English: Experimental spark radio transmitter at Poldhu, UK used by Guglielmo Marconi to make the first transatlantic radio transmission 12 December 1901. Designed by Prof. John Ambrose Fleming of University College, London, and built in secret in 1900-1901 it was an inductively-coupled transmitter with two cascaded spark gaps firing at different rates, and three resonant circuits, powered by a 25 kW alternator turned by a combustion engine. It fed a vertical monopole antenna consisting of 50 wires in a fan shape suspended from a cable between two 160 foot poles. Marconi estimated the radiated power was around 10-12 kilowatts. The operating frequency is not known, since Marconi never measured frequency or wavelength but it is estimated to be around 500 kilohertz. On 12 December 1901 Marconi reported he had received the radiotelegraphic signals on Signal Hill, St. John's on the coast of Newfoundland, 2,100 miles (3,400 kilometres) from the transmitter, using an unamplified, untuned coherer receiver attached to a 400 foot wire antenna held aloft with a kite.