||This article needs additional citations for verification. (July 2009)|
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.
Early spirit levels had two banana-shaped curved glass vials at each viewing point and were more complicated to use. In the 1920s Henry Ziemann, the founder of Empire Level Mfg. Corp., invented the modern level with a single vial. These vials, common on most ordinary levels today, have 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 travels away from the center position, which is usually marked.
Alcohols such as ethanol are often used rather than water. Alcohols have 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.
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 with a circle at the center. It serves to level a surface across a plane, while the tubular level only does so in the direction of the tube. Where a spirit level must be usable upside-down, the banana-shaped tube is replaced by a barrel-shaped tube.
Checking and adjustment 
To check the accuracy of a carpenter's type level (i.e. whether the level indicates that a truly horizontal surface is, in fact, level), it is placed on a flat and roughly level surface and the reading on the bubble tube is noted. This reading indicates to what extent the surface is parallel to the horizontal plane, according to the level, which at this stage is of unknown accuracy. The spirit level is then rotated through 180 degrees in the horizontal plane, and another reading is noted. If the level is accurate, it will indicate the same orientation with respect to the horizontal plane. A difference implies that the level is inaccurate.
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 the magnitude of the reading remains constant when the level is flipped. Note that a perfectly horizontal surface is not needed.
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. This is done in two horizontal perpendicular directions.
The sensitivity is an important specification for a spirit level; its accuracy depends on its sensitivity. The sensitivity of a level is given as the change of angle or gradient required to move the bubble by unit distance. If the bubble housing has graduated divisions then the sensitivity is the angle or gradient change that moves the bubble by one of these divisions. 2mm is the usual spacing for graduations; on a surveyor's level the bubble will move 2 mm when the vial is tilted about 0.005 degree.
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
- Slip or Skid Indicator
- Bull's eye level
A spirit level is usually found on the head of combination squares.
Surveyor's leveling instrument 
Tilting level, dumpy level or automatic level  are terms used to refer to types of leveling instruments as used in surveying to measure height differences over larger distances. It has a spirit level mounted on a telescope (perhaps 30 power) with cross-hairs, itself mounted on a tripod. The observer reads height values off two graduated vertical rods, one 'behind' and one 'in front', to obtain the height difference between the ground points on which the rods are resting. Starting from a point with a known elevation and going cross country (successive points being perhaps 100 meters apart) height differences can be measured cumulatively over long distances and elevations can be calculated. Precise levelling is supposed to give the difference in elevation between two points a kilometer apart correct to within a few millimeters.
See: Spirit leveling
Carpenter's level 
A traditional carpenter's spirit level looks like a short plank of wood and often has a wide body to ensure stability, and that the surface is being measured correctly. 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 is 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 
An engineer's precision level permits leveling items to greater 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.
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 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.
See also 
- Horizontal and vertical
- Laser line level
- Physical geodesy
- Plumb line
- Plumb Site
- Turn and bank indicator
|Wikimedia Commons has media related to: Spirit levels|