Heliograph
From Wikipedia, the free encyclopedia
This article is about the signalling device. For other meanings, see heliograph (disambiguation).
A heliograph uses a mirror to reflect sunlight to a distant observer. By moving the mirror, flashes of light can be used to send Morse code. The heliograph was a simple but highly effective instrument for instantaneous optical communication over 50 miles or more in the 19th century. Its major uses were for military and survey work. It was still in serious use at least until 1935.
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[edit] Description
Heliograph equipment varied somewhat from country to country. The U.S. Army heliograph had two mirrors mounted on a tripod. A shutter for interrupting the flashes was mounted on another tripod. If the sun was in front of the sender, its rays were reflected directly from a mirror to the receiving station. The sender used the sighting rod to align the flash with the receiver. If the sun was behind the sender, its rays were reflected from one mirror to another, to send the beam on to the target receiver. The British army version had a single mirror with a small sighting hole in the middle and a keying mechanisms that tilted the mirror up a few degrees at the push of a lever at the back of the instrument. Auxiliary mirrors mounted on separate tripods were used to redirect the sunlight if it was not coming from a favorable angle.
The heliograph had some powerful advantages. It allowed long distance communication without a fixed infrastructure, though it could also be linked to make a fixed network extending over hundreds of miles, as in the fort-to-fort network used in the Geronimo campaign. It was highly portable, required no power source, and was relatively secure since it was invisible to those not near the axis of operation. However, anyone with the correct knowledge could, in theory, intercept signals without being detected.
The distance that heliograph signals could be seen depended on the clarity of the sky and the size of the mirrors used. A clear line of sight was required, and since the earth's surface is curved, the highest convenient points were used. Under ordinary conditions, a flash could be seen 30 miles (48 km) with the naked eye, and much farther with a telescope. The maximum range was considered to be 10 miles for each inch of mirror diameter. Mirrors ranged from 1.5 inches to 12 inches or more. The record distance was established by a detachment of U.S. signal sergeants by the inter-operation of stations on Mount Ellen, Utah, and Mount Uncompahgre, Colorado, 183 miles (295 km) apart on Sept 17, 1894, with Signal Corps heliographs carrying mirrors only 8 inches square.
[edit] History
The first recorded use of the heliograph was in 405 BC, when the Ancient Greeks used polished shields to signal in battle. In about 35 AD, the Roman emperor Tiberius, by then very unpopular, ruled his vast empire from a villa on the Isle of Capri. It is thought that he sent coded orders daily by heliograph to the mainland, eight miles away.
The German professor Carl Friedrich Gauss, of Georg-August University of Göttingen, outlined a first design for a formal heliograph in 1810. His device directed a controlled beam of sunlight to a distant station. It was meant to be used for geodetic survey work.
Sir Henry Christopher Mance (1840-1926), of British Army Signal Corps, developed the first apparatus while stationed at Karachi, Bombay. The Mance Heliograph was easily operated by one man, and since it weighed about seven pounds, the operator could readily carry the device and its tripod. During the Jowaki Afridi expedition sent out by the British-Indian government in 1877, the heliograph was first tested in war.
Major W. J. Volkman of the US Army, demonstrated in Arizona and New Mexico the possibility of carrying on communication by heliograph over a range of 200 miles. The network of communication begun by General Miles in 1886, and continued by Lieutenant W. A. Glassford, was perfected in 1889 at ranges of 85, 88, 95, and 125 miles over a rugged and broken country, which was the stronghold of the Apache and other hostile Indian tribes.
The simple and effective instrument that Mance invented was to be an important part of military communications for the next 40 years. Limited to use in sunlight, the heliograph became the most efficient visual signalling device ever known. In pre-radio days it was often the only means of communication that could span ranges of up to 100 miles with a lightweight portable instrument.
The last great use of the heliograph was during the Boer War in South Africa, and was used by both sides. The terrain and climate, as well as the nature of the campaign, made the heliograph the logical choice. For night communications, the British used some large Aldis lamps, brought inland on railroad cars, and equipped with leaf-type shutters for keying a beam of light into dots and dashes. In the early stages of the war, the British garrisons were besieged in Kimberley, Ladysmith, and Mafeking. With land telegraph lines cut, the only contact with the outside world was via light-beam communication, helio by day, and Aldis lamps at night.
The Red Army made use of chains of heliograph stations to efficiently disseminate intelligence about basmachi rebel movements in Turkestan during and after the Russian Civil War.[citation needed]
The heliograph remained standard equipment for military signallers in the Australian and British armies until the 1960s, where it was considered a "low probability of intercept" form of communication. Canada was the last major army to keep the heliograph as an issue item. By the time the mirror instruments were retired in 1941, they were seldom used for signalling. Still, the army hated to see them go as "They made damn fine shaving mirrors." As recently as the 1980s, heliographs were used by Afghan forces during the Soviet invasion of Afghanistan. They are still included in survival kits for emergency signalling to search and rescue aircraft.
