How does a spirit level show whether something is level?

1. This shelf looks straight, but a ball put on it rolls to one end. One side must be lower. Our eyes cannot always tell.
2. A spirit level is a straight bar with a small window. Inside the window is a tube of liquid with one bubble. On this shelf the bubble sits off to one side.
3. The tube is called a vial. It is filled with colored liquid, except for one air bubble. The secret is its shape: it curves gently upward in the middle, like a low arch.
4. Air is much lighter than the liquid, so the liquid sinks and pushes the bubble up. Tip the vial, and the bubble slides along the arch to whichever end is higher.
5. When the level is perfectly flat, the top of the arch is exactly in the middle. The bubble settles there, between two lines. Bubble in the middle means level.
6. So raise the low end of the shelf until the bubble sits between the lines. Now the shelf is level, and the ball stays right where you put it.

Tap each part. The big tube at the top shows the vial in the window, magnified.

• Body. The body is a long, stiff bar, often made of aluminum. It has to be perfectly straight, because it carries the vial along the thing you are checking.
• Flat edge. The bottom edge is made very flat and smooth. You rest it on the shelf, so the level leans exactly as much as the shelf does.
• Vial. The vial is a clear, sealed tube. It curves upward very slightly in the middle. On a real level the curve is so gentle you can hardly see it.
• Liquid. The liquid is alcohol with yellow-green dye. Alcohol is used instead of water because it flows easily and does not freeze on cold days.
• Bubble. The bubble is a little pocket of air. Air is far lighter than the liquid, so the bubble is always pushed up to the highest spot in the tube.
• Two lines. Two lines mark the middle of the vial, just far enough apart to fit the bubble. When the bubble sits exactly between them, the level is flat.
• Upright vial. This second vial is turned a quarter of the way around. Hold the level against a wall or a post, and this bubble shows whether it stands straight up.

Drag the right end of the shelf up and down. Which way does the bubble go? Which way does the ball roll? Then make the shelf level.

Experiment: When you raise the right end of the shelf, which way does the bubble move?

1. Drag the right end of the shelf up. Watch the bubble.
2. Now drag it down below level. Where does the bubble go, and where does the ball roll?
3. Find the spot where the bubble sits between the lines.

What we found: The bubble always moves toward the high end, because air floats up through the liquid. The ball does the opposite and rolls to the low end. When the bubble rests between the lines, neither end is higher, and the ball stays still.

Turn on JavaScript to try it yourself.

Level the shelf using only the bubble, then press Check. The crooked picture might fool your eyes, so trust the bubble!

A spirit level looks simple: a straight bar with a little window. In the window is a sealed tube, called a vial, filled with colored liquid and one air bubble.

Air is much lighter than liquid. Gravity pulls the heavier liquid down, and the liquid pushes the light bubble up. So a bubble always ends up at the highest place it can reach. You see the same thing when you tip a bottle of water.

The clever part is the shape of the vial. It curves upward very slightly in the middle, like a long, low arch. When the level sits on something flat, the top of the arch is in the middle, so the bubble rests between the two lines. Tip the level, and the top of the arch moves toward the raised end. The bubble follows it.

Builders use levels to hang shelves, lay bricks and set floors, because a tilt too small to see can still make water run or a marble roll. Many levels have a second vial turned sideways for checking that walls stand straight up. Our bottle comparison is not perfect: a bottle is not curved the right way, so its bubble jumps about instead of stopping at one exact spot.

Key words

Level

Perfectly flat, with neither end higher than the other.

Vial

The sealed, slightly curved tube of liquid with a bubble inside.

Buoyancy

The upward push a liquid gives to lighter things in it, like a bubble.

Plumb

Standing exactly straight up and down.

Sensitivity

How far the bubble moves for a small tilt.

Check your thinking

1. Why does the bubble move to the higher end of the vial?

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   Air is lighter than the liquid, so it floats up. The heavier liquid sinks and pushes the lighter air up to the highest spot.

2. The bubble sits exactly between the two lines. What does that tell you?

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   The surface is level. The top of the arch is in the middle only when neither end is higher.

3. The bubble is near the left end. What should you do to make the shelf level?

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   Lower the left end, or raise the right end. The bubble goes to the high end, so the left end is too high.

4. What would happen if the vial were perfectly straight instead of gently curved?

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   The bubble would rush to one end at the tiniest tilt. With no arch there is no highest spot in the middle, so the bubble shoots to whichever end is a hair higher.

5. A vial's arc has a radius of 2 meters. About how far does the bubble move for a tilt of 0.5 degrees (about 0.0087 radians)?

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   About 17 millimeters. Offset is radius times angle in radians: 2000 millimeters times 0.0087 is about 17 millimeters.