Vacuum Tubes:

Figure shows how different kinds of vacuum tubes look like. Let us see how Vacuum tubes perform the switching operation. A simple vacuum tube, as shown in the figure, consists of five main components:

Now I will explain their working one by one:

Glass Tube: It’s a tube housing all the other components. It may contain a gas or simply vaccum.

Cathode: The Cathode is made up of such a material that has many loosely bound electrons. When we connect the negative terminal of a battery with the cathode, the electrons in the cathode are repelled due to which escape from the cathode.

Anode: This component is comparatively at high potential (positive) as compared to the cathode i-e it is connected with the positive terminal of the battery. So, the electrons coming from the cathode are attracted by the anode.

Grid: In between the cathode and the anode is a grid. It has its own power supply and it is the grid which decides whether electrons will flow towards the anode or not.

When the grid is negatively charged, it will repel the electrons from the cathode and no electron will flow to the anode. But, if there is no charge on the grid, the electrons flow towards the anode. Actually, the grid performs the switching operation.

We know that the cathode releases electrons which are attracted towards the anode. In their path is the grid. The grid has a power supply of its own. This power supply charges the grid due to the flow of current. When the power supply is ON, the grid is charged negative (due to the current). When the power supply is OFF, the grid is uncharged.

When we want the electrons to flow from the cathode to the anode, we turn OFF the grid current. Turning on the grid current, will charge the grid, thus, repelling the electrons. This is how a Vacuum tube acts like a Switch.

For a better clarification, let’s check vacuum tube by connecting it in a circuit.

When the grid current is ON, the bulb will not glow. Why? Because the negative charge on the grid will repel the electrons, so the electrons (or the current, since, current is the flow of electrons) will not flow to the anode and the circuit is open. When we turn the grid current OFF, the electrons move to the anode and a current flows in the circuit. Thus, the blub glows.

You might have seen the first figure in which I showed different vacuum tubes. Those tubes were round. Actually, I am trying to keep things simple and straight. The figure I explained to you is just for understanding. The circular vacuum tube is shown below (I was feeling it a little difficult to explain). The figure used for explanation is just a cross-section cut out of the circular tube, but again it’s just for understanding.