English: Circuit of Guglielmo Marconi's spark gap radio transmitter built at Poldhu, UK, the world's first powerful radio transmitter, with which he achieved the first transatlantic radio transmission 12 December 1901. Designed by John Ambrose Fleming, it was powered by a 25 kilowatt alternator (D) turned by an internal combustion engine. Fleming estimated the radiated power was between 10 and 12 kW. It drove an aerial (A) consisting of 50 wires suspended in a fan shape from a cable between two 160 foot poles. The frequency used is not known precisely because Marconi did not measure frequency or wavelength, but it was between 166 and 984 kHz, probably around 500 kHz. Marconi received the signal with a coherer receiver and a 400 foot wire antenna suspended from a kite at St. John, Newfoundland, Canada, a distance of 2100 miles (3500 km).

The circuit was a complicated inductively coupled radiotelegraphy spark gap transmitter with two spark gaps (S1 and S2) firing at different rates and three tuned circuits. The alternator produced current at 125 Hz. The iron core transformer T1 produces the high voltage to charge the first capacitor C1. The first spark gap and resonant transformer (S1, C1, T2) produced higher voltage damped waves to charge the second capacitor C2. The second spark gap, capacitor and resonant transformer (S2, C2, T3) generated even higher voltage damped waves which excited the antenna. The spark rate was low: the second spark gap which produced the output, fired at a rate as low as 2-3 sparks per second. An iron core inductor H1 in the primary circuit of the supply transformer allowed the transmitter to be keyed by the telegraph key K without arcing; when the key was up the high impedance of the coil turned the supply current off. The second inductor H2 was used to control the power of the transmitter by controlling the current. This curious circuit was not used in subsequent transmitters; later transatlantic transmitters used simpler single spark gap circuits. Information from Sarkar, T. K.; Mailloux, Robert; Oliner, Arthur A. (2006) History of Wireless, John Wiley and Sons, pp. 387−392 ISBN: 0471783013.

Changes to image: Replaced drawing of rectangular metal plate antenna in the original diagram, which was used for testing the transmitter when this diagram was drawn, with fan-shaped wire aerial (A) used in final transmitter.