Clinical Thermometer vs Laboratory Thermometer | Class 6 CBSE

What Is a Clinical Thermometer?

A clinical thermometer is used to measure the temperature of the human body. Its range is 35°C to 42°C.

Doctors and nurses use clinical thermometers to check if a patient has a fever. The word "clinical" comes from "clinic" — a place of medical care.

Features of a clinical thermometer:

• Range: 35°C to 42°C (94°F to 108°F)
• Has a kink just above the bulb — prevents mercury from falling back
• Mercury bulb — placed under the tongue or in the armpit
• Smaller and portable — designed for regular use outside a lab
• Must be jerked downward before each use to reset the mercury
• Can be read after removal from the patient's body — because the kink holds the reading

What Is a Laboratory Thermometer?

A laboratory thermometer is used to measure temperature in science experiments. Its range is −10°C to 110°C.

Laboratory thermometers are used in science labs to measure the temperature of liquids, gases, or solids during experiments.

Features of a laboratory thermometer:

• Range: −10°C to 110°C — much wider than a clinical thermometer
• No kink — mercury falls back as soon as it is removed from the substance
• Must be read while still in the substance — because there is no kink to hold the reading
• Longer than a clinical thermometer
• Held upright during measurement
• Bulb must be fully submerged in the liquid being measured

Clinical vs Laboratory Thermometer — Comparison Table

Feature	Clinical Thermometer	Laboratory Thermometer
Purpose	Measuring human body temperature	Measuring temperature in experiments
Range	35°C to 42°C	−10°C to 110°C
Kink	✅ Yes — just above the bulb	❌ No kink
Reading after removal	✅ Yes — kink holds mercury in place	❌ No — mercury falls back immediately
Jerking before use	✅ Yes — to reset mercury below 35°C	❌ Not needed
Position during use	Under tongue or in armpit	Bulb submerged in liquid, held upright
Size	Smaller, portable	Longer
Used by	Doctors, patients, households	Scientists, students in lab

The Kink — Explained in Detail

A kink is a small bend or constriction in the glass tube of a clinical thermometer, located just above the bulb. Here is exactly what it does:

1. You place the thermometer under the tongue. As the body temperature heats the bulb, mercury expands and rises up the tube past the kink.
2. You remove the thermometer from the mouth. Normally, mercury would contract and fall back into the bulb — the reading would disappear.
3. The kink prevents this. It stops mercury from flowing back down. The column of mercury stays in place above the kink.
4. You can now read the temperature clearly, even though the thermometer is no longer in contact with the body.

💡 A laboratory thermometer has no kink. This is why you must read it while the bulb is still inside the liquid — the moment you remove it, mercury falls back and the reading is lost.

Why Can't You Use a Laboratory Thermometer for Body Temperature?

1. No kink: A laboratory thermometer has no kink, so mercury falls back immediately when you remove it from the mouth. You cannot read the temperature.
2. Wrong range: A laboratory thermometer is not calibrated with precision for the narrow 35°C–42°C body temperature range. Its wide range makes it far less accurate for detecting small changes in body temperature.

⚠️ Common mistake: "We can use any thermometer to measure body temperature." — This is wrong. Only a clinical thermometer should be used for measuring body temperature.

How to Use Each Thermometer Correctly

Clinical thermometer — step by step:

1. Wash and wipe the thermometer clean with antiseptic.
2. Jerk it downward to bring mercury level below 35°C.
3. Place the bulb under the tongue (or in armpit). Keep mouth closed.
4. Wait 1–2 minutes.
5. Remove and read at eye level.
6. Record the temperature, then wash and store safely.

Laboratory thermometer — step by step:

1. Hold the thermometer upright by the upper end — never by the bulb.
2. Submerge the bulb fully into the liquid being measured.
3. Wait for the mercury to stabilise.
4. Read the temperature at eye level while the thermometer is still in the liquid.
5. Remove carefully and store safely.