Thermometer

The Italian scientist Galileo (see pages 31 and 33) came very close to inventing the thermometer in 1592. He made an instrument called a ‘thermoscope’. In it, an open-ended tube was held over a container of water. The level of water in the tube varied with the temperature in the room. But unfortunately, the level also changed when the air pressure varied.

Duke Ferdinand II of Tuscany was fascinated with Galileo’s apparatus and experimented with it in the early seventeenth century. In 1644 he sealed the unit from the surrounding air, and thereby removed the influence of the air pressure.

But the first thermometer that resembled the ones we know today was the mercury thermometer. This was perfected by a Dutch instrument-maker, D.G. Fahrenheit, in the early eighteenth century.

This type of thermometer works because substances expand as they get hotter. The thermometer consists of a narrow glass tube with a bulb at the bottom.

The bulb is filled with a liquid such as mercury. When the temperature goes up, the liquid expands and is forced up the tube.

Fahrenheit also introduced a scale of temperature measurement that was named after him. The Fahrenheit scale was widely used in the past, but today the Celsius or centigrade scale, developed by Swedish astronomer Anders Celsius in 1742, is preferred.