Spirit Level

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 upper image is a plain precision level used in the engineering field to level machines or workpieces, the lower image shows an adjustable precision level that has an accuracy of 1:10000. The adjustable nature of this level can also be used to measure the inclination of an object.

The accuracy of a spirit level can be checked by placing it on any flat surface, marking the bubble's position and rotating the level 180°. The position of the bubble should then be symmetrical to the first reading.

Both levels have a "vee" groove machined along the base which enables the level to sit on a round bar while remaining parallel with the bar's axis. They also have a smaller cross level to enable the second axis to be roughly checked or corrected.

While a precision level may be used to check and correct the twist in a machine (or workpiece), its presence does not necessarily need to be corrected.

• A machine such as a mill or lathe does not have to be perfectly level to operate correctly but may in fact have a known twist introduced to the machines bed. This twist is often introduced to ensure that a worn lathe turns parallel work, by realigning the bed (that is worn) to the spindle axis (unworn).
• Leveling a ship's lathe would be pointless due to the nature of the ships base - floating on water. Correcting any twist in the bed however would be essential for accurate work to be reproduced from the lathe.