Thermometers measure temperature. The first thermometer was an air thermometer used by Avicenna in the early 11th century. This was followed by the thermoscope several centuries later. Different versions of the thermoscope were invented by several inventors around the same time. The first to put a numerical scale on the thermoscopes was the Italian inventor Santorio Santorio for use in medicine in the 16th century. In 1593, Galileo Galilei invented a rudimentary water thermometer (using the contraction of air to draw water up a tube). He also discovered that liquids of lesser density than water could be suspended within it and that they would float at different heights depending on the temperature; hence Galileo's Thermoscope--Galileo's use of alcohol enclosed in glass spheres floating in a column of water in order to measure their differences in temperature. In 1714, Daniel Gabriel Fahrenheit invented the first mercury thermometer. In 1866 Sir Thomas Clifford Allbutt invented a clinical thermometer that produced a body temperature reading in five minutes as opposed to twenty.
This history of the thermometer, from its invention (an achievement attributed to several scientists, including Avicenna and Galileo) through various changes and applications over the centuries, includes controversy about its invention, the story of different scales, from Fahrenheit and Anders Celsius to the now-forgotten Newton, Réaumur, Delisle, and Christin scales, and the history of the gradual scientific then popular understanding of the concept of temperature. Not until 1800 did people interested in thermometers begin to see clearly what they were measuring, and the impetus for improving thermometry came largely from study of the weather -- the liquid-in-glass thermometer became the meteorologist's instrument before that of the chemist or physicist.
The thermometer was used by the originators of the Fahrenheit and Celsius temperature scales.
Anders Celsius devised the Celsius scale, which was described in his publication the origin of the Celsius temperature scale in 1742.
Celsius used two fixed points in his scale: the temperature of melting ice and the temperature of boiling water. This wasn't a new idea, since Isaac Newton was already working on something similar. The distinction of Celsius was to use the melting temperature and not the freezing temperature. The experiments for reaching a good calibration of his thermometer lasted for 2 winters. By performing the same experiment over and over again, he discovered that ice always melted at the same calibration mark on the thermometer. He found a similar fixed point in the calibration of boiling water vapour (when this is done to high precision, a variation will be seen with atmospheric pressure). At the moment that he removed the thermometer from the vapour, the mercury level climbed slightly. This was related to the rapid cooling (and contraction) of the glass.
The air pressure influences the boiling point of water. Celsius claimed that the level of the mercury in boiling water is proportional to the height of the barometer.
When Celsius decided to use his own temperature scale, he originally defined his scale "upside-down", i.e. he chose to set the boiling point of pure water at 0 °C (212 °F) and the freezing point at 100 °C (32 °F). One year later Frenchman Jean Pierre Cristin proposed to invert the scale with the freezing point at 0 °C (32 °F) and the boiling point at 100 °C (212 °F). He named it Centigrade.
Finally, Celsius proposed a method of calibrating a thermometer:
These points are adequate for approximate calibration but both vary with atmospheric pressure. Nowadays, the triple point of water is used instead (the triple point occurs at 273.16 kelvins (K), 0.01 °C).
A candy thermometer, also known as a sugar thermometer, is a thermometer used to measure the temperature and therefore stage of a cooking sugar solution. It is similar to a meat thermometer except that it can read higher temperatures (usually 400 degrees Fahrenheit/200 degrees Celsius or more). Like a meat thermometer, there are several different kinds of candy thermometers available. These include traditional liquid thermometers, coil spring "dial" thermometers and digital thermometers. The digital thermometers tend to read the temperature more quickly and accurately, and some models have an alarm when the thermometer hits a certain temperature. Many models have markers for the various stages of sugar cooking. Please see Candy for a description of the various stages of sugar cooking. Candy thermometers are also used to measure hot oil for deep frying.
A meat thermometer is a thermometer used to measure the internal temperature of meat and other cooked foods. This information is used to achieve the desired level of cooking, especially in steaks. It also helps to ensure that the meat has reached a temperature sufficient to kill pathogens that may otherwise cause foodborne illness.
A typical meat thermometer consists of a rod, usually metal, which is placed into the meat to be measured, and a display, usually a dial or LCD display. Many have markings of appropriate temperatures for certain meats. Some newer thermometers include light-emitting diodes or buzzers to alert the user when desired temperature is reached.
Another variety commonly used on turkey is the pop-up timer, which uses a spring held in by a soft material that "pops up" when the meat reaches a set temperature.
The meat thermometer is similar to a candy thermometer except that it reads lower temperatures.
The Galileo thermometer shows the temperature by the movement of the spheres rising and falling in the column. As the ambient temperature rises, spheres descend one by one and those above ambient temperature are left floating at the top of the fluid. The actual temperature, indicated by a numbered medallion hanging from the bottom of each sphere, is that of the lowest sphere still floating.