Asked in ScienceMeteorology and WeatherFood Spoilage
How does a six's thermometer work?
June 11, 2012 6:32PM
It consists of a U-shaped glass tube with two separate temperature scales set along each arm of the U. One of these is for recording themaximum temperature encountered and the other for the minimum temperature. The arms of the U-shaped tube terminate in sealed glass bulbs. The bulb at the top of the minimum reading scale arm is full of alcohol, the other contains a vacuum (or low pressure alcohol vapour). 
In the bend of the U is a section of mercury. This is pushed around the tube by the thermal expansion and contraction of the alcohol in the first bulb as it responds to the external temperature. The near vacuum in the other bulb allows free movement of the alcohol and mercury. It is the alcohol which measures the temperature; the mercury indicates the temperature reading on both scales.
The thermometer shows a reading at the top of the mercury section on both the maximum and minimum scales; this shows the current temperature
The maximum and minimum readings are recorded by two small steel markers which are sprung into the capillary tube so that they cannot easily slide unless a small force is applied to them.
Before a maximum or minimum reading can be taken, both markers must be returned to the top of the mercury, usually by hand using a small magnet to slide them along the tube. Any change in temperature after that time will push one of the markers along with it.
If the temperature is rising, then the maximum scale marker will be pushed. If it falls after the reset, then the moving mercury will push the minimum scale marker. Often both things happen one after another as the temperature changes, say through a twenty-four-hour period (see diurnal temperature variation). The markers thus record the furthest point reached by the mercury in each arm of the tube. They record the extremes of temperature experienced by the device since it was last reset. The thermometer is usually reset every day, but if left for longer the readings would show the highest and lowest temperatures encountered since the instrument was last reset.
To take a reading, the positions of the ends of the markers nearest to the mercury are examined. Their positions on the maximum and minimum scales show the highest and lowest temperatures encountered over the period of measurement