How does an hourglass measure time?

1. How long should the egg cook? Turn the sand timer over and it starts counting. No batteries, no numbers, just sand.
2. Between the two glass bulbs is a narrow neck. Only a thin stream of grains can squeeze through at once, so the sand takes a while to get down.
3. Here is the clever part. The sand falls just as fast when the top is full as when it is nearly empty. That steady trickle is what makes a good timer.
4. Two things set the time. A bigger hole lets the sand through faster, so the timer is shorter. More sand takes longer to run through.
5. When the last grains drop, the time is up. The same sand through the same neck takes the same time, every time you flip it.

Tap each part of the hourglass. The round window shows the neck up close.

• Upper bulb. The sand starts in the upper bulb. Its sloping glass sides guide the grains toward the neck.
• Sand. The grains are tiny, dry and nearly all the same size, so they flow smoothly and do not stick together.
• Neck. The narrow neck lets only a thin stream of sand through. Its width decides how fast the timer runs.
• Falling stream. The sand falls in a thin, steady stream. It is just as thick at the start as at the end.
• Lower bulb. The sand lands in the lower bulb and builds a cone-shaped pile. When all the sand is here, time is up.
• Frame. Two flat ends and some posts protect the glass and let the timer stand either way up.

Change the size of the opening and the amount of sand in the yellow timer. Race it against the blue one-minute timer.

Experiment: You put twice as much sand in the timer but keep the same opening. What happens to the time?

1. Set the sand to 50 grams and note the time.
2. Change the sand to 100 grams with the same opening.
3. Now try a bigger opening. Which change makes the bigger difference?

What we found: Twice the sand takes twice as long, because the sand always runs through at the same steady speed. A bigger opening makes the timer much shorter: even a little wider lets a lot more sand through.

Turn on JavaScript to try it yourself.

Build a sand timer that runs for the target time. Choose the opening and the sand, then test it.

People used sand timers hundreds of years ago, long before small clocks were common. Sailors used them to time their turns on watch and to measure the speed of their ships. A sand timer works on a rocking ship, in the cold and in the dark, and it never needs winding.

The timer has two glass bulbs joined by a narrow neck. When you flip it, gravity pulls the sand down, but only a thin stream of grains can squeeze through the neck at once. The grains above have to wait their turn, like a crowd at a narrow door.

Here is what makes sand special. If you fill a bottle with water and poke a hole in the bottom, the water squirts out fast at first and then slows down as the bottle empties. Sand does not do that. The grains rub and lean against each other and against the glass, so the grains at the neck are not squashed harder when there is more sand on top. The stream stays the same all the way through.

Because the stream is steady, the time depends on just two things: how much sand there is, and how wide the neck is. Twice the sand takes twice as long. A slightly wider neck lets through a lot more sand, so the timer is much shorter. The crowd idea is not perfect: people can push and hurry, but grains of sand never do.

Key words

Hourglass

A timer made of two glass bulbs with sand that runs from one to the other.

Neck

The narrow opening between the bulbs. It sets how fast the sand runs.

Flow rate

How much sand passes through the neck each second.

Granular material

Stuff made of lots of small solid bits, like sand, rice or sugar.

Jam

When grains wedge together into an arch over a hole and block it.

Check your thinking

1. How do you know an hourglass has finished?

   Show the answer

   All the sand has run into the lower bulb. The timer measures how long the sand takes to run through. When the top is empty, the time is over.

2. Two timers hold the same sand. One has a bigger hole. Which finishes first?

   Show the answer

   The one with the bigger hole. A bigger hole lets more grains through each second, so the sand is gone sooner.

3. Why is sand better than water for a simple timer?

   Show the answer

   Sand runs at the same speed from start to finish. Water slows down as it runs out. Sand keeps a steady trickle, so every part of the time is measured evenly.

4. A timer runs for 1 minute. You want one that runs for 3 minutes with the same neck. What should you do?

   Show the answer

   Use three times as much sand. The sand runs at a steady speed, so three times the sand takes three times as long.

5. Coarse grains 1 millimeter across are put in a timer with a 3 millimeter opening. What is likely to happen?

   Show the answer

   The grains form an arch and jam. When the opening is only about three grains wide, the grains can lock together into a stable arch that blocks it.