In the earliest days of the medical profession, there was no device available to measure a patient's temperature. Patients were either too hot, normal, or too cold, depending upon the doctor's personal observation methods. It was not until nearly the seventeenth century that scientists devised an instrument that could detect the changes in air temperature, and many years later that a medical thermometer was constructed.

The first thermometers were created to measure the changes in atmospheric temperature. The most famous of these was invented by Galileo in 1592; called an air-thermoscope (or air thermometer). It consisted of a long glass tube with a wide bulb at one end. The tube was heated, causing the air within to expand and some to be expelled. While still warm, the open end of the tube was placed into a flask of water; as the tube cooled the warm air would contract, drawing...