Mercury Thermometer Principle

This type of thermometer was developed by a German based physicist named Daniel Gabriel Fahrenheit. It consists of mercury as a liquid filled in a glass tube. On the body of the glass tube, calibrated marks are provided which facilitates the reading of temperature. A bulb is formed at one end of the thermometer which contains the largest part of mercury. The expansion and contraction of this mercury size is then further increased in the extremely thin bore of the glass tube. It aids in increasing the sensitivity of the thermometer. In general, the area over the mercury is filled with inert gases like nitrogen. However, this area can be left evacuated too.

Various types of mercury-in-glass thermometers are available. “A maximum thermometer is a unique kind of mercury thermometer which functions by having a constriction in the neck close to the bulb. The mercury is forced up through the constriction by the force of expansion as the temperature increases. When there is a decrease in the temperature, the column of mercury breaks at the constriction and cannot return to the bulb and will remain stationary in the tube.”By means of a maximum thermometer, one can measure the maximum temperature over a predetermined time span. Resetting of the maximum thermometer is a very simple process which just requires the sharp swinging of the thermometer.

The freezing point of mercury is - 38.83°C at which it gets solidified. However, it doesn’t result in expansion upon solidification and hence there will be no danger of glass breakage of the thermometer tube. Whenever there is rise in temperature, the nitrogen gas filled above the mercury in glass tube usually comes down the column and gets stuck there. This whole action may affect the functioning of thermometer. To prevent this difficulty, one must manages to get mercury thermometers inside when the temperature drops to - 37°C. In areas, where the upper temperature limit is - 38.83°C, one may employ a mercury-thallium alloy thermometer having a freezing i.e. solidification point of - 61.1°C.

The mercury in steel thermometer system, allows for rugged construction and is used extensively in industrial applications. The thermometer consists of a steel bulb, a steel capillary tube and Bourdon tube.

An advantage of the mercury in steel thermometer is that measurements can be taken a distance away from the application, as the steel tube can be made fairly long and flexible . The whole system is completely filled with mercury under pressure and sealed off. When the temperature around the bulb increases, the mercury inside the bulb will expand. The effect of the mercury trying to increase its volume within a confined space, will be an increase in pressure, transmitted via the capillary tube, to the coiled Bourdon tube.

The increase in mercury volume and pressure inside the coiled Bourdon tube, will result in the Bourdon tube starting to uncoil, proportional to the temperature. A pointer, linked to the free end of the Bourdon tube, will subsequently move over the scale to indicate the temperature.