The maximum and minimum thermometer is how to work?

Maximum and minimum thermometer is also called the thermometer is used to record in a day or a period of time the maximum temperature and minimum temperature in the air. Measuring range is - 40+50C。 Mainly used for breeding of vegetable greenhouses or factory. It works, within about two stick of glass wool stoma were blue indicating needle, when the temperature changes, the blue indicator needle by mercury push up; When the temperature drops, blue indicating needle is not affected by the temperature drop and change; Thus indicates the maximum and minimum temperature. As shown in the left of the indicating needle to the lowest temperature, the right of the instruction pointer is the highest temperature. Maximum and minimum thermometer reading method is: the left number: the above red is minus temperature to the right of the said values: the following red values are below zero temperature.