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540 RICHARDSON'S BLUEBOOK OF PROJECTION
then simple electrical arrangements with nothing mysterious about them. The grid circuit arrangements are unusual, but not puzzling. The valve action of the tube depends upon the fact that the plate current (the flow of negatrons from cathode to anode) is extremely sensitive to the degree of negative charge upon the grid.
The reason for this sensitivity lies in the construction of the tube, which is so made that negatrons moving from cathode to anode must pass close to the metal of the grid. They do not touch that metal since the negative charge repels them, deflecting them sufficiently to cause them to pass through the empty spaces provided for that purpose. But if the negative charge is high many negatrons that would otherwise reach the plate do not pass through the grid at all. They are repelled when they come close to the grid and driven back again to cathode. If the charge on the grid were made sufficiently high the plate current would be zero, but it is not made that high. The grid bias is set at a very critical balance, so that a negatron approaching the grid is attracted by the distant positive plate, and repelled by the negative grid just ahead, with almost equal force. Under these circumstances a very small change in the grid charge makes a vast difference in the number of negatrons that are able to reach the anode.
Possibly an illustration will help make these simple facts even clearer, and prevent their ever being forgotten. Let us imagine a pair of very delicate, carefully balanced scales, with ten ounces in each pan. A one-ounce weight is then added to one of the two pans, and the other immediately rises. Ten ounces have been lifted by one ounce because the one-ounce weight upset a critical balance.
It is the same inside an amplifying tube. Whenever a few additional negatrons are added to the charge of the grid, a great many negatrons are prevented from reaching the plate, and repelled toward the cathode. Whenever a few negatrons are taken from the charge of the grid, a large number of negatrons that would otherwise have been repelled toward cathode are able to pass through the spaces of the grid and add themselves to the plate