Plumb bob

<p>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 <a href="/facts/Vertical_direction/Hhsu9C8x">vertical direction</a> as a reference line, or plumb-line. It is a precursor to the <a href="/facts/Spirit_level/fXQH69hY">spirit level</a> and used to establish a vertical <a href="/facts/Datum_reference/pwGglGnJ">datum</a>. 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.
</p><p>The <a href="/facts/Measuring_instrument/gfkMHBNR">instrument</a> has been used since at least the time of <a href="/facts/Ancient_Egypt/BDsEGK4g">ancient Egypt</a> to ensure that constructions are "plumb", or vertical. It is also used in <a href="/facts/Surveying/qchGcnRC">surveying</a>, to establish the <a href="/facts/Nadir/ARbgL1VG">nadir</a> (opposite of <a href="/facts/Zenith/ISuRFp05">zenith</a>) with respect to <a href="/facts/Gravity_of_Earth/IXe9Bh8k">gravity</a> of a point in space. It is used with a variety of instruments (including <a href="/facts/Level_(instrument)/9HA0E7zQ">levels</a>, <a href="/facts/Theodolite/G4ADP1bC">theodolites</a>, and <a href="/facts/Tape_measure/uBIP6A7v">steel tapes</a>) to set the instrument exactly over a fixed <a href="/facts/Survey_marker/tHWSQXeD">survey marker</a> or to transcribe positions onto the ground for placing a marker.
</p>

Plumb bob open-in-new

Plumb bob