If we ponder over which physical quantity is measured the most, then it would be none other than temperature. Even slight changes in the value of temperature impacts the ecological processes of the world in a significant way. Before the invention of temperature measuring devices, hot and cold could be perceived only by sensory organs.
The devices to measure temperature have been slowly developed since the times of Galileo, who first invented the documented thermometer in about 1592. The modern-day thermometers are based on Fahrenheit and Celsius scale units.
Read more at Buzzle: http://www.buzzle.com/articles/farenheit-to-celsius-conversion.html