A thermionic valve with three electrodes. Electrons produced by the heated cathode flow to the anode after passing through the negatively biased control grid. Small voltage fluctuations superimposed on the grid bias cause large fluctuations in the anode current. The triode was thus the first electronic device capable of amplification. Its role has now been taken over by the transistor, except where high power (radio-frequency transmitters producing more than 1kW in power) is required.