Triode

It would be noted that for a diode, the flow of current out of the anode to the external circuit cannot be controlled. Once it starts pouting out the current, there is no limit to its rate of flow, unless the supply voltage is cut off or removed totally. Since the flow of this current depends in the amount charge  of the vacuum tube , then these electrons can be controlled by introducing a third electrode. This is the basis of another component or device called triode  valve. As the name implies, it consists of three electrodes namely, cathode, anode and control grid. The triode is similar to the diode in construction; the only major difference is the introduction of a grid in the triode. This control grid in the triode valve is a circular spiral element made from a fine wire. This is normally used to surround the cathode, which will continue to control the amount of electrons that are "boiling" out of it. The triode is used for amplifying current.