This story originally appeared in the Heat issue of Popular Science. Current subscribers can access the whole digital edition here, or click here to subscribe.

Mercury encapsulated in glass tubes has been reliably reading temperatures since the early 18th century. Modern instruments use various methods to make checkups simpler, more accurate, and a lot less likely to poison you when they break. Here’s how they work.  

Liquid Metal

The Equate Glass Oral Thermometer swaps the traditional mercury for a nontoxic blend of metals. It needs about three minutes to reveal the temp, but the analog method means you’ll never worry about a dead battery. Equate

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The Kinsa Quickcare uses electrical resistance to measure temperature. A weak current runs through the gadget’s metal probe during its 8-second test; the slower those electrons move, the higher your fever. Kinsa

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The iHealth Infrared Forehead Thermometer measures radiation coming off the skin 100 times per second. Two auxiliary sensors track the device’s distance from the patient and the ambient temperature to prevent faulty data. iHealth

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