Mercury-in-glass thermometer

The mercury-in-glass or mercury thermometer is a thermometer that uses the thermal expansion and contraction of liquid mercury to indicate the temperature.

Construction A basic mercury thermometer is a precisely crafted piece of tube-shaped glass enveloping a mercury-filled reservoir connected to an extremely thin channel, called the capillary bore, that provides a chamber the mercury from the reservoir can expand into. The shorter, bulbous end of the tube containing the reservoir is called the bulb and the longer, narrower end with the bore is called the stem. Etched into the stem or on a carefully aligned plate next to it is a graduated temperature scale. Lower temperatures are near the bulb and higher temperatures near the top of the stem. The space above the mercury may be filled with nitrogen gas or it may be at less than atmospheric pressure, a partial vacuum.

Theory of operation As the temperature of the surrounding environment changes, the mercury thermally expands and contracts, causing it to move out of, or into, the reservoir and, at the same time, rise or fall through the bore. Although changes to the mercury's volume are slight‍— about 0.018% for each degree Celsius— the small volume of the bore compared to the bulb's volume visually amplifies the change. This design feature results in clearly visible movement of the mercury up or down the scale, enabling precise temperature readings.

Calibration In order to calibrate the thermometer, the bulb is made to reach thermal equilibrium with a temperature standard such as an ice/water mixture, and then with another standard such as water/vapour, and the tube is divided into regular intervals between the fixed points. In principle, thermometers made of different material (e.g., coloured alcohol thermometers) might be expected to give different intermediate readings due to different expansion properties; in practice the substances used are chosen to have reasonably linear expansion characteristics as a function of thermodynamic temperature, and so give similar results.

History

The earliest documented use of mercury in some kind of thermometer is from the 1620s, when Athanasius Kircher, a Jesuit scholar, used quicksilver for his air thermometer, the precursor to in-glass thermometers. Later, in the 1650s, failed experiments were run to determine if mercury might be a superior substitute for spirits in an enclosed glass thermometer. In 1659, the astronomer Ismael Boulliau abandoned using mercury when he determined that it was not as responsive to changes in temperature as spirits. In 1713, Daniel Gabriel Fahrenheit began experimenting with mercury thermometers. By 1717, he was making them commercially. The superiority of his mercury thermometers over alcohol-based thermometers made them very popular, leading to the widespread adoption of his Fahrenheit scale, the measurement system he developed and used for his thermometers. Anders Celsius, a Swedish scientist, devised the Celsius scale, which was described in his publication The origin of the Celsius temperature scale in 1742. To define his scale Celsius used two fixed temperature points: the temperature of melting ice and the temperature of boiling water, both under atmospheric pressure of the standard atmosphere. This was not a new idea, since Isaac Newton was already working on something similar. The distinction of Celsius was to use the condition of melting and not that of freezing. 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 to water vapour (when this is done to high precision, a variation will be seen with atmospheric pressure; Celsius noted this). 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", that is 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 Christin 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 (100 steps). Finally, Celsius proposed a method of calibrating a thermometer:

Place the cylinder of the thermometer in melting ice made of pure water and mark the point where the fluid in the thermometer stabilises. This point is the freeze/thaw point of water. In the same manner mark the point where the fluid stabilises when the thermometer is placed in boiling water vapour. Divide the length between the two marks into 100 equal parts. These points are adequate for approximate calibration, but both the freezing and boiling points of water vary with atmospheric pressure. Later thermometers that used a liquid other than mercury also gave slightly different temperature readings. In practice, these variations were very slight and remained close to the thermodynamic temperature, once the latter was discovered. These issues were explored experimentally with the gas thermometer. Until the discovery of true thermodynamic temperature, the mercury thermometer usually defined the temperature. Modern thermometers are often calibrated using the triple point of water instead of the freezing point; the triple point occurs at 273.16 kelvins (K), 0.01 °C.

Maximum thermometer

One 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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