English: Antique
spark gap radio transmitter in
American Museum of Radio & Electricity in Bellingham, Washington, USA. Spark gap transmitters the first type of
radio transmitter, were used during the first 3 decades of radio from 1887 to 1917. The placard reads: "
Spark-gap transmitter. Radiguet & Massiot, French c. 1900. Small spark transmitter used for ship to shore communication. Range was about 10 km."
It consists of an induction coil (box in back), powered by a battery (not shown), which generates pulses of high voltage electricity which are applied to a spark gap between the two brass balls on the brass arms (top). One side of the spark gap is attached to a wire antenna suspended from the ship's mast. The other is attached to a ground connection, which in a ship was usually a wire or plate dipping in the water. Each spark between the balls excited a brief radio frequency sinusoidal oscillating current in the antenna, which declined rapidly to zero, called a damped wave The energy in the oscillating current was radiated from the antenna into space as radio waves.
The antenna acted as a resonator, so the wavelength and therefore frequency of the radio waves produced was determined by the length of the antenna. The antenna functioned as a quarter wavelength monopole, so the wavelength of the radio waves was 4 times the length of the antenna. The vibrating interrupter contact on the induction coil generated about 20 to 50 high voltage pulses per second, so the output of the transmitter was a repetitive string of damped waves, repeating at an audio rate, so the signal sounded like a buzz or whine in the receiver earphones.
To communicate information with this signal, the operator turned the power to the transmitter on and off rapidly with the
telegraph key (right foreground), a switch in the coil's primary circuit, producing different length pulses of radio waves to spell out text messages by
telegraphy in
Morse code.