Plumb-bob

From Wikipedia, the free encyclopedia

A plumb-bob or a plummet is a weight with a pointed tip on the bottom that is suspended from a string and used as a reference line that is perpendicular to the ground.

This instrument has been used since the time of the ancient Egyptians by bricklayers, masons, and carpenters to ensure that their constructions are "square", or perfectly upright. It may also be used in surveying to sight a point on the ground that is not readily visible. Small plumb bobs are included in the kits of various instruments such as levels and theodolites. They are used to set the instrument exactly over a fixed datum marker, prior to taking fresh readings.

[edit] Usage

To mark a vertical line on a wall, the following procedure may be used:

1. The plumb-bob is hung from a chalk-coated string almost against the wall (ideally with a tiny gap). A nail or other device is used to secure the string at the top, near the wall.
2. The weight may swing and/or twirl dramatically at first, it can though be steadied most of the way by hand.
3. The weight is left to find its final position (this may take quite some time).
4. Once the weight comes to rest in its final position, it may be held against the wall and the string "snapped" against the wall. This will create a vertical chalk line.
5. The chalk line may be removed when no longer needed.

The mason's or bricklayer's plumb level is placed against the vertical surface to check for plumb.

1. It is slowly rocked forward and back towards the user. That is from swinging free to resting.
2. Very quickly the plumb-bob steadies, then the string line is compared to the gauge line.
3. The dotted lines in the sketch represent possible small timber extensions.
4. They may be used for reading, say convex columns.
5. Alternatively the top piece could be longer than the bottom one, to allow sloping surfaces to be checked for consistency.

The sketch right is of another mason's tool based on the plumb-bob which tests for horizontal. Used from antiquity up to the invention of the spririt level.

To find a point vertically below another point.

1. Drop the plumb bob from the top and as it touches the floor below, lift it up and down a few times, from touching the floor to swinging free. This steadies the swing.
2. Then when it has settled slowly let it down into contact and let it lie on its side.
3. A well designed plumb-bob may roll at this stage, but it will only roll about the tip, (usually a steel point).
4. Then a single user can climb down from above and mark the floor where the tip rests.

Up until the modern age, on most tall structures, plumb-bobs were used to provide vertical datum lines for the building measurements.

1. A section of the scaffolding would hold a plumb line that was centered over a datum mark on the floor.
2. As the building proceeded upwards the plumb line would also be taken higher, still centered on the datum.
3. Many cathedral spires,domes and towers still have brass datum marks inlaid into their floors, that signify the center of the structure above.
4. The early skyscrapers used heavy plumb-bobs hung on wire in their lift wells. The weight would hang in a container of oil to to dampen any swinging movement.

[edit] Appearances in popular culture

Plumb-bob is also a term used in The Sims, The Sims 2, The Sims Stories, and related video games. The plumb-bob hovers over the head of the Sim that has been selected by the user. That Sim's date or group of friends may also have a slightly smaller plumb-bob. The plumb-bob's color changes with the sim's mood and serves as a rough barometer of the sim's mood and well-being.

There was also a series of nuclear tests in the deserts of Nevada(USA) during the 1950s which went under the name 'Operation Plumbbob'. The tests were meant to gauge how soldiers would be affected mentally and physically by nuclear warfare and a radioactive environment as well as to test the weapons themselves and measure the distribution of radiation through the troposphere and stratosphere.

Evaluating a person's mood or receptiveness to social or romantic advances has come to be called 'plumbbobing' in some circles, especially among young adults and gamers. One must be careful using this term, however, as plumbbob is also growing in popularity as a slang term refering to male genitalia, and the phrase could take on a whole different meaning to the uninitiated.

[edit] References

• 60 oz. Plumb Bob.
• String Line and Plumb Bob.