What is a hydrometer name the principal on which it works?

An instrument that measures a liquid’s density or specific gravity.

Principle:

Archimedes’ principle. A floating body displaces its own weight of liquid. At equilibrium:

Whydrometer = ρliquid g Vdisplaced

For a given hydrometer (fixed mass), lower density ⇒ larger displaced volume ⇒ it sinks deeper. Higher density ⇒ smaller displaced volume ⇒ it floats higher.

Construction (common type):

• Sealed glass float with a weighted bulb at the bottom.
• Narrow stem with a graduated scale (specific gravity, °Bé, °API, % alcohol, etc.).
• Sometimes a thermometer is built in.

How to use:

1. Fill a tall cylinder with the test liquid. Remove bubbles.
2. Gently lower the hydrometer. Let it float freely without touching the sides.
3. Read the scale at the meniscus level (usually the bottom of the meniscus for water-like liquids).
4. Record temperature and apply the instrument’s temperature correction if required (many are calibrated at 20 °C or 15.6 °C).

Why it works (intuition):

• Same hydrometer in different liquids adjusts its immersion depth until the upthrust equals its weight.
• Depth is thus a proxy for density, pre-calibrated on the stem.

Variants and uses:

• Alcoholmeter (% alcohol by volume).
• Lactometer (milk density to estimate solids).
• Battery hydrometer (electrolyte specific gravity for lead–acid batteries).
• API/ Baumé hydrometers (petroleum, chemicals).