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What Is a Plumb Bob?

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  • Written By: Sheri Cyprus
  • Edited By: Bronwyn Harris
  • Last Modified Date: 28 September 2014
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A plumb bob is a small piece of metal that weighs down a string attached to it to indicate plumb, which is an exact vertical position. Builders today consider the plumb bob a basic tool box item even though it is thought to date back to the ancient Egyptians. Research has suggested that Egyptians used plumb bobs to create the straight vertical lines of their pyramids.

A simple plumb bob may be just a plain length of string with a piece of metal tied to it as a weight. Fancy plumb bobs may have decorative, carved wooden or stylish metal sections. Some plumb bobs available today are sold in woodworking kits. The finished plumb bob may be used as a Christmas ornament as well as a working tool. There are even plumb bob ornaments that feature 24 karat gold pointed tips.

While an ornamental plumb bob may have separate tip and body sections, purely functional plumb bobs are often just a simple one piece design made to attach to a string. Basic plumb bobs are also usually made in a plain geometrical shape rather than elaborately formed. A metal such as lead, steel or brass that is smooth and triangular in shape is used for many plumb bobs, but some are made of plastic. The tip at the end is always fairly sharp as it is this pointed feature on a plumb bob that indicates the perfectly straight, or plumb, vertical direction needed by builders.

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It is the force of gravity that makes the weighted string of the plumb bob hang completely straight so that the point can indicate the plumb vertical line. Different weights of plumb bobs are available such as 6 ounces (170 grams), 8 ounces (226.8 grams) and 12 ounces (340 grams). When a plumb bob is placed on a nail on a wall by its string, it can indicate an accurate vertical line on the wall when the weight stops moving. Coating the string with chalk can make the line appear on the wall.

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