1. A clinical thermometer has a narrow constriction in the tube just above the bulb. The expanding mercury pushes its way past the constriction when the thermometer is placed in the mouth.
When the thermometer is taken out of the mouth the mecury does not immediately fall back to the bulb. The mecury is trapped at the temperature of the body it was placed in. This allows for the temperature to be read after it has been taken out of the mouth. The only way to return all the mecury to the bulb is to flick it back and forth.
A normal thermometer (a lab thermometer) does not have this constriction
2. A clinical thermometer has a much shorter range than a lab thermometer. A clinical has a range of 35C to 42C (about the range for body temp.) but a lab has a range to about 100C or 120C
3. The cross sectional shape of the tube is a pear shape. It acts as a magnifying glass so that it is easier to see the fine thread of mercury in the fine tube. A lab thermometer doesnt have this pear shape (not that I know of).
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