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Object lesson No 67

Our bodies have a built-in temperature gauge which makes us shiver when we're cold and sweat when we're hot. But temperature is an elusive thing, and nobody knew how to measure the bits in between. For centuries, it was thought that heat passed between objects in the form of invisible, weightless "caloric" and "frigorific" fluids.

Galileo realised that heat made things expand, and by 1600 he was experimenting with a thermometer made from a "small glass flask about as large as a small hen's eggI with a neck as fine as a wheat straw". In 1641, Ferdinand II, the Grand Duke of Tuscany, commissioned a "spirit" thermometer marked with 50 degrees but it was little more than an ornament.

No one had managed to measure temperature (which originally meant a "mingling" of hot and cold) until 1714 when the German scientist, Daniel Fahrenheit, successfully calibrated a mercury thermoscope between two fixed points - the temperature of melting ice (32F) and the human body (96F). The centigrade scale - with 100 degrees between melting ice and boiling water - was developed by the Swedish astronomer, Anders Celsius, in 1742, originally with zero as boiling and 100C as freezing but, te story goes, the scale was put on upside down by the manufacturers.

They might be household names today, but neither won acclaim in their lifetime. Fahrenheit's thermoscope was 10 inches long and took five minutes to register a temperature, so most doctors still used the hand-on-brow technique of temperature measurement, and the thermometer didn't catch on in general medical practice until a pocket version was produced 150 years later.

Mercury remains the medium of choice because it is visible, stable and liquid down to minus 40C. And while distances, heights, currency and weights have all gone metric, Fahrenheit and Celsius scales are still happily coexisting after nearly 300 years together.

Bakers, brewers and weather forecasters rely on accurate temperature readings to do a good job, but in most homes the thermometer is a neglected instrument. Less consulted than the clock, rarely displayed like the barometer, it is brought out only in emergencies, shoved under reluctant tongues or in armpits, then put back in the bathroom cabinet. There it continues quietly, unerringly and faithfully performing its sole function: taking the temperature.

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