Plumb bob

How it works

The cord is fixed at the upper point and the weight is allowed to hang free. When it comes to rest the cord marks the vertical through that point, and the pointed tip marks the spot directly beneath. The user transfers a point from a ceiling to a floor, or compares the cord against a post or wall to see whether the work leans.

Moxon’s 1693 account of the carpenter’s plumb-line describes trying the upright standing of posts by drawing off enough line, fixing the rest, and holding the end of the line between finger and thumb at a set distance from one corner of the post: if the line and the corner are parallel, the post is upright. Bricklayers, he says, carry a plumb rule about four feet long, with a line and a plummet of lead, to carry the work upright.

Adjustable plumb bobs, as Stanley described them in 1900, carry a reel in the top of the bob. The user drops the bob with a slight jerk to pay out line, and a spring bearing on the reel holds the bob at any point on the line.

Anatomy

Body (bob): The weight. The Met holds Egyptian bobs of travertine, limestone, hematite and carnelian; Stanley’s 1900 catalog lists bronze and iron bodies; Swanson sells solid brass bodies of 8 and 16 ounces.
Point: Stanley’s 1900 bobs have steel points; Swanson’s brass bob has a hardened steel tip with two replaceable spare tips.
Line: Cord attached through an eye at the top. Swanson’s 16 ounce bob ships with 240 inches of braided line and a steel suspension hook.
Reel (adjustable bobs): Stanley’s 1900 catalog describes a reel at the upper end with a spring bearing on it and a screw to adjust the pressure.
Cap or storage space: The Science Museum Group holds a brass plumb-bob of 1900–1950 with a screw cap, and Swanson’s bob stores its line inside the body under a removable cap.

Variants

Plain plumb bob and line: A weight on a cord; the form in all the Egyptian examples cited here and in the Science Museum Group’s mason’s plumb bob and line.
Plumb rule: A straight board or frame with a line and weight, from which the user reads whether the board is upright. The Science Museum Group holds a copy of a plumb-rule from the tomb of Sennotem, and Moxon lists a four-foot plumb rule among bricklayers’ tools.
Adjustable (reel) bob: Stanley’s 1900 No. 1 and No. 2 in bronze and No. 5 in iron, with steel points.
Combined marking line and plumb bob: Matson’s 1959 patent combines a dye-coated marking line used like a chalk line with a plumb bob stored in the same case.
Mason’s plumb line: Used with a plumb-and-level rule: Stanley’s 1900 catalog lists a mason’s plumb and level (style 35) adapted for use with a plumb bob and 42 inches of line.

Specifications

Weight · oz: 8 oz or 16 ozSwanson’s two brass bobs. Egyptian museum bobs are small: 5.6 cm and 4.7 cm high for the travertine and limestone bobs at the Met.
Line length · ft: 20 ft (16 oz bob)Swanson ships its 16 ounce bob with 240 inches of braided line.

Tips

Heavier bob for long drops and wind
Swanson says the extra mass of its 16 ounce brass bob steadies the line in wind and on long drops where a lighter bob sways. This is the manufacturer’s statement about its own product.1
Hook the line overhead, drop, and mark below
Swanson’s instruction is to hook the line to a joist, header or nail, drop the bob and mark the point directly below. The hanging bob supplies the vertical, and the tip locates the mark.1
Replace the tip when it wears
Swanson’s bob has a hardened steel tip and two threaded spare tips, because brass tips go dull and chip on stone. A worn point moves the mark off the line of the cord.1
Let an adjustable bob pay out its own line
Stanley’s 1900 catalog says to hold the ring and drop the bob with a slight jerk, which reels off the desired length of line; the spring on the reel then holds the bob at that point. The spring pressure can be increased or decreased with the screw through the reel.1
Offset the line from the post
Moxon describes holding the end of the line between finger and thumb half the diameter of the line roll away from one corner of the post. The post is upright if the line runs parallel to the corner.1

History

Earliest documented

ca. 1850–1700 B.C.

Lisht North, Egypt (excavated by the Metropolitan Museum of Art, 1920–21)

The Metropolitan Museum of Art holds a travertine plumb bob, 5.6 cm high, dated to the late Middle Kingdom (about 1850–1700 B.C.) and recorded as excavated by the museum at Lisht North in 1920–21. It also holds New Kingdom bobs of limestone (Ramesside, from Lisht North), hematite, and carnelian (with the name of Hatshepsut). Petrie’s 1917 catalog of the Egyptian collection at University College London includes a section titled “The plumb line and level.” Earlier plumb bobs, including claims for Old Kingdom examples, were not located in a source that could be read here.1 2 3 4 5

The plumb bob exists because gravity gives a vertical reference without any instrument. The Metropolitan Museum’s Egyptian plumb bobs are small objects of stone: travertine at 5.6 cm, limestone at 4.7 cm, and a hematite bob 4 cm high. The Met records one of carnelian, 1.7 cm long, with the name of Hatshepsut. The oldest of them dates to the late Middle Kingdom, about 1850 to 1700 B.C., and all are older than the metal bobs in the other sources used here.

The weight was combined early with a frame. The Science Museum Group holds a copy of an Egyptian mason’s plumb-rule from the tomb of Sennotem, and Moxon, writing in 1693, still lists a four-foot plumb rule with a line and a lead plummet among the tools of the bricklayer. The frame lets a worker read the vertical against a straight face instead of judging it by eye against a hanging cord, and the cord and weight remained as the reference.

The nineteenth-century change was one of convenience. The Tool Shed ties the Stanley plumb bob with a reel to Justus Traut’s 1874 patent, and Stanley’s 1900 catalog describes a bob with a reel and a spring that holds the line at any length so that the user need not tie it. The bodies were bronze or iron with steel points. The patent number and text for Traut’s reel have not been located.

The modern bob changes materials and combines functions. Matson’s 1959 patent (granted 1962) puts a dye-coated marking line, used as a chalk line is, and a plumb bob in one pistol-shaped case, with a removable pin in the bob that stays in the surface when the bob is lifted. Swanson, a present manufacturer, sells solid brass bobs with hardened steel tips and spare tips, and a 20-foot braided line stored under the cap; its claims about wind steadiness are the company’s own. The Science Museum Group’s brass bob with a screw cap, dated 1900 to 1950, shows the same cap-and-line storage in an earlier object.

Timeline

ca. 1850–1700 B.C.
A travertine plumb bob, 5.6 cm high, from Lisht North is now in the Metropolitan Museum of Art (22.1.110).1
ca. 1550–1080 B.C.
A hematite plumb bob of the New Kingdom, 4 cm high, bequeathed by Howard Carter in 1939, is in the Met (49.105.3).1
ca. 1473–1458 B.C.
A carnelian plumb bob, 1.7 cm long, bears the name of Hatshepsut (Met 41.160.149); the record gives the joint reign of Hatshepsut and Thutmose III.1
ca. 1300–1080 B.C.
A limestone plumb bob, 4.7 cm high, from the Ramesside cemetery at Lisht North is in the Met (15.3.627).1
undated in the record
The Science Museum Group acquires a copy of the mason’s plumb-rule from the tomb of Sennotem (object 1930-186) from the Service des Antiquités, Cairo.1
1693
Moxon describes the carpenter’s plumb-line for trying posts, and lists a four-foot plumb rule with a line and a plummet of lead among the bricklayer’s tools.1
1874
The Tool Shed captions a Stanley No. 2 brass plumb bob with reel as made under Justus Traut’s 1874 patent, produced 1874–1937.1
1900
Stanley Catalogue No. 26 lists adjustable plumb bobs with a reel: No. 1 and No. 2 in bronze and No. 5 in iron, each with a steel point.1
1900–1950
A brass plumb-bob with a screw cap of this period is in the Science Museum Group collection (1982-39).1
1962
Earl L. Matson of Portland, Oregon, is granted US Patent 3,016,616 (filed 1959) for a combined dye-coated marking line and plumb bob in one case.1
2014
Swanson adds plumb bobs, chalk lines, pencils and crayons to its marking line, according to the company’s history page.1

Gaps in the record (6)

The Old Kingdom claim for plumb bobs, sometimes made in popular accounts, was not located in a primary or museum source that could be read.
The Met’s Egyptian bobs are dated by the museum from excavation context and style; the Sennotem plumb-rule copy has no date in its Science Museum record.
Traut’s 1874 patent number and text were not located; the date rests on a caption in a collectors’ newsletter.
No standard for plumb bobs was located; standards is empty.
Safety and care have no sourced entries.
Greek, Roman and medieval evidence was not located.