Heliograph

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Image:Heliograph.jpg 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 up to 1935.

1 Description
2 History
3 See also
4 External links

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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 line the flash up 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 with no 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 in that it was invisible if you were not close to the axis of operation.

The distance that heliograph signals could be seen depended on the clearness of the sky and the size of the mirrors used. Obviously a clear line of sight was required, and the earth's surface is curved, so 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 varied 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 apart on Sept 17, 1894, with Signal Corps heliographs carrying mirrors only 8 inches square.

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History

The first recorded use of heliograph comes from 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's thought he used a heliograph to send coded orders every day to the mainland, eight miles away.

The German Professor Carl Friedrich Gauss, of Goettingen University, 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 as it weighed about seven pounds, the operator could readily carry it and the tripod on which it rests. 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 U.S 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, where both sides used it. 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 Kimberly, Ladysmith, and Mafeking. With land telegraph lines cut off, the only contact with the outside world was via light-beam communication, helio by day, Aldis lamps at night.

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 not much used for signalling. Still, the army hated to see them go as "They made damn fine shaving mirrors." However 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.