An electron tube has 2 or more electrodes separated either by vacuum or by ionized gas at low pressure . Its operation depends on generation and transfer of electrons through the tube from 1 electrode to another. The source of electrons is cathode, generally a metallic electrode which releases a stream of electrons by one of various mechanisms described below. Once the electrons have been emitted, their movement can be controlled by a magnetic field, an electric field,or both. An electric field is established by application of a voltage between electrodes in the tube, while a magnetic field can be produced outside the tube by an electromagnet or permanent magnet. In the simplest form, an electron is attracted and accelerated by positive electrode and is repelled and slowed by negative electrode . An electric field is used to change the path of electron stream, alter the number of flowing electrons, and modify their speed. The magnetic field primarily serves to control the movement of the electrons from 1 electrode to another.