Records: Birth of the Thermometer

## **A Prelude to Steam**

The concept of a thermometer, and guaging temperature had been around for more than a millennia.  Various authors have credited the invention of the thermometer to Galileo Galilei, Cornelis Drebbel, Robert Fludd, or Santorio Santorio. The thermometer was not a single invention, however, but a development. Philo of Byzantium (~280 BC) and Hero of Alexandria (10-70 AD) knew of the principle that certain substances, notably air, expand and contract and described a demonstration in which a closed tube partially filled with air had its end in a container of water. The expansion and contraction of the air caused the position of the water/air interface to move along the tube.

With guidance, members of the RANP created a mechanism to show the hotness and coldness of the air with a tube in which the water level is controlled by the expansion and contraction of the gas. It could produce this effect reliably, and the term *thermoscope* was adopted because it reflected the changes in sensible heat (the concept of temperature was yet to arise).

The disadvantage was that it was also a barometer, i.e. sensitive to air pressure. Instead, Richard made sealed tubes part-filled with alcohol, with a bulb and stem; the first modern-style thermometer, dependent on the expansion of a liquid, and independent of air pressure. The Royal Academy, now amazed, began experimenting with various liquids and designs of thermometer.

They also kept the concept of the barometer to record changes in atmospheric pressure – which became very significant regarding the weather.

While artistic, and a curiosity of nature, wasn’t particularly useful until he put a scale on it – and thus was born a thermometer. Their scale looked for a simple gradient between two very important factors in life: the freezing point of water and the boiling point of water.

As originally designed, the thermometer did not hold the temperature after it was moved to a place with a different temperature. Determining the temperature of a pot of hot liquid required the user to leave the thermometer in the hot liquid until after reading it. If the non-registering thermometer was removed from the hot liquid, then the temperature indicated on the thermometer would immediately begin changing to reflect the temperature of its new conditions (in this case, the air temperature).

To the credit of the Royal Academy, they started realizing that reading temperature over boiling water could be hazardous. Thus the drive to create a registering thermometer: designed to hold the temperature indefinitely, so that the thermometer can be removed and read at a later time or in a more convenient place.

The first registering thermometer was designed and built by the RANP that year. Mechanical registering thermometers hold either the highest or lowest temperature recorded, until manually re-set, e.g., by shaking down a mercury-in-glass thermometer, or until an even more extreme temperature is experienced. Electronic registering thermometers may be designed to remember the highest or lowest temperature, or to remember whatever temperature was present at a specified point in time.