A simple mercury barometer

Demonstration

Setting up a mercury barometer.

Apparatus and Materials

• Barometer tubes with different diameters, 2 (see Technical note 1)
• Bottle of mercury
• Small funnel
• Mercury trough
• Mercury spill tray
• Retort stand, boss, and clamp
• Translucent screen and lamp (see Technical note 2)
• Length of stout iron wire
• Metre rule

Health & Safety and Technical Notes

It is advisable to wear latex or nitrile close-fitting gloves. The gloves must still give the wearer good grip of the glassware. The room must have good natural ventilation. The volume of the spill tray should be at least 110% that of the mercury being used. It should be made from a smooth-surfaced plastic tray, not metal. Fill the spill tray with a thin layer of water.

Watches/rings etc should be removed as the mercury forms an amalgam and can damage them.

Wash hands thoroughly after handling mercury, even if gloves have been worn.

Inexperienced teachers need to be shown how to do this experiment, to avoid a large spill of mercury.

Read our standard health & safety guidance

The barometer tubes need to have very different bores for this demonstration to be effective. 4 mm and 8 mm are recommended.

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.

Filling a barometer tube takes practice and needs a steady hand but it is well worth trying to master the art.

Procedure

1. Fill the barometer tube with mercury, holding it over the tray throughout. (An easy technique for this is to pour mercury into the tube, using the funnel and a 5 cm length of rubber tubing, until it is nearly full, all but 2 cm at the open end. Close that end with a finger. Tilt the tube to run the air bubble very slowly to the other end of the tube and back again, collecting up any small sticking bubbles on the way. Then fill the tube to the top.)

2. Hold a finger on the top and invert it into the trough. Do not remove the finger until the end of the tube is below the surface. Then ask students if air can get in. They should then watch what happens when the finger is taken away. Hold the barometer in a clamp and measure the height.

3. If the height does not remain the same, air has been included and the barometer must be refilled.
4. By securing the tube with a spiral of wire, it can be conveniently inclined to show the mercury level remaining the same height above the trough.

5. Set up two barometer tubes of different diameter side by side in the same trough. It will be seen that the mercury levels are the same in each tube.

Teaching Notes

• When the tube is full, before inverting it, check that the students realize that there is no air in it and that no air can get in when the tube is inverted.
• When the supporting finger is removed then the column of mercury falls to a height which the atmosphere can support. The gap at the top of the tube contains a vacuum, the Torricelli vacuum.
• When the vertical height, not the length along the tube, of the mercury column in the inclined tube is measured, it is shown to be the same as the height of the column in a vertical tube.

This experiment was safety-tested in July 2007