A thermometer is an instrument that measures the temperature of a system. The word Analog thermometer refers to mercury thermometers commonly used in homes and clinics to measure body temperatures. The mercury used in thermometer is silvery white in color and in liquid state. Mercury is one of the 92 naturally occurring elements that become liquid at room temperature and remains in liquid state throughout a wide range of temperature. Basal thermometers are ultra sensitive thermometers and they can track body’s minutest temperature shift i.e. they can accurately measure the temperature shift by 0.1 degrees Fahrenheit. On the other hand oral thermometers can measure a minimum temperature shift of 0.2 degrees Fahrenheit. Oral thermometers are used to measure the body temperature at the time of fever while basal thermometers are used by females to chart their basal body temperature to predict their fertile period or ovulation.