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The primary component of electronics in the 1940s was of course vacuum tubes, called valves by the British. I count 87 tubes on the panel shown in this picture. For you younger people who haven't seen or used tube-based electronics, compared to a transistor, a tube is large (three to six inches high and a inch or two in diameter), a tube gets very hot, a tube uses a lot of power (at a high voltage) and a tube has a filament and so they burn out like an incandescent light bulb. During the war they never turned a Colossus computer off (it was busy decrypting Nazi messages 24 hours per day). The designers realized that filaments almost always burn out when the tube is turned on or turned off. They seldom fail while operating at a constant temperature.