Every time I give a brewer’s presentation, I must remind the various homebrewers who come to these events that a thermometer would have been an EXPENSIVE and rare instrument in the 18th Century.  Today they are ubiquitous, even my car has a built-in thermometer, but in 1750 these would have been delicate and hard to get, suitable mostly for scientific inquiry not day to day industry.  Let’s take a few minutes to explore where these devices came from.

In about 1603, Galileo Galilei developed an innovative means of measuring heat.  Galileo’s thermoscope, is not the same as a thermometer. It couldn’t measure temperature and it has no scale.  The thermoscope measures the effect of heat upon a fluid.  As the fluid becomes warmer, it expands becoming less dense.  Using Archimedes’ principle of buoyancy, Galileo found that objects weighted slightly differently but having the same volume would float higher when the fluid was cold.  This allowed him to gauge how much heat was in that fluid.

Around 1612, a Venetian named Santorio Santorio, hit upon the idea of addng a adding a scale to the thermoscope.  The scale was arbitrary but Santorio set the maximum to be the temperature after heating the thermoscope’s bulb with a candle flame and he set the minimum by immersing the bulb in melting snow.  Santorio went further and devised a means of measuring body temperatures by having the patient hold the bulb with their hand or breathe on it.

In 1701, Olaus Rømer had the idea to calibrate a scale relative to something much more accessible and universally stable, the freezing and boiling points of water.   He divided this range into 60 degrees. With fresh water boiling at 60° and freezing point of sea water at 0°.  Daniel Fahrenheit, met Rømer in Amsterdam. Their collaboration spawned the first quicksilver (mercury) thermometer, which afforded greater accuracy and precision and consistent readings.  Fahrenheit also improved upon Rømer’s scales by using equal parts water and ice that froze at 32 degrees.

Why 32? Well, that is a bit of Freemason Numerology.  The Masons have 32 degrees of enlightenment, so Fahrenheit set the freezing point of water to be 32° higher than the freezing point of equal parts salt and water (brine).  He then set body temperature to be “8 degrees of enlightenment” (96°) as calibrated by placement of the thermometer under his armpit. It all fit nicely together, and so the gauge caught on, eventually becoming temperature’s first standard scale.

The Fahrenheit Scale will become the international standard until supplanted by the metric system following the French Revolution.  Because of its “base ten” intuitive metrics, the Système International is much more intuitive and makes calculations easier. The French quickly adopted the scale developed by Anders Celsius in 1742. He made the calibration process more accurate by simply using the freezing and boiling points of water at sea level—no more salt mixture requiring its own measurements.  Originally, Celsius’ scale was upside down — 100°was the freezing point and 0° as the boiling point but Jean-Pierre Christin, a French physicist, conceived of a similar arrangement but with 100° as the boiling point the current Centigrade scale.

Of course, Lord Kelvin will have his own scale but more on that another day…