Hot and Cold
Touch a metal spoon and a wooden spoon that have been in the same drawer all night. The metal feels colder. But they have been in the same place for hours, so they must be just as warm as each other. Your hand is not a good judge. How can we measure how hot something really is?
Temperature
How hot or cold something is, is called its temperature. A hot cup of tea has a high temperature; an ice cube has a low one.
The metal spoon only feels colder because metal takes heat away from your hand faster than wood does. Your skin feels how fast it is losing heat, not the spoon's temperature. To measure temperature we need a tool that doesn't get fooled.
The thermometer
A thermometer uses something simple: most liquids take up a little more space when they get warmer, and a little less when they cool. Put a liquid in a glass bulb with a very thin tube above it. As it warms, the liquid rises up the tube; as it cools, it sinks. Marks along the tube turn the height into a number.
Many thermometers today are electronic and show a number on a screen, but they measure the same thing.
The Celsius scale
The marks need a standard, like every unit. In 1742 a Swedish astronomer, Anders Celsius, used two things anyone can make: freezing water and boiling water. He split the space between them into 100 equal steps, called degrees.
(Celsius first put 100 at freezing and 0 at boiling. Soon after his death the scale was turned the right way up.)
On the Celsius scale today:
- water freezes at 0 degrees, written 0 °C;
- water boils at 100 °C;
- a comfortable room is about 20 °C;
- the inside of a healthy person is about 37 °C. A few degrees more is a fever.
Below freezing, the numbers go below zero: a cold winter night might be -10 °C, said "minus ten degrees".
Try it at home
- Leave a metal spoon and a wooden spoon side by side for an hour, then touch them. Which feels colder? Now you know why.
- If you have a thermometer, measure the temperature of a glass of cold water from the tap and of a room. Write each one with its unit, like "15 °C".
Words to know
- temperature - how hot or cold something is
- thermometer - a tool that measures temperature
- degree (°) - the unit on a temperature scale
- Celsius scale (°C) - the scale where water freezes at 0 and boils at 100
Big idea
Temperature is how hot or cold something is, and your hand is easily fooled. A thermometer measures it in degrees. On the Celsius scale, water freezes at 0 °C and boils at 100 °C, and your body is about 37 °C.