# Opisometer

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{{Short description|Measuring tool for arbitrary curved lines}}
thumbnail|An opisometer

An '''opisometer''', also called a '''curvimeter''', '''meilograph''', or '''map measurer''', is an instrument for [measuring](/source/measurement) the [length](/source/length)s of arbitrary curved lines.

== Explanation ==
A simple opisometer consists of a toothed wheel of known [circumference](/source/circumference) on a handle. The wheel is placed in contact with the curved line to be measured and run along its length. By counting the number of teeth passing a mark on the handle while this is done, the length of the line can be ascertained:

:line length = wheel circumference × teeth counted/teeth on wheel.
<!--If TeX must be used, one should write \mathrm{line}\ \mathrm{length}, etc.
:<math>line length = wheel diameter \times teeth counted / teeth on wheel</math> -->

In more sophisticated models, sometimes called a '''chartometer''', the wheel is connected via gearing to a rotary dial from which the line length can be directly read.<ref>[https://books.google.com/books?id=ThXH7xmUskIC&dq=opisometer&pg=PA83 Gerard L'Estrange Turner, ''Scientific Instruments, 1500-1900: An Introduction''], Philip Wilson Publishers 1998, {{ISBN|0-85667-491-5}} (p.83)
</ref>

The instrument is most commonly used to measure the lengths of roads, rivers and other line features on [map](/source/map)s.  Opisometers designed for this purpose provide scales reading the measured distance in kilometers and miles.

== History of the opisometer ==
Early versions of this instrument were [patent](/source/patent)ed in 1873 by the English engineer Edward Russell Morris.<ref>[http://www.london-gazette.co.uk/issues/24044/pages/5837/page.pdf THE LONDON GAZETTE, DECEMBER 12, 1873]</ref> The instruments he produced were initially described as a ''Patent Chartometer'' although later versions were sold under the curious name of ''Wealemefna''.<ref>[http://www.mathsinstruments.me.uk/page71.html Mathematical Instruments: Map Measurers]</ref> Writing in 1881, Morris described how he had created a wholly original name in an attempt to outwit his imitators; he also refused to disclose the origin of the word.<ref>English Mechanic and World of Science: Vol. 33, London, 1881</ref>

== See also ==
* [Planimeter](/source/Planimeter)
* [Steinhaus longimeter](/source/Steinhaus_longimeter)
* [Surveyor's wheel](/source/Surveyor's_wheel)

==References==
<references />

{{commonscat-inline|Opisometers}}
{{wiktionary-inline}}

Category:Length, distance, or range measuring devices
Category:Cartography
Category:Navigational equipment

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