A liquid-in-glass thermometer is the most commonly used. It works on the principle that liquid changes its volume in accordance to the temperature. Both mercury and alcohol can be used in a liquid-in-glass thermometer. Alcohol is usually dyed red so that it is visible.
The infrared thermometers measure the infrared radiation given off by an object to determine its temperature. The thermometer contains a type of crystal that produces an electrical signal whose strength is proportional to the infrared radiation to which it is exposed. The most familiar use of such thermometers is in ear thermometers that measure infrared radiation from the tympanic membrane (ear drum) to determine body temperature. Other types of infrared thermometers include bolometers and pyrometers.
There are bimetal thermometers which are made of two metal types bonded together to make a sensor. The sensor reacts to the heat through the two types of metal. The sensor controls a metal strip coil in the thermometer. This coil will make a dial move according to the amount of heat and allows us read the temperature.
Liquid crystal thermometers are the ones that are used on the forehead. Different mixtures of liquid crystals are used, each becoming lighter in color at a specific temperature. The colors correspond to a number on a temperature scale which is printed on the thermometer.
It is now common to measure temperature with electronics. The most common sensor is a thermo resistor (or thermistor). This device changes its resistance with changes in temperature. A computer or other circuit measures the resistance and converts it to a temperature, either to display it or to make decisions about turning something on or off.