The oldest thermometers used are mercury in glass. Newer thermometers include non-mercury liquids in glass and digital and electronic devices that use sensors to measure temperature. Thermometers that check body temperature in the ear, across the forehead, or have a digital display do not contain mercury.

How does a digital thermometer work without mercury?

Digital thermometers contain a device called a thermoresistor. The thermoresistor has an electrical current running through it, and the resistance, or how freely the current can flow, changes with temperature. A small computer or other circuit measures the resistance, and converts it to a temperature.

How do you know if a thermometer has mercury in it?

How can I tell if my thermometer has mercury? Mercury is a silver-white to gray substance. If your thermometer is filled with a red liquid, your thermometer contains red dyed alcohol or mineral spirits and not mercury. These are safer alternatives to mercury fever thermometers.