To construct a mercury thermometer
One end of a length of clean capillary tubing is heated in a bunsen flame until the glass softens and seals the end of the tube. The tube is then withdrawn from the flame and a small bulb is blown at the end. By repeating this operation the size of the bulb may be increased as required.
The size of bulb needed will depend on two things, the bore of the tube and the desired temperature range. Often, in commercial practice, bulbs of a correct size are made separately and afterwards fused on to the stem. The next stage is the filling of fhe thermometer. It is placed with its open end beneath the surface of some mercury in a jar and the bulb gently heated. The air inside expands and bubbles through the mercury. On cooling, the air contracts and some mercury runs up into the bulb. The thermometer is then taken out and the bulb heated to boil the mercury. When the mercury vapour has expelled all the air
the open end is quickly inverted once more in mercury. On cooling, the mercury rises and completely fills the bulb and stem. Remember that mercury vapour is poisonous. so that all these operations must be done in afume chamber or under an extraction hood. The thermometer is now taken out and heated to a temperature somewhat higher than the maximum for which it is to be used. While at this temperature the end of the stem is rotated in a small blowpipe flame, drawn out and sealed off. The thermometer is now ready to be graduated. However, owing to the heat treatment it has received, the glass goes on contracting slowly for a considerable time. It is therefore advisable to put it aside for several months before graduation is finally carried out.