What Is a Plumb Bob Used for?

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What Is a Plumb Bob Used for?
A plumb bob uses gravity to determine straightness. (Jupiterimages/liquidlibrary/Getty Images)

A plumb bob is a ancient tool that uses the earth's gravity to gauge verticality. It consists of a weight attached to a string, and its name derives from the French word for lead, plomb, since the weight has been traditionally made of that material. Because of the unerring dependability of gravity, a plumb bob is a reliably accurate measuring device, and carpenters, masons and other tradespeople use it for a variety of purposes.

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Measuring Column Straightness

When constructing an archway or any other structure that rests on free-standing columns, a plumb bob is a reliable way to make the columns vertical. The usual method is to hang the plumb bob from an overhang or nail at the top of the column, then measure the distance from the string to the sides of the column. The bottom of the column can then be adjusted until the same reading is obtained between the string and each side of the column at the bottom. This procedure is called plumbing the column.

Setting Stairways

In order to place a stairway, a builder must pinpoint the location on the lower floor that is exactly below the edge of the landing on the upper one. He can do this by hanging a plumb bob over the landing and gently lowering it until it touches the floor. The horizontal length of the stairway is the distance between the point where the plumb bob touches and the lower landing of the staircase, and its height is equal to the length of the string plus the bob. From these measurements, the builder can determine the length of the stringer he needs to cut.

Constructing Walls

A plumb bob is an essential tool for maintaining verticality when forming masonry or brick walls. Before the worker begins construction, he sets up one or several plumb bobs along the face, then uses the lines as a guide. Carpenters also use a plumb bob to plumb vertical walls, and both masons and carpenters can use the tool to measure angles. They do this by attaching the string to a protractor. When they place the base of the protractor on an angled surface with the bob hanging down, the string's position will give an accurate reading of the angle.

Other Uses

Plumbers sometimes use a plumb bob when roughing vertical pipes into walls. In order to align the holes in the top and bottom plates of a wall, they first drill a hole in the top plate, then drop a plumb bob from the hole to the bottom plate. The point where the bob touches marks the location for the hole in the bottom plate. Electricians sometimes use the same technique when running electrical wires or conduit. Plumb bobs are also useful for levelling cabinets, squaring doorways and windows and setting foundation and fence posts.

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