A mercury-in-glass thermometer, also known as a mercury thermometer, was invented by German physicist Daniel Gabriel Fahrenheit in 1724 and is a thermometer consisting of mercury in a glass tube. Calibrated marks on the tube allow the temperature to be read by the length of the mercury within the tube, which varies according to the heat given to it. To increase the sensitivity, there is usually a bulb of mercury at the end of the thermometer which contains most of the mercury; expansion and contraction of this volume of mercury is then amplified in the much narrower bore of the tube. The space above the mercury may be filled with nitrogen or it may be less than atmospheric pressure, which is normally known as a vacuum.
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.
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.
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 special kind of mercury-in-glass thermometer, called a maximum thermometer, works by having a constriction in the neck close to the bulb. As the temperature rises the mercury is pushed up through the constriction by the force of expansion. When the temperature falls the column of mercury breaks at the constriction and cannot return to the bulb thus remaining stationary in the tube. The observer can then read the maximum temperature over the set period of time. To reset the thermometer it must be swung sharply. This design is used in the traditional type of medical thermometer.
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