Briefly describe the valve in electronics.

(N/A) vacuum tube is also known as a valve.
The valve consists of a varying number of electrodes (plates). They are named based on the number of electrodes:
- Vacuum diode: $A$ glass tube containing a vacuum with two electrodes. One is called the anode (plate) and the other is called the cathode.
- Vacuum triode: $A$ glass tube containing a vacuum with three electrodes. $A$ mesh electrode placed between the anode and the cathode is called the grid.
- Vacuum tetrode: $A$ glass tube containing a vacuum with four electrodes.
- Vacuum pentode: $A$ glass tube containing a vacuum with five electrodes.
In a vacuum tube,thermionic electrons are supplied by a heated cathode or by a filament placed near the cathode through which current is passed.
Voltage is varied between different electrodes to control the flow of electrons passing through the vacuum.
$A$ vacuum is required in the inter-electrode space; otherwise,moving electrons may lose their energy upon collision with air (gas) molecules in their path.
In these devices,electrons can flow only from the cathode to the anode (i.e.,in only one direction). Therefore,such devices are known as valves.