Francis Cooper, of Paroa School, asks :-

How many different types of thermometer are there and when were they first used?

Rod White, a physicist with the Measurement Standards Laboratory of New Zealand, who maintains New Zealand's temperature standards and calibrates thermometers for industry, responded.

There are literally millions of different uses for thermometers, in fact there are more thermometers sold than all other measuring instruments combined.

Thermometers are many and varied but all work on the same principle, that some property of materials which depends on temperature is recorded. This might be the relative expansion of mercury as compared to glass, the electrical resistance of a metal or semiconductor, the optical properties of a glass fibre or even the simple expansion of a length of metal.

The actual inventors of most types of thermometers, including the first thermometers, are unknown. But people like Fahrenheit and Celsius were amongst the first and became famous for making really good thermometers, so temperature scales have been named after them.

The first thermometers were made in Italy in the 1600's and were gas thermometers. Liquid thermometers, such as mercury or alcohol, came about 50 years later.

The longest mercury thermometer I've come across is a Jam maker's thermometer which is about two meters long. Other types of thermometer can be much longer, for example some fibre-optic thermometers are several kilometers long.

The shortest mercury thermometers I'm aware of are medical thermometers, the thermometer that doctors and nurses use.