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Reading Selection, Lesson 5

Measuring Temperature by Degrees

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A thermoscope

A thermoscope built to one of Galileo’s designs. Thermoscopes were used to compare temperatures but had no standardized scale.


Galileo built some of the earliest thermoscopes, which are thermometers without scales.

“How cold is it outside?” “Is your soup hot enough?” How many times have you been asked questions about temperature? Usually, we answer them according to how things feel to us. We compare temperatures to our own body temperature. People have always compared temperatures in this way. However, sometimes you need to know exactly how hot or cold something is. For example, if you cook a pizza in an oven that is too hot, it may burn—so you need to know the temperature of the oven.

About 400 years ago, some scientists began to tackle the problem of measuring temperature. Galileo was one of the first.

He made a thermoscope. This was a device that could be used to compare temperatures. Look at the picture of the thermoscope. Can you figure out how it worked?

It took another scientist, Olas Roemer, a Dane who was interested in astronomy and meteorology, to come up with a way of comparing temperatures measured with different devices. In 1701, Roemer calibrated his temperature-measuring devices according to the temperatures of ice water and the human body. He had made the first thermometer.


Roemer invented the first useful temperature scale.

Another scientist, this time from Holland, borrowed Roemer’s ideas. His name was G. Daniel Fahrenheit. Fahrenheit altered Roemer’s scale. He used the melting point of a salt-and-water slush as his zero point and the human body temperature as his high point. He divided the space between the two points into 96 degrees. The scale was later adjusted so that its calibration points were at 32 °F for ice melting and 212 °F for water boiling. On the adjusted scale, human body temperature became 98.6 °F. The new scale was named after Fahrenheit and is still used today.

About 30 years later, in 1742, another scientist, Anders Celsius from Sweden, came up with a new scale. Celsius designated the melting point of ice as 100 °C and the boiling point of water (at sea level) as 0 °C.
Anders Celsius

In 1742, Anders Celsius invented the Celsius (or centigrade) scale.

After Celsius’s death, the scale was reversed so that the melting point of ice became 0 °C and the boiling point of water (at sea level) became 100 °C. This scale, called the Celsius (or centigrade) scale, was popular because it used two temperatures that most people easily understand. It’s now used all around the world. This scale has one big problem. All temperatures below zero become negative numbers. Can you really have a negative temperature? Wouldn’t it be better to start a scale at the lowest possible temperature and work your way up?

William Thomson, aka Lord Kelvin

William Thomson started his temperature scale with the lowest possible temperature.

About 100 years later, in 1848, British physicist William Thomson could see the advantage of just such a scale. By that time, work done by Thomson and other scientists on how energy behaves in the universe led him to develop a scale that placed the absolute lowest possible temperature at zero. This temperature is the same as –273 °C and is called absolute zero. An object at absolute zero contains no heat energy. Thomson borrowed the divisions on Celsius’s scale and made the melting point of ice 273 degrees. What happened to the Thomson scale? It is still used by scientists around the world, who consider it to be the most useful temperature scale.

Thomson was such a clever scientist and inventor that the British government made him a lord and gave him the title Lord Kelvin. So his scale became the Kelvin scale, and temperature is measured in kelvins (abbreviated as K).

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