A thermometer with a range reaching 200 C (400 F) or more, built to sit in a pot of hot oil and show whether it is at frying temperature. Most clip to the rim of the pot with the probe in the oil but off the bottom, where a reading would be falsely high. Dial, glass and digital versions all exist, and many are marked with a sugar scale too, so one instrument serves for deep-frying and candy-making; see candy thermometer.
Oil at 175 to 190 C (350 to 375 F) seals the surface of food quickly so it crisps rather than soaks. Too cool and the coating goes greasy; too hot and the outside burns before the centre cooks. The thermometer also shows the temperature recovering between batches, which is when most home fryers go wrong.