Laboratory Thermometer

Purpose

Used to measure the temperature of objects other than the human body (e.g., boiling water, ice, chemical reactions).

Structure

• A long, narrow, uniform glass tube.
• A bulb at one end containing liquid (alcohol colored red, or mercury).
• Range: Typically -10°C to 110°C.

Finding the Least Count

To measure accurately, you must know the value of the smallest division (Least Count).

Method:

1. Find the difference between two big marks (e.g., $10^\circ C$ and $20^\circ C$). Difference = $10^\circ C$.
2. Count the number of divisions between them (e.g., 10 divisions).
3. Calculate: $$\text{Smallest Division} = \frac{10}{10} = 1^\circ C$$

Proper Usage Techniques (Activity 7.5)

Unlike a clinical thermometer, a laboratory thermometer generally does not have a “kink” to hold the mercury level. Therefore:

1. Keep it Vertical: Do not tilt the thermometer.
2. Immersion: The bulb must be fully immersed in the substance (e.g., water).
3. No Contact: The bulb should not touch the bottom or sides of the container.
4. Read Inside: Read the temperature while the thermometer is still in the water. If you take it out, the level falls immediately.