A plumb-bob or a plummet is a weight with a pointed tip on the bottom that is suspended from a string and used as a vertical reference line.
This instrument has been used since the time of the ancient Egyptians by bricklayers, masons, and carpenters to ensure that their constructions are "plumb", or perfectly upright. It may also be used in surveying to sight a point on the ground that is not readily visible. Small plumb bobs are included in the kits of various instruments such as levels and theodolites. They are used to set the instrument exactly over a fixed datum marker, prior to taking fresh readings.
Up until the modern age, on most tall structures, plumb-bobs were used to provide vertical datum lines for the building measurements. A section of the scaffolding would hold a plumb line that was centered over a datum mark on the floor. As the building proceeded upwards the plumb line would also be taken higher, still centered on the datum. Many cathedral spires, domes and towers still have brass datum marks inlaid into their floors, that signify the center of the structure above.
The early skyscrapers used heavy plumb-bobs hung on wire in their lift wells. The weight would hang in a container of oil to dampen any swinging movement, functioning as a shock absorber.
Often, figure drawing students 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 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 metal washer. This plumb line is important for lining up anatomical geometries and visualizing the subject's center of balance.
US Patent Issued to Kone on Aug. 16 for "Arrangement and Method for Positioning the Alignment Plumb Lines of the Guide Rails of an Elevator" (Dutch Inventors)
Aug 18, 2011; ALEXANDRIA, Va., Aug. 18 -- United States Patent no. 7,997,000, issued on Aug. 16, was assigned to Kone Corp. (Helsinki)....
The Gender Difference in Orienting Liquid Surfaces and Plumb-Lines: Its Robustness, Its Correlates, and the Associated Knowledge of Simple Physics
Sep 01, 1996; Abstract In comparison between male and female psychology students, a greater proportion of female students fail to represent...