Plumb Bob

A plumb bob is a weight, usually pointed at its lower end, suspended from a string to establish a true vertical reference line, traditionally made of stone, wood or lead. Ancient Egyptian builders used plumb bobs to keep walls and structures vertical, attaching the line to a tool shaped roughly like the letter E so that, when the tool was held against a wall, the hanging line showed at a glance whether the surface was truly plumb; the plumb bob has been used for this purpose since at least ancient Egyptian times and predates the spirit level, which serves a related purpose. In surveying, a plumb line is also used to find the nadir relative to gravity, and in Europe an A-frame tool called an archipendulum, with a plumb line hung from its apex, was used for leveling work until the mid nineteenth century; builders of early skyscrapers hung heavy plumb bobs on wire inside elevator shafts to check that the structure was rising true. The word plumb comes from the Latin plumbum, meaning lead, the traditional material for the weighted bob itself.