Why Must a Fever Thermometer Be Shaken?
A fever thermometer is made so that the mercury in the tube will remain at the highest point it registers, so it can be read accurately even after it starts to cool. When your temperature is taken, your body’s warmth makes the mercury expand and rise in the hollow tube.
There is a pinched-in place in the tube where the thread of mercury breaks instead of shrinking back into the bulb as the temperature is read.
If it were to be used again on a second patient with a lower temperature, the thermometer would still show the temperature of the first patient. The mercury stays in position until it is shaken down with a snap of the wrist. After being sterilized, the thermometer is ready for use again.