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It used a primitive radio wave detector called a coherer, invented in 1890 by Edouard Branly consisting of a glass tube with metal filings between two electrodes. The coherer was connected to a wire antenna, and also a circuit with a battery (left) and a relay (cylinder, center). When a radio signal from the antenna arrived at the coherer's electrodes, the metal filings became conductive. The current from the battery flowed through the coherer and energized the relay, closing its contacts, and applying the current from the battery to the electromagnet of the electric bell (right) which pulled its arm over to give the bell a tap. To restore the coherer to its receptive state, the filings needed to be disturbed by tapping it, so when the bell arm sprang back it tapped the coherer, automatically resetting it.