CHECKING FOR AIR IN THE COLUMN
Before hanging the barometer on its backboard, it should be tested to ensure that
no air has entered the vacuum space at the top of the mercury column.
To do this the barometer should be slowly raised to a vertical position with the
scale uppermost. This is to be the position in which it will finally be used.
It should now be possible to see that there is a space between the top of the
mercury column and the top of the glass tube. If there is no space visible, unscrew
the milled-headed screw at the base of the cistern several turns. This will lower the
mercury column and create the required space.
Next slowly incline the instrument away from the vertical until the mercury
touches the end of the tube. When this happens a sharp metallic click should be
heard, if only a dull thud or no sound at all is heard it is indicative that air has
entered the column. If this is so, please refer to NOTES section.