A plumb bob is a tool used to ensure that a building structure like a door frame or a shelving support is as vertical as possible. It is also used to measure if an object is placed directly under a point above it, such as placing a shower drain relative to some point on the ceiling, or putting something right under a lighting fixture.
This isn’t a tool used by plumbers, as some people might think, in fact it’s name has nothing to do with plumbing at all. The term plumb comes from the Latin word for lead, plumbum, because such tools were typically made of lead, and the adjective plumb developed by extension.
The plumbob consists of a pointed weight attached to a line of nylon or silk, fastened to a slot in the center of its cap. The weight can be anywhere from 1 1/2 ounces to 4 pounds. The pointed end is commonly made to be replaceable, as it will no longer give a true reading if bent or damaged. It can be unscrewed and replaced easily.
You can make your own DIY plumb bob in a pinch if you need one in a hurry. Find a wooden toy spinning top, tie fishing line or string around the top handle, or drill a small hole through it if there is room, to attach the string through.
Use some metal washers threaded onto the string for adding weight. It might not be as precise as a fancy new electronic plumb-bob from Home Depot, but it’ll get the job done.
To use the plumb bob, hold the end of the line at the desired reference point, or alternately, nail the string to the ceiling, leaving enough length of string so that the bob’s point is as close as possible to the point you wish to measure.
Allow it to settle, waiting until it stops swinging of it’s own accord. It will then be forming a perfect vertical line. Now you can carefully mark the point below the end of the bob with a pencil or chalk.
These tools have a long history of use, going back to ancient Egypt, but they will work just as well in today’s digital age. Up until the modern times, on most tall structures, plumb-bobs were used to give builders a vertical datum line for the building measurements.
There would be a portion of the scaffolding that would hold a plumb line 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.
Early skyscrapers used heavy plumbobs hung on wire in their lift wells, and the weight would hang in a container of oil to dampen any swinging movement.