Marconi rotating beam transmitter at Inchkeith , Firth of Forth , Scotland . This revolving wirelss beam apparatus projects a horizontal ray of short wireless waves which sweeps round the horizon in about 2 minutes sending out distinctive signals indicating the direction in which the beam is pointing at the moment . A vessel desiring to know its hearing in relation to the lighthouse only requires to note on a short wave received at each revolution of the beam . This indicates absolutely the bearing of the lighthouse because only on that definite bearing can that distinctive signal be transmitted by the apparatus . Marconi Wireless Co Ltd . 1 January 1925