Spirit level
A spirit level is an instrument designed to indicate the level of a surface. The name "spirit level" has its origin in the use of spirits (eg. wine) in the glass tube.
Spirit levels feature a slightly curved glass tube which is incompletely filled with a liquid, leaving a bubble in the tube. Most commonly spirit levels are employed to indicate how horizontal or how vertical a surface is.
Often also a indicator for a 45 degree inclination is included.
Some are also capable of indicating the level of a surface between horizontal and vertical to the nearest degree. The crudest form 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 in two perpendicular directions, while the tubular level only does so in the direction of the tube.
The sensitivity of a level is given as the angle, in seconds of arc, by which the level has to be tilted to move the bubble by one graduation unit.
For precision levels it is as little as 5.
Its invention is credited to either Jean de Mechisedech Thevenot or Robert Hooke[1]. Those attributing the invention to Jean de Thevenot date it in the range of 1662[2] to 1666[3].
Often the term spirit level is used to refer to a levelling instrument as used in surveying to measure height differences over larger distances. It consists of a spirit level in the above sense, mounted on a measurement telescope containing cross-hairs, itself mounted on a tripod.
The observer reads height values off two staffs, one in front of him and one behind him, to obtain the height difference between the ground points on which the staffs are resting. By repeating, height differences can be measured cumulatively over large distances.
See also
|
