## Below Zero

As I write this, the temperature here in Madison Indiana is -4° Fahrenheit with wind chill down to around -27°. It is cold outside. It is even cooler than I would like inside, even with the heat on. To distract myself from this winter horror, I will try to think warm thoughts and write a little about temperature. What does it mean to say the temperature is 30 degrees or 100 degrees? What exactly are we measuring? Shouldn’t zero degrees be the coldest possible temperature?

People have known that some days are hotter or colder than other days since time immemorial. Before the invention of the thermometer, it was not possible to measure just how much hotter or colder. People could not quantify or measure temperature, except by personal perception, which is subjective.  A person might feel that it is getting warmer, but he could not be sure if the environment was actually getting warmer, or that he was simply feeling warmer, perhaps because he was exerting himself. Also, there was no way of determining just how warmer today was than yesterday. Notice that I am talking about temperature rather than heat. The two concepts are related but are not the same thing. In any substance the atoms and molecules that make up that substance are not standing still but are moving about. In a liquid or a gas, the atoms can move about freely, while in a solid, they are held in place but still vibrate back and forth. In a sense then, the temperature of an object is the average kinetic energy of the atoms in that object. Heat is defined by physicists as the transfer of thermal energy from a warmer body to a colder body. Heat is not measured by degrees but by joules or calories. ( The calories on food labels are actually kilocalories.)

The basic principle on which the thermometer works was actually discovered in ancient times. Hero of Alexandria knew that air expanded or contracted based the temperature and invented a thermometer of sorts by placing a closed tube with its open end in a container of water. The water would move up or down in the tube according to the temperature. Galileo constructed a similar device, as did several other renaissance scientists. None of these devices had a scale, however, so it was still not possible to quantify temperature with them. They were also sensitive to air pressure.

The first thermometer with a scale was invented by either Francesco Sagredo or Santorio Santorio around 1611-1613.

In 1714, Daniel Gabriel Fahrenheit invented a thermometer which used mercury in a glass tube. Once it became possible to

English: Gabriel Daniel Fahrenheit (Photo credit: Wikipedia)

manufacture thermometers on a standard design, it was also possible to develop a standard scale. Fahrenheit developed such a scale in 1724. He used three points to calibrate his scale. The temperature of a mixture of water, ice, and ammonium chloride was designated as zero. The temperature of water just as ice began to form was set at 32 and human body temperature at exactly 96. Later, it was discovered that there are about 180 of Fahrenheit’s degrees between the melting and boiling points of water so the scale was calibrated to make exactly 180 degrees so that the boiling point of water on the Fahrenheit scale is 212°. The Fahrenheit Scale is the one most used in the United States and is still widely used in Britain and Canada.

In 1742 Anders Celsius developed a scale in which there were one hundred degrees between the melting and boiling points of water. Curiously, he designated the boiling point of water as 0 and the melting point as 100 so the temperature measurement got lower as it got hotter. The Celsius scale was reversed and adopted as part of the metric system. This scale, sometimes called centigrade, is used worldwide, especially by scientists. Conversion between the two scales is easy enough. Because there are 180 degrees Fahrenheit between the melting and boiling points of water, but only 100 degrees Celsius, each degree Fahrenheit is 9/5 of a degree Celsius. Since Fahrenheit has the melting point of water at 32°, to convert from Fahrenheit to Celsius you subtract 32 and then multiply by 9/5. To convert from Celsius to Fahrenheit, multiply by 5/9 and then add 32.

Anders Celsius

The coldest possible temperature, at which the atomic motion stops, is called absolute zero. This is -459.67° Fahrenheit or -273.15°. It is not actually possible to reach absolute zero, but scientists have come close. The lowest temperature ever recorded in a laboratory is around .oooooooo1 degrees Celsius. In 1848, the British physicist William Thompson, later to be Lord Kelvin, proposed a temperature scale using degrees Celsius which began at absolute zero. The Kelvin scale is slightly different from other scales in that it does not rely on the physical properties of any materials, being based on absolute zero. Temperatures in the Kelvin scale are measured in “Kelvins” rather than degrees so that you may say that the melting point of water is 273 K. The Kelvin scale is also extensively used by scientists, especially those who work with very low temperatures.

Lord Kelvin

It’s not working. All of this writing about absolute zero is just making me feel colder.