Electrical Fields

There are two fields in electricity: the electrostatic and the magnetic field. These are found at right angles to each other around the electron. The electrostatic field, although present between the terminals of an e. m. f. source, is seldom used to do work in electrical circuits. A common location of this type of field is found between the plates of a charged condenser.

A current in a coil cause a magnetic field around the conductor. If the current is alternating, the field will build and collapse with the current alternations. When another coil is brought into the field of the first coil, because magnetic lines of force are being cut by a conductor periodically, an e. m. f. is developed in the second coil. The intensity of a field varies inversely as the square of the distance traveled. If that distance is doubled, the intensity is one-fourth the original. The extent or strength of a field depends upon the magnitude of the current.

Triodes

When a third electrode is inserted between the filament and the plate, the vacuum tube is enabled to perform other operations. This electrode is called the control grid and is generally in the shape of laterally wound wire or mesh whose turns are spaced so as to permit a current flow from filament to plate. It is also insulated from other electrodes, its purpose being to control the flow of plate current. Because the grid is placed very close to the source of electron emission, a small change in any voltage supplied to the grid will have as great a controlling effect on the plate current as a relatively large change in plate voltage.

The quantity of electrons attracted to the plate depends upon the combined effect of the plate and grid charges. When the plate is positive, normally, and the grid voltage is made more and more negative, the plate is less able to attract electrons and the plate current decreases. The grid being made less and less negative, the plate can more easily attract electrons and the plate current increases.

Pentodes

When still another grid is inserted between the screen grid is inserted between the screen grid and the plate, a five-element tube is formed. Its construction is similar to that of the control grid, and it is often connected to the cathode inside the tube. This third grid is called the suppressor grid. There are many combinations of these electrodes in other tubes, the tube itself varying only in the number and position of its electrodes.

As higher and higher voltages are applied to the tube, the electrons strike the plate with such velocity that they dislodge electrons from the plate material. This secondary emission wastes energy and lowers the efficiency of the tube.

To eliminate this waste, the suppressor grid is erected between the screen grid and the plate. It is generally connected to the cathode, which gives it a low negative potential. Electrons emitted by the plate are repelled back to the plate or suppressed.

All types of vacuum tubes are derived from one or more of the foregoing classes. Design and operation characteristics are merely altered to fit the tube for special types of service.