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ELECTRICAL APPARATUS 95
either to pass on to anode, or to return to cathode, by a very slight change in the voltage acting upon them. Because of the grid's location a very weak voltage applied to the grid by means of the C circuit may make an enormous difference in the number of electrons reaching tinanode, that is to say, in the amperage oi the space current. If the grid voltage be a.c. or fluctuating d.c. the space current will fluctuate in exact response to every slight change in grid voltage. The pattern of the grid fluctua tions (which may perhaps resemble the bottom line ol Fig. 8, page 36) will be reproduced in every detail, though on a much greater scale, in the plate current, with the result that that pattern has been amplified by the action of the tube.
(96) A tube of the type shown in Fig. A-2 is called an amplifying tube of the three-element type, the threeelements being anode, cathode and grid. The cathode may be directly heated, as in Figs. 41 and 42, or indirectly as in Fig. 40. Another name often given this type of tube is triode, meaning it has three electrodes.
(97) The tube may have four elements and then is known as a tetrode. In such tubes an additional grid, called the screen grid, is mounted between the control grid and the anode. Since the anode and control grid are conductors separated by the insulation of a vacuum, they often act like a small condenser, with undesirable results that will be explained hereafter. To break up this condenser effect a screen grid is placed between them. The screen grid is positivly charged at a potential somewhat lower than the anode potential.
(98) The pentode or five-element tube in addition haa negatively charged suppressor grid which entirely surrounds the anode. Its purpose is to suppress secondary emission from the anode (page 93). In spite oi it own negative charge, usually derived from connection to the cathode, the effect of this grid in suppressing secondary emission permits a greater flow of current through the
tube.
(99) The beam power tube is a tetrode with a cathode so shaped, and so flanked by negatively charged vanes.