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, the founder of Empire Level Mfg. Corp., 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 a horizontal inch at the center per vertical inch at either end. 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.
Read more about Spirit Level: Checking and Adjustment, Sensitivity, Types of Level, History
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