A small piece of iron wire (diameter 0.5–1 mm, length 1–5 mm) is sealed into a polythene tube of suitable diameter, so that water cannot enter. In order to do this, the polythene tube is first sealed at one end by squeezing it together with a heated forceps, whose points have been bent outwards (see Fig. 1). It is often difficult to obtain a watertight seal if forceps with sharp instead of with rounded ends are used, since the polythene tube at the ends of the wire can be damaged when it is compressed with the heated forceps. It is important to heat the forceps to the correct temperature: if they are too cold the seal is imperfect, while overheated forceps may easily set the polythene tube on fire. The wire is introduced into the open end of the polythene tube so that about 3 mm of polythene tube projects beyond the end of the wire. The other end is then sealed with the heated forceps. Finally the surplus polythene at both ends is removed with scissors.