Chapter 2
Temperature

We have several ways of measuring temperature. This chapter will explain some of them.

Celsius and Fahrenheit
One of the easiest ways of measuring temperature is by the use of the fact that everything around us responds to temperature by changing size. The substance in a thermometer, for example, rises when the temperature goes up and it goes down when the temperature decreases.

We use different scales to determine temperature. In the United States we mostly use the Fahrenheit scale (F) to measure temperature. This scale was created by a scientific instrument maker in Amsterdam named Gabriel Daniel Fahrenheit in 1714. He used the reference points in which water freezes and boils. These reference points are 32°F in which water freezes, and 212°F in which water boils.

The Celsius scale (C) is used all around the world and for all scientific work. This scale was devised by a Swedish astronomer named Andres Celsius who lived from 1701 to 1744. This scale used to be called the "centigrade scale," but was changed in 1948 when an international conference of scientists decided the name should be changed to Celsius. The Celsius scale was also based on when water freezes and boils. The reference point in which water freezes is 0°C, and the point water boils is 100°C.

Another way of to measure temperature is by checking the electrical resistance of most metals. One more way of measuring temperature is measuring the pressure of gas in a container. The pressure of this gas is called vapor pressure.

Absolute Zero
In the late 1800s scientists began to understand that there was a connection between temperature and energy. They thought that a temperature scale based on energy rather than the physical properties of a substance would be more useful.

A nineteenth century English physicist named William Thompson, more commonly known as Lord Kelvin, devised a scale base upon energy. Lord Kelvin’s scale used zero as the coldest possible temperature. The coldest temperature is called absolute zero. There is no limit of how hot something can get, but there is a limit of how cold something can get. To understand absolute zero you must understand that the more energy (heat) there is, the faster the molecules move. But as you lower the temperature the molecules slow down, and at -273.15°C the molecules stop completely. We cannot obtain absolute zero because there would not be anything there if the molecules were not moving. This is only a theory. Lord Kelvin’s temperature is called the Kelvin scale (K). The Kelvin scale is based on absolute value, and uses 0 Kelvin as absolute value. Since

0K = -273.15°C

to change a number from the Kelvin scale to the Celsius scale you must add -273.15°C.

Scientist are able to get very close to absolute zero, but are not able to reach absolute zero. The Third Law of Thermodynamics states that absolute zero cannot be attained. The lowest temperature achieved so far is 0.00000003K.

"Superconductivity is perhaps the most remarkable physical property in the universe"
- David Pines, University of Illinois Urbana-Champaign Center for Advanced Study Professor of physics and electrical and computing engineering