To measure the cubic expansivity of air at constant pressure
A glass tube about 30 em long and about 1.5 mm bore has one end sealed by heating in a bunsen flame. A short pellet of concentrated sul phuric acid is then introduced a little more than a third of the way down the tube by means of a quill tube drawn out into a capillary and used as a pipette as shown in Fig. 16.5 (a).
Some people use a mercury pellet, but sulphuric acid has the advantage of drying the air inside the tube. The process of sealing the tube invariably introduces moisture into it, and it is essential in this experiment that the air is dry, since the presence of water vapour will give results which are abnormally high. Care must be taken to avoid spilling concentrated sulphuric acid or getting it on one’s fingers: it is advisable to enlist the help of someone experienced in handling it.
The tube is now inserted, with its open end upwards, through a hole in a bung and placed inside a glass tube with a bung at its lower end and packed with ice shavings (Fig. 16.5 (b)). When the air inside the experimental tube has fully acquired the temperature of the melting ice it is raised until the top of the pellet is just visible above the top of the bung, and the distance from the open end of the tube to the top of the pellet is measured. This distance plus the length of the pellet is afterwards
subtracted from the total length of the tube from the open end to the inside of the closed end in order to find the length of the air column at 0 °C.
The ice and water are now removed from the tube and the experimental tube pushed well down. Steam is now passed in through the top and out at the bottom for several minutes. The experimental tube is again adjusted until the pellet is just visible above the top of the bung and measurements are taken as before to find the length of the air column at the steam temperature. A final check reading is taken to make certain that the air has fully reached the steam temperature, assumed to be 100°C.
Assuming the bore of the tube to be uniform, and neglecting the small error arising from the curved inside end of the bore and acid pellet, wl? take the lengths 10 and 1100 of the air column at the ice and steam temperatures as being proportional to their volumes Vo and V100• The cubic expansivity of the air at constant pressure, y, is given by,
The value obtained is approximately mor 0.003 66/K and, moreover, practically the same result is obtained for all gases. The original experimental work on this subject was carried out towards the end of the eighteenth century by the distinguished French scientist, Jacques Charles, whose researches were doubtless inspired by an interest in flight by hot-air balloons (Fig. 16.6). He later decided in favour of hydrogen balloons, since these had greater lifting power. The result obtained in the experiment just described is generally known as Charles’s law. The volume of a fixed mass of gas at constant pressure expands by 2~3 of its volume at 0 °C per kelvin rise in temperature.