A plumb bob's weight hanging at the end of a line pulls the string it is attached to taut. This creates a straight vertical line that can be used as an alternative to a level. Plumb bobs only read accurately if they hang freely and the weight is completely still. SFGate recommends that they be used indoors or on windless days.
To use a plumb bob, hammer a nail into a ceiling joist. Allow the weight to hang down, making sure nothing is in its way. Once the weight stops swinging, measure the distance from the vertical surface to the top of the string. Match this distance to the distance from the vertical surface to the tip at the bottom of the weight to find level.
Ranging in weight from an ounce to as much as several pounds, the weight at the end of a plumb bob varies according to the use. Plumb bobs are often used in construction. Surveyors use plumb bobs for lining up points or transferring them. They are used in excavation and foundation construction. They help establish vertical lines for walls and doors. True vertical can be established for a chimney flue by using a plumb bob. They can also be used by homeowners for jobs such as hanging wallpaper or stacking decor items. Some bobs have pointed tips that can get bent. The weight should be replaced if this happens.