English: A battery of Leyden jar capacitors used in a large wireless telegraphy spark-gap transmitter around 1915. The first practical use for Leyden jars was in the spark gap transmitters used during the first 3 decades of radio, from the 1890s to World War 1. It consisted of a long narrow glass jar plated on the inside and outside with a copper coating, with a metal electrode making contact with the inner coating. The coatings stopped well short of the mouth of the jar so the high voltage charge (5000 - 75,000 V) would not arc between the coatings. The long narrow shape evolved to make better use of space.