Sterling audio amplifier valve, c 1925, and printed circuit board, c 1950.
© Science Museum / Science & Society Picture Library
By the 1920s valves were effective amplifiers and could be used in the home to drive loudspeakers. This amplifier gave out as much power as a modern transistor radio. Printed circuit boards are composed of an electronic circuit based on thin strips of a conductor on an insulating board, made by etching.