To disinfect a digital thermometer, you can use an alcohol-based wipe with at least 70% concentration, or rubbing alcohol. You can also clean glass thermometers or rectal thermometers with soap ...

To clean an oral or rectal digital thermometer, do the following: Using a cotton ball or pad soaked in rubbing alcohol, or an alcohol or bleach wipe, rub the entire length of the thermometer ...

The thermometer has a small bulb filled with either mercury (in older thermometers) or alcohol (in modern ones). When the temperature rises, the liquid expands and moves up the narrow tube.