Daniel Gabriel Fahrenheit

1686-1736

German-Dutch Instrument Maker

Daniel Fahrenheit is famous for the temperature scale that bears his name and for developing the first mercury thermometer. He also established quantitatively that boiling-point temperatures vary with pressure, and he discovered supercooling of water.

Daniel Gabriel Fahrenheit was born in Danzig (now Gdansk) on May 24, 1686 to a wealthy merchant family. Demonstrating a gift for learning, he was to have attended the Danziger Gymnasium. However, his parents died before he matriculated. Much against his will, his guardians sent him to Amsterdam to complete a business apprenticeship. He failed to complete it, and a warrant for his arrest was issued with the intent of sending him to the East Indies (1707).

Thus pursued, Fahrenheit traveled throughout Europe, visiting scientists and instrument makers. He later settled in Amsterdam (1717) and established himself as a manufacturer of thermometers, barometers, and areometers. Throughout his life, Fahrenheit devoted all his resources to inventing instruments, including a mercury clock for determining longitude at sea and a pumping device intended for draining Dutch polders. Though highly esteemed, he died impoverished on September 16, 1736.

In 1708 Fahrenheit visited Ole Römer (1644-1710). Since at least 1702 Römer had been making alcohol thermometers with two fixed points and a scale divided into equal increments. He impressed upon Fahrenheit the scientific importance of reproducible, intercomparable measurements by different thermometers. Inspired, Fahrenheit eventually devised a method for commercially producing such thermometers.

Fahrenheit conducted experiments to determine the best thermometric substance. Settling on mercury, he produced the first mercury thermometer (1713). Further investigations led him to discover that glass expansion varies depending on its composition. He eventually devised a method for determining expansion coefficients for different glasses (1729), but had by then developed a method for calibrating thermometers regardless. Fahrenheit accurately determined boiling points for many liquids and quantitatively established their variation with barometric pressure (1723). Based on his discoveries he constructed the first hypso-barometer—an instrument for measuring barometric pressure from changes in water's boiling point (1724). Fahrenheit also discovered the supercooling of water—a phenomenon whereby water is chilled a few degrees below its freezing point without solidifying.

The present Fahrenheit temperature scale evolved from Römer's. Römer calibrated his standard thermometer in melting ice and boiling water, marking the ice point 7½ and boiling point 60. Römer took precautions to avoid negative measurements. However, Römer noticed temperature measurements never went above 20°. Since most of the thermometer's length was thus unnecessary, he calibrated shorter thermometers. Fixing the ice point as before, he placed shorter instruments in 22½° water-baths, as determined by his standard thermometer, to set the upper point.

Observing this procedure, and unaware Römer's second fixed point was actually that of boiling water, Fahrenheit took the upper fixed point to be 22½°, which was approximately normal body temperature. Fahrenheit applied Römer's 22½°-scale to his own thermometers but subdivided it into quarters to yield a 90°-scale, which he later altered to 96°. The freezing point was fixed at 32° and upper at 96°, which was calibrated by placing a thermometer in the mouth of a healthy person. Depending on the barometer reading, Fahrenheit measured water's boiling point from 205½° to 212½°.

Fahrenheit communicated his results to the Royal Society in 1724. Though he clearly never took water's boiling point as fixed, the Royal Society officially established it as such (1777), fixing the value at 212°, which occurred at a barometric pressure of 29.8 inches-Hg. Normal body temperature on this scale is 98.6°. The revised scale was quickly adopted in England and Holland and became the standard throughout the English-speaking world.

STEPHEN D. NORTON