The thermometer is the instrument to measure temperature. Use numeric values determined on fixed points, such as boiling water and melting ice. Among the different types of thermometers are those of mercury, resistance, thermistors, thermocouples and optical pyrometer. The first are the most common and consist of a glass tube that has inside a capillary, also of glass, connected to a mercury-filled blister. To contact with the heat, the mercury in the bulb expands and rises in the capillary. The latter are of great precision, and have a coil of platinum wire connected to an instrument for measuring electrical resistance. Thermistors and thermocouples used sensing elements that respond rapidly to changes in temperature, while the optical pyrometer used to measure temperature of solids in excess of 700 º C.
Galileo is credited with the invention of the thermometer. But mercury thermometers and alcohol were created in the eighteenth century by the German physicist Gabriel Fahrenheit.
The best known are the Celsius and Fahrenheit. On a scale created by the Swedish astronomer Anders Celsius, known as the centigrade scale, the freezing point is 0 ° C and the boiling point of 100 º C. While on the Fahrenheit scale the boiling point is 212 º F and the freezing, 32 º F.