Electron Valve

Electron Valve

One of the first applications of the vacuum tube, we have seen, was to produce X-rays. Here, electric current is transformed into light rays. But the vacuum tube can also be adapted to the reverse purpose to transform light into electric current. A familiar example is the device for opening a door when a person approaching it intercepts a fixed beam of light. This beam is focused on a selenium cell which is connected in series with a circuit passing through the grid of a valve. The cell varies in conductivity with the amount of light falling upon it, and when the fixed beam is intercepted a marked change occurs in the grid current. This in turn has an amplified effect on a power current passing through the terminals of the tube, sufficient to operate a switch and turn on an electric motor that opens the door.

Another way of converting light to current is to make the cathode of a valve out of light sensitive material. Then light replaces heat as the stimulator of electronic emission. Variance in the amount of light, and thus in the electron flow, causes a corresponding variance in the flow of current through the grid circut. This device is used in motion picture projection, to convert the "sound track" on film, which is band of varying shades of light and dark, into actual sound. It is also the basis of the "scanner" used in television.

The simple diode, without a grid, makes the best rectifier to change alternating into direct current. The triode, or diode with grid, can be made to perform the reverse function by a simple link that feeds part of the outgoing current back to the grid. Then direct current fed in comes out as alternating current. A very important application has been to step up frequencies to millions of cycles per second, far beyond the range of previous methods. Radio transmitters use "oscillators" on this principle, and so can operate at frequencies never touched by wireless telegraph equipment.

Probably one of the largest fields opened up by "electronics" will be the applications of ultra high frequency currents. Already they have been utilized to build a radically new type of calculator. All mathematical computations can be reduced ultimately strange as it may seen to simple addition. For example, you can multiply any number by 7,593,028 by adding it to itself that number of times. Naturally, that is not practicable method for the human mind. But circuits of electron valve can be arranged to translate the numbers into electrical impulses, and to "count" the number of impulses in successive additions. The earliest "electronic counters" made could aount to 5,000 in less than a second, and later machines are stepping up the pace by hundreds of time.

Another vast field of electronics is the precision control of industrial operations by electron valves. A control circuit is connected to the grid, so that the amount of current flowing through it is governed by time, speed, temperature, or whatever other variable is vital. The circuit operating the machine goes through the tube. Then variations in the grid circuit cause corresponding or compensating changes in the operating current. Already this arrangement has found extensive use in governing motor speeds and controlling tools where great precision is required.