Telemeter

The original meaning of telemeter, and still one of the main uses of the word, was a device used to measure distances to remote objects, that is, a rangefinder. Originally optical devices used in surveying, they soon found military applications in rangefinding, especially naval gunnery.

More generally, a telemeter is a device used to remotely measure any quantity. It consists of a sensor, a transmission path, and a display, recording, or control device. Telemeters are the physical devices used in telemetry. Electronic devices are widely used in telemetry and can be wireless or hard-wired, analog or digital. Other technologies are also possible, such as mechanical, hydraulic and optical.

The first rangefinder telemeter was invented by James Watt in 1769 and put to use in 1771 in surveying canals. Watt called his instrument a micrometer, a term now used with a different meaning in engineering. It consisted of two parallel hairs in the focal plane of a telescope eyepiece crossing an upright hair. At the point to be measured, two sliding targets on a surveyor's rod were adjusted to align with the hairs in the telescope. The distance to the rod could then be determined from the distance between the targets on the rod by trigonometry.

Several others have been credited with the invention of the rangefinder telemeter at one time or another. The Royal Society of Arts gave an award to W. Green for its invention in 1778, even though they were made aware of Watt's priority.Alexander Selligue is often mistakenly credited with the invention. Selligue did invent a telemeter in 1821 and is responsible for coining the term.

In 1781 Georg Friedrich Brander invented the coincidence telemeter. Two mirrors set a distance apart horizontally and forming two images in the same eyepiece. This rangefinder does not require a measuring rod at the target and could perhaps be considered the first true telemeter. In 1790 Jesse Ramsden invented a half-image range finder. In 1899 Carl Pulfrich at Carl Zeiss AG invented the stereo telemeter.

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