How did Farenheit decide there should be 180 degrees between freezing and boiling, let alone 212 for boiling and 32 for freezing?

Asked by: Tom Vandor

Answer

There are many suggestions as to how the scale was derived I am not sure if any one really knows.

My favourite story is that Farenheit made his thermometer without a scale. He then placed it in a beaker of ice and salt, waited for the mercury to fall and marked this point as '0'. He then measured his own body temperature, marked this point as '100' and then divided the distance between them evenly. By extending the scale upwards the boiling point of water occurred at the 212th graduation. A variation on this tale is that 0 degrees was the temperature of his cellar in the winter.

The salt/ice mixture & body temperature reference points where chosen as these where seen to be constant (an early use of referring to a 'universal' constant accessible to anyone who needs to check the calibration of their instrument). Farenheit thought that the ice/salt mixture was as cold as you could get and that everyone's body temperature was constant. We now know that both these theories where flawed. Indeed, it is thought that Farenheit was ill when he measured his body temperature as the temperature of a healthy person is approx 97.8F.
Answered by: Martin Collins, B.S., Chemist, Hull UK