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538 RICHARDSON'S BLUEBOOK OF PROJECTION
Grid of the Amplifier Tube
Placed between anode and cathode, the grid consists of a metal skeleton or mesh with plenty of empty space between the turns of the spiral. It may be a ladder-like construction of metal, with empty space between the wire "rungs." It may be a loosely-woven fabric of metal screen, or "cloth." Occasionally it consists of a metal plate in which many holes have been drilled. The object of its construction is to compel negatrons moving through the tube from cathode to anode to pass close to the metal of the grid, without, however, interposing any physical barrier in their path.
"Space Current" of an Amplifying Tube
(3) The flow of negatrons from cathode to anode, across the empty vacuum of the tube, is called the "space current," (4) or plate current. If there were nothing else in the tube but the anode and cathode (no grid) the size of the plate current in milliamperes would depend wholly upon the emission from the cathode and the positive attraction of the anode.
(5) Amplification as obtained in a vacuum tube is the process of using a weak variable grid voltage to cause strong variations in the plate current. The tube, in short, is a valve, operated by a weak current and controlling a strong one. This has been said before. It cannot be repeated too often. When once that much about a tube is thoroughly understood it ceases to be any kind of mystery and becomes a very simple piece of electrical apparatus, with circuits that behave like any of the ordinary electrical circuits with which everyone is familiar.
How the Tube Amplifies
(6) Consider first the plate circuit. It has two wires, like any other circuit. One of these wires connects to the cathode of the tube, one to the anode. The positive wire goes to the anode. When the cathode is heated negatrons flow across the tube from cathode to anode, closing the