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646 RICHARDSON'S BLUEBOOK OF PROJECTION
(15) In order to reduce flicker, scanning is interlaced, the tip of the scanning beam following a path of the type shown in Figure 228. The entire field is scanned twice for each frame. Though there are 30 frames per second the field is scanned 60 times a second. The first scanning may take "lines'' 1, 3, 5, 7, 9, etc., all in l/60th second, and the second scanning take in "lines" 2, 4, 6, 8, 10, etc. Each such scanning constitutes one-half of a complete frame, the entire frame being scanned in l/30th second.
(16) In order to make the electron beam follow a path of the type shown in Figure 228, "saw tooth" frequencies must be applied to the deflecting vanes of Figure 224. If the deflecting frequencies were "pure" or sinusoidal a.c. of the type diagrammed in Figure 8, Page 36, the beam would follow a sinusoidal path. To make it follow a saw tooth path like that of Figure 228, "saw tooth" frequencies must be used. This type of frequency is characterized by a slow increase in voltage followed by a sudden, trigger-like reversal of voltage, which causes the electron beam to "fly back" almost instantly. There are several circuits that can be used with an oscillating tube to make it generate a saw tooth a.c. One popular device for this purpose is a gas-filled triode with a condenser connected across from plate to cathode. While this condenser is in process of charging the voltage across it builds up for a time very regularly. When the voltage reaches a critical point the gas ionizes and in consequence there is a sudden rush of current through the tube between cathode and plate. This surge of current reduces the plate voltage below the point necessary to maintain ionization of the gas. Then the gas returns to normal (deionizes), the excessive space current is reduced, and the plate voltage rises again until after a fraction of a second the gas again ionizes and another surge of current passes through the tube. The charge upon the condenser connected between plate and cathode undergoes a saw tooth fluctuation accordingly. The frequency of the saw tooth a.c. is governed by the grid bias applied to the tube, by the capacitance of the plate