Spark gap of a ship's wireless transmitter, circa 1900. The front panel has been swung down to reveal the device but would be closed in operation for protection against noise and ozone. The electrical oscilliations inherent in sparks were conveyed to the antenna through a transformer. The hemispheres were energized from an induction coil. The development of this technology was driven equally by naval and civil maritime needs, but its culmination cane in the First World War for the transmission of wireless telegraphy.
From T. Devereux, Messenger God of Battle, Brassey's, London, 1991.