Do you know how long this thin, fragile rod has been around? And why there are so many different temperature scales? Whether used to determine a fever or to help decide whether to wear a coat or a tank top, nearly everyone is familiar with thermometers . . . but are you familiar with its origins? Early History The word thermometer comes from the Greek words thermos ("warm") and meter ("to measure"). The Greeks, brilliant and curious as they were, invented the earliest temperature-measuring gizmos around the first century B.C. The first thermometer was a simple device consisting of a tube filled with air and water. As the air grew heated, it would expand and cause the water to rise. There was no system of measurement applied to these crude devices, and they were used primarily to observe changes in temperature. Thermoscopes In 1592, Galileo Galilei improved upon the Greek creation and invented the 'Thermoscope' - a bulb with a long thin neck, upturned over a container of water. When the bottle was heated, the air expanded to push the water level in the neck down. When the bottle was cool, the water rose. Galileo also later invented the 'Galileo Thermometer' - a large glass cylinder containing several bulbs of varying density. As the temperature of the water inside the cylinder increased, the density decreased, changing the buoyancy of the inner bulbs. The lowest bulb would then indicate the temperature. In the early 17th century, an Italian physician, Santorio Santorio was the first to add a scale to a thermoscope, in order to quantify changes in temperature. Designed to be placed in a patient's mouth, Santorio's invention is considered the first clinical thermometer. The Fahrenheit Scale Though many people created versions of the thermometer, each with unique scales, only a few were able to survive the test of time. German physicist, Daniel Gabriel Fahrenheit, was the first to use mercury in a thermometer. Daniel Fahrenheit became well-known during the early 18th century for the accuracy and quality of his scale, which used 96 F, "blood heat" (In 1835, it was discovered that blood heat was actually 98.6°F) and 32 F, the melting point of ice, as its two fixed calibration points. The Celsius Scale Scientists and inventors, aware that the Celsius thermometer was influenced by atmospheric pressure, looked for ways to trounce its effects. In 1742, Andres Celsius, a Swedish astronomer, created the centigrade scale which used the temperatures measured when the thermometer bulb was dipped in melting snow (100 C) and when it was immersed in boiling water (0 C) as its fixed calibration points. By the end of the 20th century, the Celsius scale had replaced the Fahrenheit scale throughout most of the world.