A thermometer is an instrument that measures temperature or a temperature gradient. It can measure the temperature of a solid, such as food, a liquid, such as water, or a gas, such as air. A thermometer has two important elements: a temperature sensor in which some change occurs with a change in temperature, and some means of converting this change into a numerical value. The temperature sensor can be the bulb of a mercury-in-glass thermometer or the pyrometric sensor in an infrared thermometer. The means of converting the change can be the visible scale that is marked on a mercury-in-glass thermometer or the digital readout on an infrared model. Thermometers are widely used in technology and industry to monitor processes, in meteorology, in medicine, and in scientific research.

The invention of the thermometer is generally credited to the Italian mathematician-physicist Galileo Galilei. In his instrument, built about 1592, the changing temperature of an inverted glass vessel produced an expansion or contraction of the air within it, which in turn changed the level of the liquid with which the vessel’s long, open-mouthed neck was partially filled. In the early 21st century, mercury thermometers were supplanted by electronic digital thermometers, which were more accurate and did not contain toxic mercury. Digital thermometers use a thermistor, a resistor with a resistance that varies with temperature. To measure body temperature, infrared thermometers that focus infrared light onto a detector that measures the amount of light received and convert the electrical signal produced by the detector into a temperature were also used. Electrical-resistance thermometers characteristically use platinum and, like thermistors, operate on the principle that electrical resistance varies with changes in temperature. However, they can measure a much greater temperature range than thermistors. Thermocouples are among the most widely used industrial thermometers. They are composed of two wires made of different materials joined together at one end and connected to a voltage-measuring device at the other. Other thermometers operate by sensing sound waves or magnetic conditions associated with temperature changes.

The most common thermometer is the bulb thermometer, which comprises a large bulb filled with a liquid and a narrow glass tube through which the liquid rises. All liquids expand when heated and contract when cooled, which explains why the liquid within a thermometer rises as the temperature increases and falls when it decreases. Technology has come a long way since Galileo’s day, but his thermometer to this day has a futuristic look to it. Another futuristic thermometer that is available today is the CorTemp thermometer. Developed by Dr. Leonard Keilson o...