Plumb bob

A plumb bob is a weighted point suspended from a string, used to establish a true vertical line. Gravity pulls the weight straight down, and the string it hangs from defines an exact vertical reference — a plumb line.

How it works

You hold or suspend the string at the point you want to reference, and the weight pulls the string taut in a perfectly vertical line. By marking where the string falls at the bottom, you transfer a vertical reference from one point to another. The pointed tip of the bob lets you mark with precision.

Types

• Conical plumb bob: The classic shape — a pointed brass or steel cone, typically 4 to 8 ounces. The cone shape is self-centring and resists wind.
• Self-retracting plumb bob: A flat, disc-shaped weight on a tape that retracts into a case. Convenient for one-person use.
• Plumb rule: A long, straight board with a plumb bob attached to the top. Used in masonry and timber framing to check vertical alignment over longer distances.

Uses

• Masonry: Ensuring walls are plumb as they are built.
• Timber framing: Checking that posts and columns are vertical.
• Surveying: Establishing vertical references for measurements.
• Hanging doors: Ensuring the door frame is plumb before setting the door.
• Chalk lines: A chalk-filled string pulled taut and snapped against a surface creates a straight line — the string is often made plumb with a plumb bob first.

Why it works

The plumb bob exploits the simplest force in physics: gravity. Because gravity acts uniformly downward, a freely suspended weight always hangs in a true vertical line. This has been true for thousands of years — ancient Egyptian builders used plumb bobs to align the blocks of the pyramids.