Hydrometer
device · 400 · Ancient world
A float with a scale: the denser the liquid, the higher it floats. It is used to measure the density of milk, syrup or battery acid.
What it is made of
- Mercury — has part. Old hydrometers used mercury or lead shot as the weight to keep them upright.
Principles and materials it uses
- Archimedes’ principle — uses. A hydrometer floats: the buoyant force depends on the liquid's density, so how deep it sinks shows the density.
- Glass — is made of. A glass tube with a weight at the bottom and a scale at the top.
Where it is used
- Lead–acid battery — is used in. A car battery's charge is checked with a hydrometer by the density of its acid.
Things built in a similar way
- Cream separator — works like. Both rely on density: a separator removes the lighter cream, a hydrometer checks by density whether milk has been watered down.