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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.
Français : Émetteur radiotélégraphique à étincelles à bobine d'induction de Ruhmkorff. En exposition au American Museum of Radio & Electricity à Bellingham, Washington, USA. «Émetteur à étincelles français de Radiguet et Massiot, fait à Paris en 1900. Ce petit émetteur à étincelles d'une potée d'environ 10 km était utilisé pour les radiocommunications navire-terre." Il se compose d'une antenne hertzienne dipôle constitué de deux tiges de laiton avec un éclateur entre eux, alimentés par de fortes impulsions de tension à partir d'une bobine d'induction (boîte à l'arrière). Le manipulateur morse de télégraphe (au premier plan à droite), un commutateur dans le circuit primaire de la bobine, a été utilisé par l'opérateur pour éteindre l'émetteur et le rapide, l'envoi de messages en télégraphie en morse. La longueur de l'antenne, qui fonctionne comme un dipôle demi-onde, détermine la fréquence des ondes produites; cette antenne est d'environ 1/2 mètre de long, et ainsi de générer des ondes serait de 1 mètre de longueur d'onde, avec une fréquence d'environ 300 MHz .
Date
Source originally posted to Flickr as Spark gap transmitter
Author Rob Flickenger
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current06:13, 25 January 2013Thumbnail for version as of 06:13, 25 January 20133,312 × 3,890 (4.14 MB)F1jmmRetouche image coupé
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