The hydrometer
A hydrometer is used to measure the density of liquids and solutions — and it is built on Archimedes' principle.
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Its parts
It has a cylindrical stem and a bulb, both made of glass.
Mercury or lead shot is put inside the bulb. This is done so that it can float upright in a liquid.
The stem carries a scale for reading off the density.
How it works
When a hydrometer is put into a liquid, it sinks until the liquid applies an upthrust equal to its weight, and then floats. Since by Archimedes' principle the upthrust equals the weight of the liquid displaced, when it floats the weight of the liquid displaced equals the weight of the hydrometer.
The weight is fixed; the volume is not
The hydrometer weighs the same whatever liquid it is put in. So the weight of liquid it must displace is always the same too — what changes is the volume that weight occupies.
In a denser liquid
The amount of liquid equal to the hydrometer's weight occupies a small volume. So the hydrometer sinks to a smaller depth.
In a less dense liquid
That same weight of liquid occupies a larger volume. So the hydrometer sinks to a greater depth.
How the scale is read
Put the liquid whose density is to be measured into a vessel and let the hydrometer float in it; the density can then be read from the hydrometer scale at the level to which it has sunk.