Vacuum tube
The computer's first switch: a light bulb that could say yes or no, and that broke about as often as one.
A vacuum tube controls the flow of electrons between a heated cathode and an anode inside an evacuated glass envelope. A third electrode grid regulates that flow, giving both amplification and switching — the basis of radio and of the first computers alike. The technology arrived in 1904 and dominated electronics for fifty years.
The problems were the filament and the glass. Tubes ran hot, drew a lot of current and burned out like lamps. ENIAC with its 18,000 tubes suffered a failure roughly every other day, handled by never switching the power off — most tubes fail during warm-up. The transistor solved all of this at once, and the computers of the 1960s were an entirely different kind of machine.