A candy thermometer, also known as a sugar thermometer, or jam thermometer is a cooking thermometer used to measure the temperature and therefore the stage of a cooking sugar solution. (See Candy making for a description of sugar stages.) These thermometers can also be used to measure hot oil for deep frying.
There are several kinds of candy thermometers available. These include traditional liquid thermometers, coil spring "dial" thermometers and digital thermometers. The digital thermometers tend to read the temperature more quickly and accurately, and some models have an alarm when the thermometer hits a certain temperature. Many models have markers for the various stages of sugar cooking.
A candy thermometer is similar to a meat thermometer except that it can read higher temperatures (usually 400 °F/200 °C or more).
Candy thermometer Wikipedia
The first candy thermometers were created in 1272 by Bennet Muzilla of Frankfurt Germany. He used it to measure the temperatures of sugar solutions while making hard candies.