It measures the maximum temperature in a day. Mercury is used as thermometric liquid.
- There is a steel index inserted inside the capillary tube. Spring connected to the index prevents it from moving back to the bulb.
- With increasing temperature, mercury expands and pushes the steel index along the capillary tube. The index will be left at the furthest position to which mercury has expanded. Lowest point of the index (convex meniscus of mercury) reads the highest temperature within the day.
- The index can be reset using a magnet.