Spirit level

A spirit level is a precision instrument used to determine if a surface is horizontal, known as being level, or vertical, known as being plumb.

These tools are essential in many industries. Carpenters, stonemasons, and bricklayers rely on them to ensure structures are straight. Surveyors use them to map the land. Even photographers and videographers use levels to keep their cameras steady. The device is often called a bubble level or simply a level. It earns the name "spirit level" from the liquid inside its vial. This liquid is commonly an alcohol, often referred to as a spirit.

The mechanism relies on the behavior of an air bubble within a liquid. Most levels use alcohol, such as ethanol, rather than water. Alcohol is chosen because it has low viscosity and low surface tension. This allows the bubble to move quickly through the tube. It also settles accurately with very little interference from the glass. Alcohol has a much wider temperature range than water. This means it will not freeze and break the glass vial. To make the bubble easier to see, a colorant like fluorescein is often added. This dye usually turns the liquid a bright yellow or green.

There are two primary designs used to achieve different goals. The first is the tubular or linear level. These have a glass vial that is often slightly curved. This curve ensures the bubble naturally rests at the highest central point. If the surface tilts, the bubble moves away from the center marks. Some tubular levels use a barrel-shaped tube with a wider middle. This design allows the tool to work even when held upside-down or on its side. The second design is the bull's eye level. This is a circular, flat-bottomed device with a convex glass face.

While a tubular level only measures in one direction, a bull's eye level can level a surface across an entire plane.

We can trace the history of this invention back to the 17th century. The French scientist Melchisédech Thévenot invented the instrument before February 2, 1661. We know this date from his letters to the scientist Christiaan Huygens. Within a year, Thévenot shared his invention with Robert Hooke in London and Vincenzo Viviani in Florence. By 1666, the Académie Royale des Sciences recommended using "levels of the Thévenot type" for an expedition to Madagascar. In the United States, William B. Fell changed precision leveling in 1939. Working in Rockford, Illinois, he invented the Fell All-Way precision level.

This bull's eye level could show tilt on both x and y axes at the same time. It was so accurate that it was restricted from export during World War II. It provided a resolution of .0005 inches per foot.

Precision levels are defined by their sensitivity. Sensitivity is the change in angle or gradient required to move the bubble by a specific unit of distance. For a surveyor's level, the bubble might move when the vial is tilted by only 0.005 degrees. In high-end machinist levels, the movement is even more specific. Machinists use a unique term called "5 tenths per foot." This refers to a length of 5 tenths of 1 thousandth of an inch.

To check if a level is accurate, a user can perform a rotation test. They place the level on a surface and note the bubble position. Then, they rotate the level 180 degrees and check it again. If the bubble does not show the same orientation, the level is inaccurate. The user can then adjust the bubble tube within its housing to fix the error.

Many specialized versions of the level exist for specific tasks. A torpedo level is a small, torpedo-shaped tool used in tight spaces.

It often contains vertical, horizontal, and 45-degree tubes. A line level is a lightweight tool designed to hang on a builder's string line.

Surveyors use even more complex tools like the tilting level or dumpy level. These combine a spirit level with an optical telescope mounted on a tripod.

By reading height values from two graduated rods, a surveyor can calculate elevation differences over long distances. These measurements can be accurate to within a few millimeters.

Today, technology has introduced digital and electronic alternatives. Digital spirit levels are common in civil engineering and steel construction. They can display precise numeric angles within 360 degrees. These tools often have an accuracy of 0.1 to 0.05 degrees. They are much easier to read from a distance than a small glass bubble. Many modern electronic levels are also waterproof and impact-resistant. Even smartphones have become levels. They use a built-in sensor called an accelerometer to detect orientation. Whether using an old glass vial or a new digital sensor, these tools remain vital for building a stable world.