. The principles underlying radio communication . in space charge effect whichwould result if electrons were emitted more rapidly, or. moreexactly, for any extra electrons emitted, an equal number ofthose in the space are repelled back into the filament. If nowthe plate voltage is increased to a new value, Fa, the plate-current curve will rise higher before bending over as shown bythe dotted portion of the curve in Fig. 255, because it takesa larger space charge to offset the effect of the plate at thehigher voltage. 190. The Three-Electrode Electron Tube.—Between the filamentand plate of a tu
. The principles underlying radio communication . in space charge effect whichwould result if electrons were emitted more rapidly, or. moreexactly, for any extra electrons emitted, an equal number ofthose in the space are repelled back into the filament. If nowthe plate voltage is increased to a new value, Fa, the plate-current curve will rise higher before bending over as shown bythe dotted portion of the curve in Fig. 255, because it takesa larger space charge to offset the effect of the plate at thehigher voltage. 190. The Three-Electrode Electron Tube.—Between the filamentand plate of a tube we may insert another piece of metal. Thisthird electrode inteiT>osed in the stream of electrons betweenfilament and grid is usually in the form of a metallic gauze ora grid of fine wires, and is generally called the grid. Atube which contains a filament, plate, and grid is called athree-electrode tube and is capable of many more uses than thetwo-electrode tube. Fig. 256 shows the construction of a THE THREE-ELECTRODE TUBE. 457 GRID GRID. Fig. 256.—Construction of the three-electrode electron tube. 458 ELECTRON TUBES. three-electrode tube of American manufacture. The additionof the third electrode makes it possible to increase or decreasethe current between plate and filament through wide limits. Ifa voltage is impressed upon the grid by means of a third batteryconnected between the filament and grid, shown at C in Fig,257, the space charge effect will be modified. The electronstraveling from filament to plate pass between the wires formingthe grid. If the grid is given a potential which is negativewith respect to the filament the grid will repel the electrons, but
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