VACUUM TUBES

are devices used in radio communication, and distance wire telephony, for producing, amplifying and rectifying alternating currents. It has been known for turies that metals at high temperatures exhibit peculiar electrical characteristics, but it remained for Profe or in 1905 to apply the thermoionic sion theory to radio work. He by the earlier observations of and of Elster and Geitel, and constructed ve RS vacuum, his ‘Two Element Fleming Valve,’ whick an air acted as a rectifier for receiving circuits. may This valve consisted of a tube or bulk air containing an incandescent lamp fila. state. ment, and an electrode in the form of a by plate, enclosed in a gla bulb with the among air exhausted. When the filament is heated by the pa age of an electric current from a battery or other source it emits negative electrons. The latter devices are attracted to the plate, owing tc used the positive charge which it maintains from by virtue of its connection with the posietc. tive side of the circuit, consisting of the rotary plate, a battery and one end of the which filament. This circuit, although not a nozzles, closed metallic circuit, carries current moment. when the filament is incandescent, and finely electrons are being emitted. If the plate air to were negatively charged, it would repel such the electrons; hence the apparatus acts is as a rectifier, or electrical check valve, to be permitting the current to flow in one the direction, but not in the other. This i: which is the action desired in the detector of a filtered radio receiving set. Three element vac bag as uum tubes, called ‘Triodes,’ were conceived by Dr. Lee De Forest. They were are of the same general construction as Fleming’s valve, but contain a third machines electrode in the shape of a grid of wire utilize located between the filament and the on plate. The addition of this grid permits elect he tube to be used as an amplifier wlator. This type of tube is in wide usé the in radio transmitting and receiving sets be and also in long distance telephone driven systems. or this town, on Danube, Hungary; bp.’s see and trade in grapes. Pop. 17,200. ft. of VAGINA, DISEASES OF. See Grne COLOGY. with a