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A spirit level is a critical instrument for construction efforts. The body of the level is a very precisely molded or extruded plastic or metal bar, with longer ones stiffened with flanges to ensure they do not twist or bend under their own weight. The functional part of the level, however, is a small cylindrical capsule mostly filled with alcohol (the "spirit", as water would freeze more easily), leaving only a small air bubble left over. The capsule will have two marks centered precisely in the capsule and separated by a distance equal to the size of the air bubble.

To use a spirit level, set the body against the surface that you want to level. Since liquids naturally seek their own level, the air bubble in the capsule will be perfectly centered between the marks on the capsule when the spirit level is level. If the bubble floats to the left, raise the right side, and vice-versa. Some spirit levels use a curved, and therefore more demanding, capsule to increase the necessary precision to center the bubble. Since most things need to be level in two dimensions, a standard level will need to be used twice, at 90 degree angles to each position, to get the surface level both left-to-right and back-to-front. Some levels come with two capsules set at 90 degrees to each other so this can be done in one step.

Spirit levels come in a variety of lengths for different tasks. The longer the level is, the more expensive it is, but the more accurate it will be. An 8-foot level is overkill for most applications, however, and a 1-foot long level is plenty for simple tasks like leveling mirrors or pictures hanging on the wall. Longer levels, 3 or 4 feet long, are typically used to level appliances or furniture such as a washing machine or table. Very long levels, 8 feet or longer, are used in construction for tasks like wall framing.

Many spirit levels will have multiple capsules to check level, plumb, and sometimes a 45 degree angle. Some come with magnets on one side of the body so they can be used to plumb flagpoles, electrical conduit, plumbing lines, and other round, metal objects. Many modern levels come with a battery-powered laser to extend their useful range, and this laser will often move back and forth quickly to define a straight line across another surface. These are useful to ensure two objects are on the same level with each other across a wall, for example two pictures that should be aligned with each other. A tripod-mounted spinning laser level can mark all four walls of a room with a level line for installing ceiling tile grid or wainscoting.

An alternate type of spirit level is called a bullseye level. Rather than being a cylindrical capsule mostly filled with alcohol, a bullseye level is round. Bullseye levels are used to level things in two dimensions without having to reposition the level. A theodolite will typically have a bullseye level built-in, and some electric drills have a bullseye level on the back to help the user drill a perfectly vertical hole.

The Debutante reminds me of another kind of level called a digital level. A digital level uses an accelerometer to measure the pull of gravity and can be used with such devices as the Apple iPhone and the Nintendo Wii controller. Their primary advantage is that they can be calibrated more easily.

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