Spirit level

For other uses, see Spirit level (disambiguation).

A tubular spirit level

A bull's eye spirit level

A spirit level or bubble level is an instrument designed to indicate whether a surface is horizontal (level) or vertical (plumb). Different types of spirit levels may be used by carpenters, stonemasons, bricklayers, other building trades workers, surveyors, millwrights and other metalworkers, and in some photographic or videographic work.

Original spirit levels had two banana-shaped curved glass vials at each viewing point and were much more complicated to use. In the 1920s, Henry Ziemann invented the modern level with a single vial. These vials, common on most ordinary levels today, feature a slightly curved glass tube which is incompletely filled with a liquid, usually a colored spirit or alcohol, leaving a bubble in the tube. At slight inclinations the bubble will travel away from the center position, which is usually marked in some manner.

Alcohols such as ethanol are often used rather than water for a variety of reasons. Alcohols generally have very low viscosity and surface tension, which allows the bubble to travel the tube quickly and settle accurately with minimal interference with the glass surface. Alcohols also have a much wider liquid temperature range, and won't break the vial as water could due to ice expansion. A colorant such as fluorescein, typically yellow or green, may be added to increase the visibility of the bubble.

Some spirit levels are capable of indicating the level of a surface between horizontal and vertical to the nearest degree. An extension of the spirit level is the bull's eye level: a circular, flat-bottomed device with the liquid under a slightly convex glass face which indicates the center clearly. It serves to level a surface across a plane, while the tubular level only does so in the direction of the tube. The most sophisticated spirit levels are guaranteed accurate to five-ten-thousandths (.0005) of an horizontal inch at the center per vertical inch at either end^{[citation needed]}. Where a spirit level must also be usable upside-down, the banana-shaped tube is replaced by a barrel-shaped tube. The upper internal surface of the tube is thus always of an appropriate shape.

Checking and adjustment

To check the accuracy of a simple carpenter's type level(ie when placed on a truly horizontal surface, does the level indicate the surface is level), a flat and approximately level surface (note it does not have to be level) is chosen and the reading on the bubble tube is noted. This reading indicates to what extent the chosen surface according to the level (which at this stage is of unknown accuracy) is, parallel to the horizontal plane. The spirit level is then rotated through 180 degrees (in the horizontal plane), whereupon the level, if it is in fact accurate will still indicate exactly the same state of orientation with respect to the horizontal plane. If the bubble indicates a different state of orientation, then the level is inaccurate and must be adjusted.

Adjustment of the spirit level is performed by successively rotating the level and moving the bubble tube within its housing to take up roughly half of the discrepancy. Until when the level is flipped the buble does not move its relative position. Note that a perfectly horizontal surface isn't required.

A similar procedure is applied to more sophisticated instruments such as a surveyor's level or a theodolite and is a matter of course each time the instrument is set up. In this latter case, the plane of rotation of the instrument is levelled, along with the spirit level. Furthermore this is done in two horizontal perpendicular directions.

Types of level

There are different types of spirit levels for different uses:

Surveyor's leveling instrument
Carpenter's level (either wood, aluminium or composite materials)
Mason's level
Torpedo level
Post level
Line level
Engineer's precision level
Electronic level
Inclinometer
Slip or Skid Indicator

A spirit level is also usually found on the head of combination squares.

Surveyor's leveling instrument

Tilting level, dumpy level or automatic level ^[1] are terms used to refer to types of leveling instruments as used in surveying to measure height differences over larger distances. It consists of a spirit level in the above sense, mounted on a telescope containing cross-hairs, itself mounted on a tripod. The observer reads height values off two staffs, one 'behind' and one 'in front', to obtain the height difference between the ground points on which the staffs are resting. Starting from a point with a known (absolute) elevation, by repeating 'behind', 'front' and intermediate 'front' readings, (relative) height differences can be measured cumulatively over large distances and elevations can be calculated of intermediate points.

See: Spirit leveling

File:Tool-level.jpg

Carpenter's level

A traditional carpenter's spirit level looks like a short plank of wood and often has a wide body to ensure both stability, and that the surface is being measured correctly. Embedded in the middle of the spirit level is a small window where the bubble and the tube is mounted. Two notches (or rings) designate where the bubble should be if the surface was level. Often an indicator for a 45 degree inclination is included.

Line level

A line level is a level designed to hang on a builders string line. The body of the level incorporates small hooks to allow it to attach and hang from the string line. The body is lightweight, so as not to weigh down the string line, it is also small in size as the string line in effect becomes the body; when the level is hung in the center of the string, each leg of the string line extends the levels plane.

Engineer's precision levels

Main article: Engineer's spirit level

An engineer's precision level permits leveling items to a greater degree of accuracy than a plain spirit level. They are used to level the foundations, or beds of machines to ensure the machine can output workpieces to the accuracy pre-built in the machine.

History

The spirit level was invented by Melchisedech Thevenot (born in either 1620 or 1621; died 1692). Thevenot was an amateur scientist and patron of many scientists and mathematicians. He was wealthy and well-connected, later becoming the Royal Librarian to King Louis XIV of France. Thevenot invented the instrument some time before February 2, 1661. This date can be very accurately established from Thevenot's correspondence with scientist Christiaan Huygens. Within a year of this date the inventor circulated details of his invention to others, including Robert Hooke in London and Vincenzo Viviani in Florence. It is occasionally argued that these bubble levels did not come into widespread use until the beginning of the eighteenth century, the earliest surviving examples being from that time, but Adrien Auzout had recommended that the Académie Royale des Sciences take "levels of the Thevenot type" on its expedition to Madagascar in 1666. It is very likely that these levels were in use in France and elsewhere long before the turn of the century.

Thevenot is often confused with his nephew, the traveler Jean de Thevenot (born 1633; died 1667). There is evidence to suggest that both Huygens and Hooke later laid claim to the invention, although only within their own countries.