Heat the tube in boiling water, drying oven or with a hot air gun and place open end in clean mercury. Allow tube and the air in it to cool, drawing a thread of mercury about 5mm long up into the tube. Allow all to cool to room temp. To move the thread of mercury further down the tube, to ensure it is not expelled when the tube is heated to 100°C during the experiment, pass a short length of nylon fishing line (approx 0.5mm diam) down the tube and through the mercury. This allows the air below the mercury to leak out around the line and when the mercury has fallen to the required position, remove the nylon line.
See also: Law of Charles and Gay-Lussac, Constructing school science lab equipment