Thermometers are commonly used to determine if someone is suffering from a fever. Some women use thermometers to determine ovulation by recording changes in body temperature. Formerly, glass thermometers were commonplace in both homes and doctor's offices. Since the dangers of mercury have become known, a variety of digital thermometers are now the popular choice. Digital thermometers are used orally, rectally and under the arm; while tympanic thermometers are used in the ear and Forehead thermometers are used over the forehead. It's important to know how to use a thermometer to insure you get accurate readings so always read the instructions.