Which instrument uses a float in a glass tube to measure the specific gravity of a liquid?

Which instrument uses a float in a glass tube to measure the specific gravity of a liquid?

• A. Hydrometer
• B. Ground
• C. Diode
• D. Field (magnetic field)

Answer: (A)

Buoyancy and density determine how far a float sinks in a liquid, and this is exactly how a hydrometer works. A hydrometer is a weighted float inside a glass tube with a calibrated stem. When you place it in a liquid, the liquid’s density sets how much of the float is submerged: in denser liquids it rises higher, in lighter liquids it sinks deeper. The stem is marked with a scale, so where the liquid’s surface touches the stem tells you the liquid’s specific gravity relative to water. In other words, the reading directly reflects how dense the liquid is compared with water. Other items don’t fit because they aren’t instruments that measure density using a floating buoyant mechanism. A ground is a reference point in circuits, a diode is a semiconductor device, and a magnetic field is a physical quantity, none of which involve a float in a glass tube to gauge specific gravity.

Buoyancy and density determine how far a float sinks in a liquid, and this is exactly how a hydrometer works. A hydrometer is a weighted float inside a glass tube with a calibrated stem. When you place it in a liquid, the liquid’s density sets how much of the float is submerged: in denser liquids it rises higher, in lighter liquids it sinks deeper. The stem is marked with a scale, so where the liquid’s surface touches the stem tells you the liquid’s specific gravity relative to water. In other words, the reading directly reflects how dense the liquid is compared with water.

Other items don’t fit because they aren’t instruments that measure density using a floating buoyant mechanism. A ground is a reference point in circuits, a diode is a semiconductor device, and a magnetic field is a physical quantity, none of which involve a float in a glass tube to gauge specific gravity.