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{{Short description|Weighted line used as a vertical reference}}
{{About|the tool|the nuclear testing program|Operation Plumbbob||}}
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[[File:Plumb bob.jpg|thumb|upright|A plumb bob]]

A '''plumb bob''', '''plumb bob level''', or '''plummet''', is a weight, usually with a pointed tip on the bottom, suspended from a string and used as a [[vertical direction]] as a reference line, or '''plumb-line'''. It is a precursor to the [[spirit level]] and used to establish a vertical [[datum reference|datum]]. It is typically made of stone, wood, or lead, but can also be made of other metals. If it is used for decoration, it may be made of bone or ivory.

The [[measuring instrument|instrument]] has been used since at least the time of [[ancient Egypt]]<ref name="Stocks2003">Denys A. Stocks. ''[https://books.google.com/books?id=oLDuHvQODoIC&pg=PA180 Experiments in Egyptian archaeology: stoneworking technology in Ancient Egypt]''. Routledge; 2003. {{ISBN|978-0-415-30664-5}}. p.&nbsp;180.</ref> to ensure that constructions are "[[wikt:plumb|plumb]]", or vertical. It is also used in [[surveying]], to establish the [[nadir]] (opposite of [[zenith]]) with respect to [[Gravity of Earth|gravity]] of a point in space. It is used with a variety of instruments (including [[Level (instrument)|levels]], [[theodolite]]s, and [[tape measure|steel tapes]]) to set the instrument exactly over a fixed [[survey marker]] or to transcribe positions onto the ground for placing a marker.<ref name="Brinker1995">{{cite book |title=The surveying handbook |url=https://archive.org/details/surveyinghandboo00robi |url-access=limited |pages=[https://archive.org/details/surveyinghandboo00robi/page/n124 93]–94 |editor1-last=Brinker |editor1-first=Russell Charles |editor2-last=Minnick |editor2-first=Roy |isbn=978-0-412-98511-9 |year=1995 |publisher=Springer}}</ref>

== Etymology ==
[[File:Cc&j-fig23--plumb rule.png|upright=0.4|thumb|left|A plumb rule from ''[[Cassells' Carpentry and Joinery]]'']]
[[File:Cc&j-fig24--plumb square.png|thumb|A plumb square from ''Cassells' Carpentry and Joinery'']]

The ''plumb'' in ''plumb bob'' derives from Latin ''[[wikt:plumbum|plumbum]]'' ('[[lead]]'), the material once used for the weighted ''[[Bob (physics)|bob]]'' at the end.{{r|McGeough}}  The adjective ''[[wikt:plumb|plumb]]'' developed by extension, as did the noun ''[[wikt:aplomb|aplomb]]'', from the notion of "standing upright".

==Use==
Until the modern age, plumb bobs were used on most tall structures to provide vertical [[datum]] lines for the building measurements. A section of the scaffolding would hold a plumb line, which was centered over a datum mark on the floor. As the building proceeded upward, the plumb line would also be taken higher, still centered on the datum. Many cathedral [[spire]]s, [[domes]] and [[tower]]s still have brass datum marks inlaid into their floors, which signify the center of the structure above.

[[File:Lot hg.jpg|thumb|Plumb bob with scale as an inclinometer]]
[[File:Plumb bob with string.jpg|thumb|Plumb bob with string in masonry work]]

A plumb bob and line alone can determine only a vertical reference.  However, if they are mounted on a suitable scale the instrument may also be used as an [[inclinometer]] to measure angles to the vertical.

[[Ancient Egypt]]ians used a plumb line attached to the top outer part of a tool resembling a letter ''[[E]]''; when placed against a wall, the plumb line would indicate a vertical line. An [[archipendulum]], an [[A-frame]] level with a plumb line hung from the vertex was also used to find horizontal; these were used in Europe until the mid–19th century. A variation of this tool has the plumb line hung from the top of an inverted ''[[T]]'' shape.<ref name="McGeough">{{cite web |last1=McGeough |first1=Joseph A. |last2=Hartenberg |first2=Richard S. |title=Hand tool {{!}} Measuring and defining tools {{!}} Plumb line, level, and square |url=https://www.britannica.com/technology/hand-tool/Screw-based-tools#ref39230 |website=Encyclopaedia Britannica |language=en |date=2019}}</ref>

The early [[skyscraper]]s used heavy plumb bobs, hung on wire in their elevator shafts.{{Explain|date=October 2020}}

A plumb bob may be in a container of water (when conditions are above freezing temperatures), molasses, very viscous oils or other liquids to dampen any swinging movement,<ref>Staley, W. W.. ''Introduction to mine surveying'',. 2nd ed. Stanford University, Calif.: Stanford University Press, 1964. Print. 138.</ref> functioning as a [[shock absorber]].

=== Determining center of gravity of an irregular shape ===
Students of [[figure drawing]] will also make use of a plumb line to find the vertical axis through the [[center of gravity]] of their subject and lay it down on paper as a [[frame of reference|point of reference]]. The device used may be purpose-made plumb lines, or simply makeshift devices made from a piece of string and a weighted object, such as a [[washer (hardware)|metal washer]]. This plumb line is important for lining up anatomical geometries and visualizing the subject's [[center of mass|center of balance]].

== See also ==
*[[Vertical position]]

==References==
{{Reflist}}

== External links ==
{{Commons category|Plumb bobs}}
* [http://www.antiquetools.com/10-tools/10-Tools/Plumb-Bob.html 60 oz. Plumb Bob].
* [https://web.archive.org/web/20070828055441/http://www.builderbill-diy-help.com/string-line.html String Line and Plumb Bob].
{{Measuring and alignment tools}}
{{Authority control}}

{{DEFAULTSORT:Plumb Bob}}
[[Category:Inclinometers]]
[[Category:Measuring instruments]]
[[Category:Surveying instruments]]
[[Category:Woodworking measuring instruments]]
[[Category:Orientation (geometry)]]
[[Category:Carpentry tools]]
[[Category:Stonemasonry tools]]
[[Category:Weights]]