The least count of a thermometer is the smallest temperature change it can measure. A standard laboratory mercury thermometer has a least count of 1°C. A clinical thermometer typically has a least count of 0.1°C (or 0.2°F). Digital thermometers can have least counts of 0.1°C or even 0.01°C.
Least count of standard laboratory thermometer = 1°C.
Least count of clinical thermometer = 0.1°C.
Least count = smallest scale division = full range / total number of divisions.
Clinical range: 35–42°C; Lab range: −10°C to 110°C (typical).
Normal body temperature = 37°C = 98.6°F.
Type of Thermometer → Least Count Laboratory mercury thermometer → 1°C Clinical mercury thermometer (Celsius) → 0.1°C Clinical mercury thermometer (Fahrenheit) → 0.2°F Digital clinical thermometer → 0.1°C Precision laboratory thermometer → 0.1°C or 0.2°C Infrared thermometer → 0.1°C (typical) Platinum resistance thermometer → 0.01°C or better
Definition: Least count = smallest division on the thermometer scale
For a lab thermometer with range 0–100°C and 100 divisions: LC = 100°C / 100 = 1°C
For a clinical thermometer (35–42°C, 70 divisions for 7°C range): LC = 7°C / 70 = 0.1°C
Least count determines precision: • Lower LC → can detect smaller temperature changes → more precise. • A thermometer with LC = 1°C cannot distinguish 36.5°C and 37°C. • A thermometer with LC = 0.1°C can detect the 0.5°C difference.
Clinical thermometer: • Range: 35°C to 42°C (or 94°F to 108°F) • LC = 0.1°C — important for detecting fever accurately. • Normal body temperature = 37°C (98.6°F) • Fever threshold = 37.5°C–38°C • Fine graduation needed to distinguish fever from normal.
Laboratory thermometer: • Range: −10°C to 110°C (common) • LC = 1°C — sufficient for most lab experiments. • Cannot be used clinically.
Reading rule: Estimating between divisions: A thermometer with LC = 1°C can be estimated to ±0.5°C by a careful observer.
Steps to read a mercury thermometer:
Parallax error: • Reading from above → gives a lower reading. • Reading from below → gives a higher reading. • Always read with eyes perpendicular to the scale.
Kink in clinical thermometer: The constriction (kink) in a clinical thermometer prevents mercury from falling back — allowing a stable reading after removal from the body. It must be shaken down before reuse.
Conversion: °F = (°C × 9/5) + 32 °C = (°F − 32) × 5/9
Laboratory thermometer: 1°C. Clinical thermometer: 0.1°C. The least count equals the smallest scale division.
0.1°C (or 0.2°F for Fahrenheit scale). The clinical range is 35–42°C, subdivided into 0.1°C divisions.
LC = Range of thermometer / Number of divisions. Example: lab thermometer 0–100°C with 100 divisions → LC = 1°C.
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