English: A 2 kW shipboard quenched spark-gap transmitter manufactured by the Marconi Wireless Telegraph Co. around 1917. Spark-gap transmitters, the first type of radio transmitter, used from 1888 to about 1920, generated radio waves by an electric spark. Unlike modern radio stations that transmit audio (sound), spark transmitters communicate information by wireless telegraphy; the operator turns the transmitter off and on with a telegraph key producing pulses of radio waves to spell out text messages in Morse code. One of the first uses of radio was on ships, to keep in touch with shore and call for rescue if they were sinking. Marconi had a virtual monopoly on ship wireless, since its operators were forbidden to handle messages from wireless systems of other manufacturers.

The Model "P-4" was a high-power transmitter with a daylight range of 450-650 miles and a night range of 1500-2500 miles, designed for use on large-tonnage oceangoing ships. It was a type called a "quenched-spark transmitter". It transmitted at three standard wavelengths, 300 metres (1 MHz), 450 metres (660 kHz) and 600 metres (500 kHz) with a spark rate of 1000 sparks/sec so its signal sounded like a high-pitched tone in earphones.

Since the transmitter required AC, it has a motor-generator (22) to convert the ship's 110V DC current to a constant 140 VAC at 500 Hz, which was stepped up to about 12,500 V by a transformer (60). This was applied to a bank of 6 Leyden jar capacitors (56), charging them. At the peak of each AC cycle, the voltage would jump across a quenched spark gap on the front panel (13), flowing through a flat pancake-shaped coil of wire called the oscillation transformer (38,39). The coil and capacitor together formed a tuned circuit, and the charge flowing back and forth rapidly between the plates of the capacitor through the coil and spark gap generated a pulse of oscillating radio frequency current. This induced oscillating currents in the secondary coil of the oscillation transformer, which was connected through a loading coil to a long wire antenna suspended between the ship's masts. The quenched spark gap consisted of microscopic gaps between about 20 flat cylindrical electrodes in series. The function of the quenched gap is to extinguish the spark quickly after the energy from the primary is transferred to the secondary, reducing the energy loss in the spark, allowing the secondary to resonate with the capacitance of the antenna, producing long "ringing" waves. There was a backup rotary spark gap (21,22) on the rotor of the motor that could be used, in case the quenched spark gap on the front panel failed. The narrow vertical rack layout was necessary so the set could be maneuvered through narrow ship hatches.

Alterations to images: combined separate images of the front and back of the transmitter, increased contrast and lightened background to bring out labels. Labeled parts: