1715 – Netherlands
‘The kelvin scale is more suitable for scientific purposes and the Celsius scale is neater, based on decimals. The advantage of using the Fahrenheit scale is that it is designed with everyday use in mind, rarely needing negative degrees’
Even as late as the start of the eighteenth century, scientists had no reliable means of accurately measuring temperature and a uniform scale by which to describe the limited measurements they could make.
GALILEO had used the knowledge that air expands when heated and contracts when cooled to build a primitive instrument. Using a cylindrical tube placed in water, he noted that when the air in the device was hotter, it pushed the level of the water downwards, just as it rose when the air-cooled. He realised that readings from the device were unreliable because the volume and therefore the behaviour of the air also fluctuated according to atmospheric pressure. Gradually scientists began using more stable substances to improve the accuracy of the reading, with alcohol being introduced as a possible substitute late in the seventeenth century.
Fahrenheit knew that the boiling points of different liquids varied according to fluctuations in atmospheric pressure; the lower the pressure, the lower the boiling point. A producer of meteorological instruments, he first achieved progress in 1709 with an improved alcohol thermometer. Building on the work of GUILLAUME AMONTONS (1663-1705) who investigated the properties of mercury, Fahrenheit took the measurement of temperature into another domain. He produced his first mercury thermometer, particularly useful in its application over a wide range of temperatures, in 1714.
In 1715 he complemented his breakthroughs in instrument making with the development of the Fahrenheit temperature scale. Taking 0degrees to be the lowest temperature he could produce (from a blend of ice and salt), he used the freezing point of water and the temperature of the human body as his other key markers in its formulation.
In his initial calculations, he placed water’s freezing point at 30degrees F and the body’s temperature at 90degrees F.
Later revisions changed this to 32degrees for the freezing point of water and 96degrees for the body temperature of humans. The boiling point of water worked out to be 212degrees F, giving a hundred and eighty incremental steps between freezing and boiling.