The spirit level is a very old tool, used by carpenters, builders, and even folks at home trying to hang a painting, which helps you to determine straight or plumb lines. These are also called bubble levels because the goal when you line one up in a horizontal fashion is that you want the bubble in a small amount of liquid in the center of the level to be in the center, between two lines. This will let you know with a good deal of accuracy whether your line is straight. There are also spirit level types for measuring level vertical lines, which may operate by somewhat different standards.
As mentioned above, the spirit level is verifiably ancient. The first was invented by Melchisedech Thevenot in the 17th century CE. However, they may not have been widely used until the early 18th century, though Thevenot shared the design with other scientists and philosophers like Robert Hooke of England. The original spirit level had several vials with liquid, which would help a person measure a straight line horizontally. The design, which is now commonly seen, and certainly in use in plenty of homes, that contains a single vial with a bubble in it wasn’t created until the 1920s, and Harry Zeiman is credited with its invention.
The name spirit in the spirit level comes from the use of the liquid contained inside the vial, often ethanol. Those that contain ethanol usually have yellow colored fluid in the center vial. Some prefer the intensely more accurate blue fluid filled spirit levels, which may have a slightly different shaped glass tube. Many also claim these are easier to read, and are of value because of their accuracy, which can be extremely important when building or leveling things.
A number of people have switched to the laser line level, which can help measure straight lines along three planes, and combines some aspects of the spirit level. These project a laser light so that the straight line is visualized across a long surface. They’re in wide use in the construction industry, and are becoming a popular home improvement tool too, with a few models much less expensive than professional ones.
Levels aren’t simply used in home building or home improvement projects. Engineers use variants of the spirit level or laser level to make sure that machine components are completely level. Even slight inaccuracies in the placement of the foundation for a machine can result in problems with accurate function.