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In page Leyden jar:

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Beginning in the late 18th century it was used in the medical field of electrotherapy to treat a variety of diseases by electric shock. By the middle of the 19th century, the Leyden jar had become common enough for writers to assume their readers knew of and understood its basic operation.[citation needed] Around the turn of the century it began to be widely used in spark-gap transmitters and medical electrotherapy equipment.