Measuring History

November 26, 2001

Hot and Cold
Winter is coming to the northern hemisphere, and with it, talk of central heating, gloves and scarves, snowfall, and record low temperatures. Monitoring temperatures is part of our everyday life — whether we're talking about the weather, our bodies, central heating, or cooking.

In the United States, we typically measure temperature in degrees Fahrenheit. But in other countries, such as England, you'll get the weather forecast in degrees Celsius. Recipes will advise you to bake a pie at 205慢 and not 400慚. In fact, the Celsius scale is the most commonly used temperature scale in the world. This week, we have special cause to wonder about the history of this scale, because November 27 is the 300th anniversary of the birthday of the man who invented it.

Anders Celsius was born in Sweden in 1701. His parents didn't know that their son's work would one day make their family name an everyday word used by millions of people around the world.

Meet Professor Celsius
The world Anders lived in was very different than the world we know. Much of the knowledge that we now take for granted — such as the motion of Earth with respect to the Sun and planets — was only beginning to be understood then. In Europe, radical and brilliant scientists, such as Galileo and Sir Isaac Newton, had been developing new and revolutionary ways of understanding the workings of the world and the universe. The era was known as "the Enlightenment," and it was a good time for someone of Anders's ability to be working in the field of science.

Anders's talent came from his family of academics: one of his grandfathers was a mathematician and the other an astronomer, and his father, Nils, was a professor of astronomy. As a child, Anders showed a natural flair for mathematics, but he developed a keen interest in astronomy and became a professor at the age of 29. So, how did a clever astronomer come to develop a temperature scale?

The answer lies with the weather. When making observations of the night sky, Anders monitored the weather conditions, including the temperature. At the time there were a lot of different kinds of thermometers with different scales — by the time Anders started working on the problem of temperature measurement, around 35 different scales existed. (Compare this number with today, when three main scales are in use — Celsius, Fahrenheit, and Kelvin.) It is thought that Anders may have realized the need to have an international standard, and also to make temperature measurement as accurate as possible.

An Astronomer's Work
Anders Celsius is best remembered for his temperature scale, but as an astronomer, he also worked on many other important projects.

Anders was the first to measure the brightness of stars with tools, and he was also the first to see a connection between the Northern Lights and Earth's magnetic field.

Scientists of the day argued over whether Earth was perfectly spherical or flattened at the ends (geoid-shaped). Anders played a part in settling this dispute by taking part in a 1636 expedition to north Sweden. He and his cohorts measured the length of a degree along a meridian close to the pole and compared their results with the measurements of a similar expedition to the opposite end of Earth, at Peru. They concluded that Sir Isaac Newton's theory about flattening at the poles was true: Earth is a geoid.

Making a Scale
To make any scale, you need to begin with two fixed points — that is, two known markers. Anders chose:

The boiling point of water
The melting point of ice

The boiling point of water had already been suggested by scientists such as Newton. Anders was the first to use the melting point of ice, because he was able to easily monitor the temperature of snow as it melted. This is the same temperature at which water freezes; in fact, it's the point where ice and water mix together and are in an equilibrium. Here is a question for you to consider:

Why do you think Anders chose to base his temperature scale on the physical properties of water? Click here to see the answer.

Anders's main challenge was to make sure that the boiling point of water and the melting point of ice were the same no matter where you were on Earth. Only then could he use those temperatures as reliable, constant points on his scale. Anders put a mercury thermometer into snow and marked the point on the thermometer at which the snow melted. He wrote, "I have repeated this experiment many times during two years in all winter months and all kinds of weather, and during different barometric changes, and always found precisely the same point on the thermometer." The temperatures he recorded in Sweden were the same as those recorded by another scientist, Reamur, in Paris. So, Anders determined that snow (or ice) always melted at the same point on his thermometer.

What about the second fixed point on the scale, the boiling point of water? Anders knew that it varies a little with air pressure. To allow for this, he had to monitor air pressure carefully when recording the boiling point of water on different days and in different locations. In addition, he noted, "it is well known that once water has begun to boil, it will not take up any considerable degree of heat even if one continues boiling for long; thus the mercury in the thermometer will end at the same point." In other words, once water boils, it will remain at the same temperature — it doesn't get hotter if boiled for longer.

Through such careful experimentation, Anders was able to fix the boiling point of water and the melting point of ice (or snow) more accurately than his predecessors had. He marked each point on his thermometer and put 100 marks in the temperature interval between them. Interestingly, he put the melting point of ice at 100, and the boiling point of water at 0. Compare this with the version of the scale that we use today:

How would you express 30 degrees on Anders's original scale? Or 12 degrees? Or -5 degrees?

Anders's precise experimentation with fixed points and the design of his scale set his work apart and made it such an important achievement. In 1742, one year before his death, Anders described his scale in a paper entitled "Observations on two persistent degrees on a thermometer." The scale did not become popular until some time later when the values of the fixed points were switched — in other words, when the scale was changed to the one we know today.

Where Do You Put Zero?
The question of where to put zero on a scale had been dealt with by another scientist whose name we also associate with temperature — Daniel Gabriel Fahrenheit (1686-1736). Fahrenheit was a German physicist who made mercury thermometers using his own scale.

On Fahrenheit's scale, the melting point of ice is at 32 degrees and the boiling point of water is at 212 degrees. Because zero is outside the ordinary temperature region, it means you are less likely to deal with negative numbers when using the Fahrenheit scale.

A third scale which is used by scientists sets zero at a much lower temperature than either the Celsius or Fahrenheit scale. Zero on the "Kelvin" scale is equivalent to -273.16 慢. This temperature is known as "absolute zero," and in theory it is the coldest temperature that can possibly be reached. Scientists believe that this is the point at which molecular motion almost comes to a standstill.

Anders did not name his new scale after himself. Because the scale was made up of 100 steps, he originally called it the "Centigrade" scale (centi means 100, and "grade" comes from gradus, the Latin for "step.") The name of the scale was changed to the "Celsius scale" in 1948 in honor of the scientist, although you will still hear it being referred to by its original name today.

If you were developing your own temperature scale, what would you consider using as fixed points? Would body temperature be a good fixed point?

Try developing your own unusual temperature scales. You might try one that's based on more extreme temperatures (such as the surface temperatures of different planets). Is your scale practical? Why or why not?

Anders Celsius died of tuberculosis not long after he introduced his temperature scale. He was just 42 and never knew how useful his work would be to millions of people in the centuries to follow. Happy birthday Professor Celsius!


		Negative Integers Join Dijit as he works with negative temperatures and negative integers in this Destination Math tutorial.

		Converting Temperatures Use matrices to convert a table of temperatures from Fahrenheit to Celsius.

		Liquid Nitrogen Read about a world of super cool temperatures in this Riverdeep archive article.