Vacuum tube
A device that controls a flow of electrons inside an evacuated glass tube. Before the transistor, it did the work of amplifying and switching.
Electrons boiled off a heated electrode travel through the vacuum to an electrode opposite. In 1906 Lee de Forest placed a mesh electrode in between, creating the triode, in which a small voltage on the mesh produces a large change in the current[1]. It could serve as an amplifier for weak signals or as an on-off switch, and it underpinned radio, long-distance telephony and the first computers.
Its weaknesses were that the heated electrode consumes power, gives off heat and eventually burns out. ENIAC, in 1946, used about 17,000 tubes and was a constant battle against failures. The search for something smaller and more reliable led to the invention of the transistor.
Related terms
- TransistorA semiconductor device in which a small signal controls a larger current, so it can amplify or switch. The basic component of all electronics.
- Flip-flopA circuit that stays in one of two stable states and switches to the other when signalled. The smallest component that can remember one bit.
- Integrated circuitA circuit in which transistors and other components, together with the wiring between them, are all built into a single piece of semiconductor — a chip.