This type of minimum and maximum thermometer uses a U-shaped tube. Both sides should measure the same temperature. However, the scale is inverted. There are also needles inside the tube. As the temperature drops, the mercury pushes the left hand side needle upwards. This then measues the minimum temperature because a magnet at the back holds the needle in position. As the temperature increases to a maximum, the right hand side needle is pushed upwards to record a maximum when it stops. To reset it, the red centre button is pushed and the needles drop.