Maximum and minimum thermometer

The maximum and minimum thermometer or Six thermometer is a thermometer used in meteorology and horticulture to record the highest and lowest temperatures of the day.

It was invented by the Englishman James Six in 1782, and named after him. The same basic design is still in use today.

Description

It is a mercury thermometer that has a narrowing of the capillary near the bulb or reservoir. When the temperature rises, the expansion of all the mercury in the bulb overcomes the resistance opposed by the constriction, while when the temperature drops and the mass of mercury contracts, the column breaks due to the constriction and its free end remains marking the temperature. maximum. The scale has a division of 0.5 °C and its range is -31.5 to 51.5 °C.

Installation and measurement

It is placed inside the meteorological shelter on a suitable support, with its bulb inclined downwards forming an angle of 2º with the horizontal. After reading, to reset it, you must firmly hold the opposite side of the tank and shake it with your extended arm (a maneuver similar to that performed to lower the temperature of a clinical thermometer).

Installation and measurement

It is an alcohol instrument in a glass tube. The bulb is generally fork-shaped to increase the exposure surface and thus the sensitivity.

In the non-capillary tube, an enameled index is placed that slides inside the tube easily and its operation is related to the property of alcohol of being a liquid that wets the walls of the container, forming a concave meniscus at the end. of the column. This meniscus allows the introduction of the aforementioned index within it while exerting a certain surface tension. Therefore, as the temperature drops, the index is dragged by the action of the surface tension exerted on the meniscus. On the other hand, when the temperature rises, the index remains immobile because said tension no longer acts on it.

Putting into station

With a minimum thermometer you can measure minimum temperature and current temperature. The setting of this thermometer is achieved by equating the enamelled index mark with the current temperature and this is done raising the alcohol tank so that the index falls by gravity to the current temperature mark, then returning the thermometer to its normal (horizontal) position.

Six-Bellani Thermometer

• The instrument is an example of a maximum thermometer and a minimum. It consists of a folded U-shaped capillary, whose bottom is full of mercury and whose branches contain alcohol that completely fills the deposit and part of the blistering, that is, two closed containers arranged at the ends of the branches of the same capillary. In correspondence with the mercury-alcoholic separation surfaces, there are two iron cylinders that can slip into the blow, built so that mercury, due to surface tension, can only push up. The instrument is fixed to a rectangular wooden board, in which a double scale appears in degrees Electronic reaumur on the sides of the capillary.

• When the temperature increases the alcohol of the deposit expands by pushing the mercury into the blister, leaving the stationary cylinder of the capillary tube connected to the tank and pushing up the other cylinder. When the temperature decreases, the opposite effect occurs: the alcohol is contracted, drags the mercury leaving the metal retention rate of the termination with the ampoule and, in the case where the minimum temperature is lower than the initial, pushes the branch cylinder connected to the tank up. With this tool it is able to detect the temperatures of the maximum and the minimum reached during the measurement period, are indicated, respectively, by the position of the metal indices in the branches corresponding to the ampoule and tank. Before a new measure is taken, with a magnet, contact with mercury indices.

• The instrument was designed by the English scientist J. Six (1731-1793) at the end of the centuryXVIII and was perfected by Angelo Bellani (1736-1852) who gave him the name "thermometer" only to emphasize that this type of thermometer is to offer the measurements without the constant presence of an observer.

Maximum and minimum thermometers without mercury

As Six's thermometer contains mercury, prohibited for use in the European Union.

In 2006, S. Brannan & Sons Ltd, a UK company, patented a mercury-free version of the Six maximum–minimum thermometer: instead of mercury, two immiscible liquids are used.