A triode is a type of vacuum tube that consists of three electrodes, a heated cathode, a control grid, and an anode. It was the first electronic amplifier and was used in early radio receivers. A magnetron is a type of vacuum tube that uses a magnetic field to control the flow of electrons. It is used in microwave ovens and radar systems. A tetrode is a type of vacuum tube that has four electrodes, a cathode, a control grid, a screen grid, and an anode. It is used in audio amplifiers and oscillators.