Thermometer: Definition, Types, Parts, and How to Use
A thermometer is a scientific instrument used to measure the temperature of an object or a living body. Temperature is the measure of the degree of hotness or coldness of an object. Most traditional thermometers work on the principle that liquids expand when heated and contract when cooled. In school science, students mainly learn about clinical thermometers used at home or clinics, and laboratory thermometers used in science experiments.
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Main Parts of a Thermometer
A standard liquid-in-glass thermometer consists of a few simple, carefully designed parts that work together to show accurate temperature readings.
- Bulb: The small, rounded bottom part that holds the measuring liquid such as mercury or colored alcohol.
- Capillary Tube: A very narrow glass tube through which the liquid rises or falls as the temperature changes.
- Glass Stem: The thick outer glass tube that protects the inner capillary tube and makes it easy to hold.
- Scale: Marked lines and numbers along the stem that allow you to read the temperature in degrees Celsius or Fahrenheit.
- Kink or Constriction: A narrow bend found just above the bulb in clinical thermometers to stop mercury from falling back before a reading is taken.
Difference Between Clinical and Laboratory Thermometer
Clinical and laboratory thermometers are built differently because they serve different purposes. Here is a direct comparison between the two main types studied in school.
| Feature | Clinical Thermometer | Laboratory Thermometer |
|---|---|---|
| Main Purpose | Measures human body temperature | Measures temperature of chemicals and materials |
| Temperature Range in Celsius | 35 degrees C to 42 degrees C | -10 degrees C to 110 degrees C |
| Temperature Range in Fahrenheit | 94 degrees F to 108 degrees F | 14 degrees F to 230 degrees F |
| Presence of Kink | Present just above the bulb | Absent |
| Reading Position | Can be read after removing from body | Must be read while kept inside the substance |
| Liquid Used | Mercury or digital sensor | Mercury or colored alcohol |
| Least Count | Usually 0.1 degree C or 0.2 degree C | Usually 1 degree C |
| Cleaning Method | Washed with cold water or antiseptic | Washed with normal laboratory water |
| Sterilization with Hot Water | Never allowed, bulb may burst | Allowed within its scale limits |
| Common Place of Use | Homes, clinics, and hospitals | School and research laboratories |
| Standard Normal Reading | 37 degrees C (98.6 degrees F) | Room temperature (around 20 to 25 degrees C) |
Types of Thermometers Used in Daily Life and Science
Different kinds of thermometers are designed based on where and how the temperature needs to be measured.
- Clinical Thermometer: Specially designed to measure human body temperature with a short, safe scale.
- Digital Thermometer: An electronic thermometer with an LCD display that does not contain toxic mercury.
- Laboratory Thermometer: A long glass thermometer with a wide scale used in science experiments.
- Maximum-Minimum Thermometer: Used by weather forecasters to record the highest and lowest temperatures of the day.
- Infrared Thermometer: A non-contact thermometer that reads body or surface temperature from a distance using infrared light.
How to Read a Clinical Thermometer Correctly
Reading a clinical thermometer requires gentle handling and simple safety precautions to get an accurate measurement.
- Wash the thermometer before and after use with an antiseptic solution or clean cold water.
- Hold the thermometer firmly and give a few gentle jerks to bring the mercury level below 35 degrees Celsius.
- Place the bulb under the tongue or under the armpit for at least one minute.
- Take out the thermometer and hold it horizontally at eye level to clearly see the level of the mercury thread.
- Read the marked number aligned with the top edge of the mercury column.
Important Precautions While Using a Laboratory Thermometer
A laboratory thermometer does not have a kink, so its mercury falls immediately once removed from the liquid.
- Keep the thermometer upright and completely immerse the bulb in the liquid without touching the sides or bottom of the container.
- Read the temperature while the thermometer is still immersed in the liquid.
- Keep your line of sight straight and level with the top of the mercury column to avoid parallax errors.
- Never use a laboratory thermometer to stir liquids, and handle it carefully to avoid glass breakage.
Common questions
- What is the normal temperature of a healthy human body?
- The normal temperature of a healthy human body is 37 degrees Celsius or 98.6 degrees Fahrenheit.
- Why is there a kink in a clinical thermometer?
- The kink prevents the mercury from dropping back into the bulb immediately after being taken out of the mouth, allowing enough time to take an accurate reading.
- Why is mercury commonly used in thermometers?
- Mercury expands uniformly with heat, stays liquid over a wide temperature range, does not stick to the glass tube, and has a shining silvery appearance that is easy to see.
- Why should we not wash a clinical thermometer with boiling water?
- A clinical thermometer measures only up to 42 degrees Celsius. Boiling water is at 100 degrees Celsius, which causes the mercury to expand beyond the tube and burst the glass bulb.
- Why are digital thermometers preferred over mercury thermometers today?
- Digital thermometers are safer because they do not contain toxic mercury, do not break easily, and show clear numeric readings on an electronic screen.