Francis Cooper, of Paroa School, asks :-

How are mercury thermometers made and how do we check them?

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.

Mercury thermometers are made from two pieces of glass. The first piece, the bulb, contains most of the mercury while the second piece is the stem which is a hollow tube.

A mercury thermometer works because in general two materials, for example glass and mercury, expand by different amounts when the temperature is raised. The volume of the mercury increases more than the glass does and so the mercury column becomes longer. The temperature depends on the length of the column of mercury.

When the thermometer is being made the two pieces are joined together, the air is sucked out of the bulb and stem, the thermometer is filled with mercury and the other end of the stem is sealed. The scale is then printed on the stem of the thermometer. Note that this scale is usually chosen to be correct when the lower 76mm of the thermometer are immersed in the liquid to be measured.

To check a thermometer, put it in a well stirred mixture of ice and water (ice melts at exactly 0 degrees celsius) and then into boiling water (close enough to 100 degrees celsius.)

If a thermometer needs to be checked accurately over a wide range of temperatures then it is usually sent to our laboratory for calibration.

Alcohol, dyed red so it can be seen easily, is often used in thermometers which must work at very low temperatures because mercury becomes solid at 39 degrees celsius below zero.