Minimum Thermometer

The left-hand (minimum arm) bulb is full of alcohol. This bulb measures the temperature by the expansion and contraction of the liquid. The right-hand (maximum arm) bulb contains alcohol and a bubble of low-pressure gas or alcohol vapour. This bulb accommodates the expansion in the other bulb and allows the train of alcohol and mercury to move in the U-shaped tube as the temperature changes. The thermometer shows a reading at the top of the mercury section on both the maximum and minimum scales; this shows the current temperature and should be the same on both scales. If the two readings are not the same, then the instrument scales are not correctly positioned or the instrument is damaged, The maximum and minimum readings are recorded by two small steel markers which are sprung into the capillary tube so that they can slide, but only if a force is applied to them, either by being pushed by the mercury or under the influence of an external magnet.