Early radios produced little power, so extra amplifiers were often added. There was no industry producing them on a large scale, but they could always be built at home, like this magnificent creation of the late 1920s made by PFW Bush. Big amplifiers need plenty of raw power to work on, and this one had a hefty power supply. It also used two valves to drive the loudspeaker, one pushing, the other pulling. A similar arrangement is still used in modern amplifiers.
© Science Museum / Science & Society Picture Library