Types of Thermometer
Simple Explanation
A thermometer measures temperature by using some property of a material that changes predictably with temperature. A liquid-in-glass thermometer uses the thermal expansion of a liquid (like mercury or alcohol) in a thin tube; a thermocouple uses a tiny voltage that appears between two different metals at different temperatures; an infrared thermometer measures the infrared radiation an object emits, without touching it at all.
Why Do We Need It?
Different thermometer types suit different situations — a liquid-in-glass thermometer is cheap and simple for everyday use, while a thermocouple can survive extreme industrial temperatures, and an infrared thermometer can measure something too hot, too far away, or too dangerous to touch directly.
Worked Example
Choose the right thermometer for a situation
A factory needs to measure the temperature inside a furnace running at 1200°C. Explain why a normal liquid-in-glass thermometer would not work, and suggest a better choice.
Why Does This Work?
Each thermometer type relies on a different physical effect that reliably changes with temperature — thermal expansion, an electrical voltage, or emitted radiation — so choosing the right one is really about matching the physical effect to what the situation can survive or requires.
Real-Life Example
A forehead thermometer
A doctor uses a non-contact infrared thermometer to quickly check a patient's temperature.
This avoids any physical contact (useful for hygiene and speed) by measuring the infrared radiation naturally emitted by the skin, which increases predictably with body temperature.
Practice
Which type of thermometer measures temperature without touching the object at all?
EasyCommon mistake
Assuming all thermometers work the same way — different designs rely on entirely different physical effects (expansion, voltage, or radiation), which is exactly why some suit extreme or hands-off situations far better than others.
Quick Review
- Liquid-in-glass: uses thermal expansion of a liquid.
- Thermocouple: uses a voltage generated between two different metals.
- Infrared: measures emitted radiation, with no contact needed.