Laboratory Thermometer — Range, Parts, Uses, Diagram and Precautions

1. Bulb — the rounded glass tip at the bottom that holds the thermometric liquid (mercury or alcohol). It is the part placed in contact with the substance.
2. Capillary tube — a very fine bore running through the stem along which the liquid rises and falls.
3. Thermometric liquid — mercury or coloured alcohol that expands and contracts with temperature.
4. Stem — the thick glass body that protects the capillary and carries the scale.
5. Scale — the markings, usually in degrees Celsius (°C), along the stem.
6. No kink — unlike a clinical thermometer, there is no constriction, so the liquid moves freely both up and down.

Range: A laboratory thermometer normally reads from -10°C to 110°C, which is wider than a clinical thermometer (35°C to 42°C). The wide range lets it measure both the freezing point of water (0°C) and temperatures above the boiling point in many experiments.

Least count: The smallest temperature that can be measured is usually 1°C (each small division equals 1°C). The least count is the value of one smallest division on the scale.

Principle: A laboratory thermometer works on the principle of thermal expansion of liquids — the thermometric liquid expands and rises in the capillary when heated, and contracts and falls when cooled. The height of the liquid column indicates the temperature.

Uses:

1. To measure the temperature of liquids and solutions in experiments
2. To find the boiling point and freezing/melting point of substances
3. To measure the temperature of the surroundings or of hot/cold objects in the lab
4. Used in science practicals in schools and colleges

Precautions while using a laboratory thermometer:

1. Keep the thermometer upright (vertical) while reading, not tilted.
2. The bulb should be fully surrounded by the substance but should not touch the bottom or sides of the container.
3. Read the temperature while the bulb is still in contact with the substance, because the reading falls back quickly once removed.
4. Keep your eye level with the top of the liquid column to avoid an error of parallax.
5. Do not use it to measure temperatures beyond its range.

Laboratory thermometer: • Range: -10°C to 110°C • Has no kink (constriction) • Reading falls immediately when removed; must be read in place • Used to measure the temperature of objects, liquids and surroundings • Usually read while kept upright

Clinical thermometer: • Range: 35°C to 42°C (covers normal body temperature, 37°C) • Has a kink that stops the mercury from falling back • Can be removed from the body and then read • Used only to measure human body temperature • Has a smaller least count (commonly 0.1°C) for accuracy