## Temperature Scales

When using a thermometer, we need to mark a scale on the tube wall with numbers on it. We have to define a** temperature scale**. A ** temperature scale** is a way to measure temperature relative to a

**starting point**(0 or zero) and a

**unit of measurement**.

These numbers are arbitrary, and historically many different schemes have been used. For example, this was done by defining some physical occurrences at given temperatures—such as the **freezing** and** boiling points of water **— and defining them as 0 and 100 respectively.

There are several scales and units exist for measuring temperature. The most common are:

## Celsius Scale – Celsius Temperature

About 20 years after Fahrenheit proposed its temperature scale for thermometer, Swedish professor **Anders Celsius** defined a better scale for measuring temperature. He proposed using the **boiling point of water** as **100° C** and the** freezing point of water** as** 0° C**. Water was chosen as the reference material because it was always available in most laboratories around the world.

**Celsius temperature scale** is also called **centigrade temperature scale** because of the **100-degree** interval between the defined points. The Celsius temperature for a state colder than freezing water is a negative number. The Celsius scale is used, both in everyday life and in science and industry, almost everywhere in the world.

**Absolute zero**, the lowest temperature possible, is defined as being precisely 0 K and −273.15 °C. The temperature of the **triple point of water** is defined as precisely 273.16 K and 0.01 °C. This definition fixes the magnitude of both the degree Celsius and the kelvin as precisely 1 part in 273.16 of the difference between absolute zero and the triple point of water.

It must be added, by international agreement the unit “degree Celsius” and the Celsius scale are currently defined by two different points: **absolute zero**, and the **triple point of water **(instead of boiling and freezing points). This definition also precisely relates the Celsius scale to the Kelvin scale, which defines the SI base unit of thermodynamic temperature.

**Temperature Conversion – Fahrenheit – Celsius**

To convert from a** Fahrenheit temperature** to a **Celsius temperature** we have to subtract **32 degrees** from the Fahrenheit reading to get to the zero point on the Celsius scale and then adjust for the different size degrees. The ratio of the size of the degrees is **5/9** so that the relationship between the scales is represented by the following equations:

**°F = 32.0 + (9/5)°C**

**°C = (°F – 32.0)(5/9)**