istory. Initially Fahrenheit fixed the zero point of his scale using a mixture of ice and salt, with the upper point being the average temperature of the human body. Fahrenheit later adjusted his scale to the more familiar freezing point of water at 32 degrees.

In 1742, the Swede Anders Celsius created another scale. Celsius set the freezing and boiling points of water as the definition of his scale. Celsius chose 0 degrees as the boiling point of water and 100 degrees as the freezing point of

water. Later, these points were inverted and the Centigrade scale was born. In 1948 the Ninth General Conference on Weights and Measures changed the name to the ‘Celsius scale’ in his honor.

The Celsius and Fahrenheit scales are both relative; their numerical values of reference are arbitrary. Because of the desire for more-grounded reference points, the additional temperature scales of Kelvin and Rankine were developed. These scales assign 0 to the thermodynamic absolute zero, the theoretical point of zero molecular kinetic energy.

With creation of universally accepted temperature scales, scientists were now free to research the effects of temperature on various substances. In 1821, Thomas Seebeck discovered that when two different metal wires are joined at two points and one of the points is heated, an electric current will circulate. It was this discovery that led to the modern development of the thermocouple, one of the most-used temperature sensors for industrial applications.

By the 20^th century, the necessity of universally accepted temperature properties of various materials was clear. This would foster consistency and repeatability within the scientific community and promote scientific advancement. The most recent ratification of temperature standards was on January 1, 1990, when these scales and values were fully standardized under the assignment of the International Temperature Scale—the ITS-90. In addition, there are localized standards used in the measurem