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By Victoria Carroll on

A Lot Of Hot Air?

How did you enjoy the hottest day of the year so far on Sunday? It got me thinking about what else we have in the collection relating to temperature.
For simplicity, I like this modern reconstruction of an apparatus which Philo of Byzantium devised back in 200 – 100 BC to indicate temperature change. A hollow, lead globe is attached to a tube, which is bent over into a container of water. You can probably guess what happens when the globe is warmed…

Reproduction of Philo's thermoscope
Reproduction of Philo's apparatus for indicating temperature change (Science Museum / Science & Society)

Philo explained:  

I assert that when the globe is placed in the sun and becomes warm, some of the air enclosed in the tube will pass out … into the water, setting it in motion and producing air bubbles, one after the other. If the globe be placed in the shade … then the water will rise through the tube and flow into the globe. 

Some seventeen or so centuries later Philo’s idea was revisited, leading to the invention of the air thermoscope. The Italian physician Santorio Santorio was one of several Europeans working on it simultaneously.

Illustration of Santorio's air thermoscope
The two pieces of string tied round this air thermoscope indicate a rise in temperature. From Sanctorii Sanctorii, ... Ars de statica medicina, etc., 1625 (Wellcome Library, London)

Santorio’s instrument is in two parts. The glass bulb and tube are heated to expel some air, and the end of the tube is inverted into the narrow vessel containing water. As the air inside the bulb cools it contracts, drawing liquid up into the tube.  Once it has been set up, the changing water level indicates rising and falling temperature.

Santorio later put a scale on the thermoscope, creating the first air thermometer.  The air thermometer was supplanted by the more familiar liquid-in-glass thermometer from the 1640s. More on that another time. 

Combined thermometer and alcohol barometer, 1719
Combined thermometer and alcohol barometer, 1719 (Science Museum / Science & Society)

One comment on “A Lot Of Hot Air?

  1. Philo’s instrument is fascinating, but have have had very limited usefulness apart from leading to better ones.

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