1. Maximum thermometer

It is used to measure the maximum temperature for a day and it contains mercury, which has a high boiling point.

When the temperature increases, the mercury is pushed past the constriction due to the pressure exerted by the expansion of the mercury. When the temperature decreases, the mercury is left in the tube as there is a break at the constriction.

A maximum thermometer

If there is further increase in temperature, more mercury will be pushed past the constriction, thus, the maximum temperature can read from the highest end of the mercury column.

The thermometer is reset by shaking it to allow the mercury to flow back to form a continuous thread of mercury from the bulb to the meniscus.

2. Minimum thermometer

It is used to measure the minimum temperature of a day and it contains alcohol, which has a low freezing point. There is a metal index in the tube held on by surface tension.

When the temperature decreases, the alcohol contracts, and its meniscus pushes the index down. When the temperature increases, the alcohol expands without affecting the position of the metal index.

A minimum thermometer

The lowest temperature of the day is given by the end of the metal index closet to the meniscus.

A minimum thermometer is reset by tilting the bulb upwards to move the index along the tube to touch the meniscus.

3. Six’s thermometer

This U-shaped instrument measures the maximum and minimum temperatures of a day. Mercury is contained in the U-bend while the upper arms are filled with alcohol. The right conical bulb contains vacuum to allow space for expansion.

When the temperature increases, both liquids expand but alcohol vapourises as it expands. The expanded alcohol in the left arm forces the mercury towards the right arm.

The mercury, in turn, pushes the index up in the right arm while the alcohol in the right arm expands and occupies the vacuum space. So, the maximum temperature is given by the bottom of the index in the right arm.

A Six's thermometer

When the temperature decreases, both liquids contract. The vapour in the right conical bulb condenses to liquid and forces the mercury towards the left arm.

The mercury, in turn pushes the index in the left arm up. So, the minimum temperature is obtained by taking a reading off the bottom end of the index in the left arm.

After readings have been taken, the thermometer is reset by using a magnet to reset the two metal indices or by shaking the thermometer to reset the metal indices.