Which instrument is specifically used to measure the specific gravity (relative density) of milk accurately?

Introduction / Context:
Measuring the quality of milk is important in dairies and households, and one key parameter is specific gravity or relative density. Different instruments are used to measure density or related properties of various substances. This question asks which specific instrument is designed for measuring the specific gravity of milk, a common general knowledge topic.

Given Data / Assumptions:
- The quantity to be measured is specific gravity or relative density of milk.- We assume standard laboratory or dairy testing conditions.- Several instruments related to measurement are listed as options.

Concept / Approach:
Relative density compares the density of a substance to the density of water at a reference temperature. Hydrometers are floating instruments designed to measure the relative density of liquids in general. A lactometer is a special type of hydrometer calibrated specifically for milk so that it can indicate whether milk has been diluted with water or adulterated. Thus while hydrometer is a general instrument, lactometer is the specialised instrument for milk.

Step-by-Step Solution:
1. Recall that hydrometers measure relative density of liquids by floating deeper or shallower depending on liquid density.2. Remember that for milk testing, a specially calibrated hydrometer, called a lactometer, is widely used.3. Examine the options and identify lactometer as the only instrument explicitly devoted to milk specific gravity.4. Recognise that hygrometers measure humidity of air, not liquid density.5. Recognise that barometers measure atmospheric pressure, unrelated to milk density.

Verification / Alternative check:
In dairy quality control, instructions often mention taking a sample of milk in a cylinder and gently lowering a lactometer into it. The reading on the lactometer scale gives the specific gravity directly. This specific usage is highlighted in many agriculture and general science textbooks, confirming that lactometer is the correct answer for milk.

Why Other Options Are Wrong:
- Hydrometer: This is a general instrument for liquid density, but the question asks specifically about milk. The special instrument for milk is the lactometer.- Hygrometer: Used for measuring humidity in the air, not density of liquids.- Barometer: Used for measuring atmospheric pressure, not relative density of liquids.

Common Pitfalls:
Students sometimes choose hydrometer because it measures liquid density in general. While that is conceptually close, competitive exam questions often expect the more specialised term lactometer when the substance is milk. To avoid confusion, remember the word root lacto, which relates to milk in many scientific terms such as lactose and lactation.

Final Answer:
The specific gravity of milk is measured accurately using a lactometer.