Ray Oliver on how expansion is used to measure temperature

9th June 2006 at 01:00
When materials are heated they tend to expand. Children will be familiar with liquid in glass thermometers, both those containing coloured alcohol and those filled with mercury. They will probably not have reflected on some of the problems associated with these simple thermometers.

Does mercury expand by the same amount between zero and 10 degrees C as from 90 to 100 degrees C? Does the glass containing the mercury expand at the same rate as well? Luckily, these basic thermometers are reasonably accurate, since mercury does expand at a fairly uniform rate. However, at the high temperatures inside a furnace or pottery kiln, one of these thermometers would explode as the liquid inside would boil.

The problem of monitoring high temperatures was solved by porcelain manufacturers at Sevres, in northern France. They used a different technique, based on the expansion of a metal bar. One end of the bar was linked to a pointer, and as it heated, the bar expanded and moved the pointer across a temperature scale.

Children can build and test their own version of the metal bar thermometer.

Fix one end of an iron bar horizontally so that it cannot move, for example using a G-clamp. The iron bar from a retort stand works well for this.

Place heat-proof mats under the whole length of the bar. Under the free end of the bar place a wooden roller - a thick pencil will do. Press into the wood a long pin to act as a pointer, initially set vertically. Heat the bar, gently at first, using a Bunsen burner flame. As the bar expands, the free end will move, turning the roller and causing the pointer to change position. Ask the children to try using a longer pointer to make small movements easier to detect. This solid thermometer will also show contraction of the bar as the temperature falls. Either wait patiently or, with great caution, pour a little cold water on the bar to accelerate the contraction of the iron.

Ray Oliver teaches science at St Albans Girls' School Hertfordshire

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