Why doesn’t a liquid flow under gravity when it’s turned upside down?
The principle of the mercury thermometer is to rely on the property of mercury to expand with heat and contract with cold. Within a certain range, the volume change of mercury is proportional to the temperature, so the temperature is obtained.
If there is air above the thermometer, then the mercury will expand with the air if it is heated, which may affect the accuracy of the reading. So the thin tube in the thermometer is a vacuum.
For a thermometer, there is a small slit between the mercury bubble and the display area, so that when the temperature rises, the mercury can expand and pass through, but if it shrinks, the mercury will break at the slit, so that the display The number is fixed so that we can read it easily.
For the cold and heat meter, the upper and lower widths are the same, so the indication of the thermometer will change continuously with the outside temperature.
But no matter what kind of thermometer, mercury will not flow because it is hung upside down. This is because mercury is non-wetting to glass, which means that mercury tends to shrink into a ball on the glass. When it is hung upside down, the force provided by the surface tension and gravity will balance each other.
