luminiferous ether. =Welding, Electric.= Welding by the use of the electric current. =Wimshurst Electric Machine.= An influence machine for producing high potential or static electricity. Thin disks of glass are mounted on insulated bearings and revolved by power. Brushes collect the frictional electricity, which is discharged into a Leyden-jar or other form of accumulator. It is of no practical use excepting in electro-therapeutics. =Wire, Flexible.= A cord of fine wire strands laid together and insulated so that it may be easily bent or wrapped. =Wiring.= Installing wires so as to form a circuit for the conveyance of current for light, heat, and power. X =X-rays.= A curious phenomenon involving the radiation of invisible rays of light, which have the power to travel through various opaque bodies. The rays are used in detecting foreign substances in the human body and for photographing invisible or hidden objects without disturbing their surroundings.