Crystal radio set. This radio receiver was built in c.1926 by the Brownie Wireless Company of Great Britain Ltd. It functions without the need for a power supply. An antenna (not seen) picks up radio signals and passes them down a wire in the form of an electric current. A crystal (most commonly lead sulphide, which is also known as galena) is housed in the metal case at upper left. This gives the set its name and allows the current from the antenna to pass through in one direction towards the headphones (upper right). The headphones contain a solenoid that moves in response to the electrical current, causing it to vibrate and produce sound.
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