用冰點同氣點做水銀溫度計既固定點 有咩優點 急問 thx

I think 冰點同氣點 refer to the freezing point and boiling point (沸點）of water, which are used as the lower and upper calibration points (or fixed points) for thermometer calibrations.

The advantage of using the freezing point and boiling point of water is that these two points are easily reproducible and are not much affected by environmental conditions. That is to say, the temperatures of these two calibration points can be set up easily by simple materials (ice and water) and methods. This makes thermometer calibrations more convenient.

這可以確保溫度計的水銀在使用範圍內是處於液態, 這樣溫度的變化和水銀的體積就會呈線性, 不用擔心水銀會變成固體或氣化, 因而不能反映真正的溫度.

I don't know if you are asking this:

Not only for mercury thermometer, basically ordinary alcohol ones will use melting point and boiling point as the fixed points too. This is to enable the manufacturer to divide the length marked between two points to 100 divisions such that 1 division represents a degree in temperature.