A vacuum tube is in a glass capsule from which the air is removed, and that leads inside metal electrodes. A simple tube of two elements (diode) comprises a cathode and an anode, connected to the positive terminal of a power supply the latter. The cathode (a small metal tube that is heated by a filament) releases electrons that migrate toward him. If an AC voltage is applied to the anode, the electrons will flow towards the anode during the positive half cycle; during the negative cycle of the voltage AC, repelera anode the electrons, preventing any current to pass through the tube. partial discharges diodes connected in such a way that only allow the positive semiciclos of an alternating current (AC) are referred to as rectifier tubes and are employed in the conversion of alternating current to direct current (DC). To insert a rack, formed by a metal wire spiral between the cathode and the anode, and by applying a negative voltage to This grille, is possible to control the flow of electrons.
If the grid is negative, it repels them and then only a small fraction of the electrons emitted by the cathode can reach the anode. This type of tube, called the triode, can be used as an amplifier. Small variations of tension that occur in the grid, such as those generated by a radio or sound signal, can generate large variations in the flow of electrons from the cathode to the anode and, consequently, into the circuitry connected to the anode.