Spirit level

How it works

A vial is sealed with a liquid and a small bubble of air. The bubble rises to the highest point of the vial, so when the vial is set parallel to the working edge of the stock and the stock lies level, the bubble sits between the marks at the center. Thévenot’s level of 1681, as described in the Dictionary of Scientific Biography, used a tube about the diameter of a little finger and seven or eight times as long as it was wide, filled with alcohol and mounted on a stone ruler with a viewing lens.

The sensitivity of the vial depends on the curvature of its inner surface. The Dictionary of Scientific Biography reports that Huygens remarked on the lack of convexity in Thévenot’s original tube, and that the difficulty of making glass tubes of constant section kept the design from common use until the middle of the eighteenth century, according to the historian Maurice Daumas.

A vial set across the stock reads plumb when the stock is held vertical. Builders’ levels sold as “plumb and level” carry both. Adjustable levels allow the vial to be reset against the base: L. L. Davis’s 1867 patent describes a level whose broken bubble could be quickly replaced and the level put back into perfect adjustment, and which could be set to any inclination read in linear measure on a scale.

Anatomy

Stock: The body and working edge; mahogany, cherry and rosewood in Stanley’s 1859 list, rosewood in an English carpenter’s level of 1830–1860, iron in Davis’s levels and machinists’ levels, ebony and brass in a 1926 London level.
Vial (level glass): A sealed tube holding liquid and a bubble; Stanley sold level glasses separately in 2- to 4-inch lengths in 1859.
Plumb vial: A second vial set at right angles to the working edge to read vertical.
Top plate and tips: Brass plates protecting the vial opening and the ends of the stock; Stanley’s 1859 list offers levels polished and tipped, and brass bound.
Adjustment: Screws or an eccentric for resetting a vial against the base; Davis’s 1867 patent uses a screw at each end of the bubble case.

Variants

A-frame (plumb-bob) level: The pre-bubble form: a plumb line hung from the apex of an A-shaped frame, level when the line falls on a mark on the crossbar. The Met holds a Late Period Egyptian amulet in the shape of a surveyor’s level.
Plumb rule: A straightedge with a plumb line hung against a raised center mark, shown in thirteenth- and fifteenth-century pictures of masons’ work, according to Lonnie Shelby’s 1961 study as summarized by Johnson Level.
Carpenter’s plumb and level: A long wooden stock with level and plumb vials; the main product of Stanley’s 1859 level list.
Machinist’s level: A short, more sensitive level for setting machinery; Stanley listed a polished iron machinist level with brass top plate in 1859.
Pocket level: A small level with a single vial; Stanley listed iron, brass-topped and all-brass pocket levels in 1859.
Adjustable level and inclinometer: A level whose vial can be reset or set to an angle; patented by L. L. Davis in 1867.

Specifications

Length · in: Stanley’s 1859 list runs from 10-inch levels to plumb-and-levels of 26 to 30 inches.
Accuracy · mm/m or in/ft: No independent or standards-body figure for builders’ level accuracy has been sourced.

Made to

DIN 877

Safety and care

Care

Davis’s 1867 patent and his 1871 advertisement give the reason for adjustable levels: a bubble glass can be put out of true with the base by accident, and should then be readjusted or replaced.

Tips

Test a level by turning it end for end
Johnson Level instructs the user to set the level on a flat horizontal surface, note the bubble and mark the level’s position, then reverse the level end for end with the leveling surface down in exactly the same position. If the bubble rests where it did before, the level is true. The military manual gives the same test, and Johnson Level adds that the working edges of the level and the surface must be free of foreign material.1 2
Test the plumb vial against a vertical surface
The same check works for the plumb vials: the working edge is placed on a flat vertical surface, the bubble position noted and the position marked, and the level reversed working edge for working edge. Johnson Level says the level is true if the bubble returns to its noted place.1
Know which vial does what
Johnson Level describes the center tube vial as finding the true horizontal and the end vials as finding the true vertical, with an angled vial on many torpedo levels for 45 degrees. The manual describes the carpenter’s level as having three vials set horizontally, vertically and at 45 degrees, with the bubble between the two etched lines when the surface is true.1 2
The bubble moves to the high side
Johnson Level says to put the eyes at the level of the tube for an accurate reading. If the bubble sits to the right of the lines, the object slopes downward from right to left; if to the left, downward from left to right.1
Read an accuracy rating as a slope
Johnson Level gives the best spirit level accuracy as plus or minus 0.5 millimeter per meter, equal to 0.0005 inch per inch or 0.029 degree, with 0.75 millimeter per meter (0.043 degree) as the next level it lists.1

History

Earliest documented

before 1661

Paris (Melchisédech Thévenot)

The Dictionary of Scientific Biography credits Melchisédech Thévenot with the bubble level, says it was probably connected with his experiments on capillarity and the siphon of 1658–1661, and records that he mentioned it in letters to Vincenzo Viviani and Christiaan Huygens in 1661 and 1662. It was publicized in an unsigned pamphlet in 1666 and described and illustrated by Thévenot in 1681. Levels without a bubble are far older: the Met holds an Egyptian amulet of 664–332 BCE in the shape of a surveyor’s level.1 2

Before the bubble, level was found with a plumb line. Johnson Level, summarizing Lonnie Shelby’s 1961 study, describes the Egyptian solution as an A-shaped frame with a weighted string hung from its apex, level when the string fell on a mark, and notes that it could only compare the two points where its legs stood. The medieval plumb rule improved on it by hanging the line against a long straightedge, so that the whole length of a course could be checked at once. The Met’s Late Period amulet in the shape of a surveyor’s level shows how familiar the form was in Egypt.

The bubble level came out of laboratory physics. The Dictionary of Scientific Biography ties Thévenot’s level to his experiments on capillarity and the siphon in 1658–1661 and records that he mentioned it in letters to Viviani and Huygens in 1661 and 1662. It also records that Hooke and Huygens improved the design. Thévenot himself proposed it for finding a horizontal plane at sea, a use the Dictionary calls exaggerated.

Glass held it back. The Dictionary reports that tubes of constant section were difficult to make, so the level was used at first simply to set a surface parallel to the horizon, and, citing Daumas, that it came into common use only in the mid-eighteenth century as construction improved. By the early nineteenth century it was a carpenter’s tool: the Science Museum Group holds a rosewood carpenter’s level of 1830–1860 from a tool chest.

Industrial manufacture in New England turned the level into a catalog line. Stanley’s 1859 price list offers plumbs and levels in mahogany, cherry and rosewood, varnished or polished, tipped or brass bound, along with iron machinists’ levels, pocket levels and level glasses sold separately for repair. The name of the company, the Stanley Rule and Level Company, records how central the tool was to its trade.

The next problem was keeping the vial true. Davis’s 1867 patent addresses broken bubbles and adjustment, and argues that mechanics work by linear measure rather than by degrees, so his inclinometer read slope against a scale of inches. His 1871 advertisement presents the iron-stocked adjustable level as the replacement for the old spirit level and plumb.

Timeline

664–332 BCE
Egyptian amulets of the Late Period take the form of a surveyor’s level; the Met holds a hematite example.1
1200s
Pictures show masons using a straightedge level with a raised semicircle and a plumb line, according to Lonnie Shelby’s study as summarized by Johnson Level.1
1658–1661
Thévenot experiments on capillarity and the siphon, the work the Dictionary of Scientific Biography connects with his bubble level.1
1661–1662
Thévenot mentions his level in letters to Vincenzo Viviani and Christiaan Huygens.1
1666
The level is publicized in an unsigned pamphlet.1
1681
Thévenot describes and illustrates the level in his “Discours sur l’art de la navigation,” in the Recueil de voyages.1
mid-1700s
Thévenot’s design comes into common use as glass construction improves, according to Maurice Daumas as cited in the Dictionary of Scientific Biography.1
1830–1860
An English carpenter’s spirit level of rosewood and glass, part of a carpenter’s tool chest now in the Science Museum Group collection.1
1857
Stanley says it begins making levels; the Stanley Rule & Level Company is founded by Henry Stanley the same year.1
1859
The Stanley Rule and Level Company, formed by the union of A. Stanley & Co. and Hall & Knapp, issues a price list of mahogany, cherry and rosewood plumbs and levels, an iron machinist’s level, pocket levels and loose level glasses.1
1867
L. L. Davis of Springfield, Massachusetts, patents an adjustable spirit level whose bubble can be replaced and readjusted and which reads inclination in linear measure (US Patent 68,961).1
1871
Davis advertises iron-stocked adjustable levels, plumbs and inclinometers in Scientific American as replacing “the old-fashioned Spirit Level and Plumb.”1
1926
A 10-inch ebony level made in London, part of the collection of the London tool dealers Charles Nurse & Co. now in the Science Museum Group.1

Gaps in the record (7)

Secondary sources report that Huygens and Hooke later claimed the invention in their own countries; the Dictionary of Scientific Biography says only that they improved it. The original correspondence (Huygens, Oeuvres complètes; the Oldenburg correspondence) has not been read, so the priority question is not entered as a dispute.
The 1666 pamphlet and Thévenot’s 1681 “Discours sur l’art de la navigation” have not been located and read directly.
Lonnie Shelby’s 1961 article is known here only through a manufacturer’s summary; the article itself has not been read.
The wooden plumb level from the tomb of Sennedjem in the Egyptian Museum, Cairo, has not been verified from a museum record.
No source has been read for the change from wood to aluminum stocks or from glass to acrylic vials in the twentieth century.
No independent accuracy test or standard text for builders’ levels has been read; DIN 877 is listed as a lead only. Buying is null.
No sourced safety entries.