A candy thermometer (aka deep fry thermometer) can be used to test the temperature of mixtures for candy, jam or jellies or of oil being heated for deep-frying. Ranges handled by different brands of candy thermometers differ, but they seem to average 37 C to 205 C (100 F to 400 F).
All are usually quite long; some have insulated handles. Some have clips that let you clip them onto the side of the pot in such a way that the bottom of the thermometer is in the hot mixture or oil without the thermometer touching the side of the hot pot and distorting the reading.
Candy thermometers are usually glass tubes, though bi-metallic coil dial and digital models are also available.
If you are using a mercury candy thermometer, keep it upright while cooling — if you lay it down, the heated mercury may separate into small beads and render the thermometer useless in the future. Always let cool completely before cleaning or it may shatter.
High altitude usage: For every 300 metres (1,000 feet) that you are above sea level, subtract 1 degree C (2 degrees F) from the temperatures given in your candy recipe.