The plumb bob is an ancient tool that is still in use today. It is made of the simplest possible parts; basically a string and a weight. A heavy weight is suspended by a string and gravity causes the attached string to establish a reference line that is perfectly vertical.
The Egyptians invented the plumb bob more than 4000 years ago. They used it to establish vertical references that were indispensable in the construction of their buildings, pyramids and canals. The ancient builders used the versatile tool in every conceivable way, including astronomy and navigation, and for vertical references in surveying instruments.
Early instances of plumb bobs used somewhat randomly shaped weights such as rocks, but as the tool developed, it became more symmetrical and was tapered to a pointed tip that greatly increased the instrument’s precision. The Egyptians mounted the plumb bob in a wooden frame with horizontal and vertical sides. The resulting instrument could be used to check the horizontal level as well as vertical plumb.
The word “plumb” was derived from the Latin word “plumbum,” for the element lead (chemical symbol Pb). The first manufactured weights that replaced stones were undoubtedly made of lead due to its density. Much later weights were made of steel and some precision types used brass. Brass is quite dense and therefore less prone to corrosion.
Introduction of the Level
In the 19th century, another instrument challenged the plumb bob as the device of choice for measurements of plumb and level. The spirit level could be mass-produced and was, for most applications, just as accurate, and easier to use. It gradually displaced the plumb bob for most of these measurements.
At about the same time, there arose a need to accurately transfer points from one place to another and to duplicate vertical lengths at different heights. Bridges, buildings and-large scale machinery being built at that time depended on this ability, and the plumb bob filled the need.
Although more advanced levels have been in widespread use for a long time now, and today there are electronic versions that yield quick and extremely accurate readings, plumb bobs are still in use. They are handier than a spirit level for establishing very long vertical lines, and can be more accurate for these measurements. As builders discovered more than a hundred years ago, plumb bobs are valuable as coordinate transfer devices.