Thermometer

A thermometer is an instrument for measuring temperature. There are many different types that vary in range and accuracy. The most common type met in schools and colleges is the liquid-in-glass type. These are glass tubes containing a liquid (coloured alcohol or mercury are most likely to be used). As the liquid heats up, it expands and rises up the tube. The glass tube has a scale marked on it, from which the temperature can be read. If you are choosing a thermometer it is important to state the range (ie between which temperatures it can measure) and the accuracy (how closely can it be read). Two popular liquid-in-glass thermometers are shown in the photograph. There is a - 10 to 110 °C ± 0.5 °C alcohol thermometer and a - 10 to 360 °C ± 0.5 °C mercury thermometer. The small red triangular plastic pieces that you can see in the photographs prevent the thermometers from rolling of the bench.

Digital thermometers are becoming more popular. They are more expensive than most liquid-in-glass thermometers, but are generally more accurate and less easy to break. The probe contains a device for measuring the temperature called a thermocouple and the box contains the electronics for converting the probe's electrical signal into a temperature. They are available in a number of ranges and accuracies. The one shown is typical of schools and colleges and has a range of - 40°C to + 150°C and an accuracy of ± 0.1°C. Some digital thermometers can be unreliable and suffer from drifting.

Just because a thermometer can read to the nearest 0.5 °C does not necessarily mean that the correct temperature will be within that range. It is important to standardize thermometers. This is usually done by placing them in pure boiling water. At normal atmospheric pressure the thermometer should read 100 °C. If it does not, then the thermometer needs adjusting. Liquid-in-glass thermometers can have the scale remarked on the tube. Digital thermometers have a control for adjusting the displayed temperature. A second standardization temperature should also be checked. The melting point of ice is usually used.