Atmospheric pressure and a mercury column

Demonstration

A mercury column supported by atmospheric pressure.

Apparatus and Materials

• Glass tube (1.5 m long, 4 mm bore, 8 mm external diameter)
• Small trough of mercury
• Length of pressure tubing, 1.5 m
• Retort stand, boss, and clamp
• Vacuum pump
• Translucent screen and lamp (see Technical note 2)
• Mercury spill tray
• Trap (to prevent the water being sucked into the vacuum pump)

Health & Safety and Technical Notes

When mercury is being used, the laboratory should be well ventilated, and equipment for dealing with spills ready to hand. The apparatus itself should be placed in a tray so that any beads of mercury can be collected easily.

Use a trap to prevent mercury being sucked into the vacuum pump.

Wash hands thoroughly after using mercury.

Read our standard health & safety guidance

It is essential to insert a trap between the pump and the tube, so that the pump is fully protected.

It is much easier for the students to see the mercury if the tube is placed in front of an illuminated sheet of translucent material. A suitable screen can be made of tracing paper in a wooden frame. A lamp behind the tube ensures that the tube is seen in silhouette.

You need enough mercury to fill the tube about up to 800 mm high, with some left in the reservoir.

Procedure

1. Set up the glass tube vertically with the lower end immersed in the mercury trough. The tube should be held in position by a clamp. Place the whole apparatus in a spill tray for safety.

2. Connect the top end of the tube to the vacuum pump by pressure tubing.

3. After pumping, measure the height of mercury in the tube above its level in the trough.

Teaching Notes

• Start by asking: "Would the same level difference be expected for a mercury filled U-tube with a vacuum on one side if the U-tube had arms of unequal size, one made of much wider tubing than the other."

• Draw a picture of several U-tubes: one with arms of equal width tubing; then one with one arm much wider than the other; the 'W' tube; and eventually an open dish and the tube.
• The glass tube dipping into an open dish of mercury is a U-tube with one arm very wide indeed. Dip a 1.5 m tall glass tube into a dish of mercury. Connect rubber tubing to the top of the tube and follow the procedure described above.
• Ask: "What makes the mercury go up? Why does it stop?" This is a barometer, which measures the pressure of the atmosphere in centimetres of mercury.

This experiment was safety-tested in July 2007