What is a Magnetic Compass, its principle, types and errors?

1. Tilt the bowl until the “filter hole” comes upper most. This hole is provided on the side of the bowl.
2. Unscrew the stud/screw provide.
3. Top up with ethyl alcohol. If this is not available distilled water would be added.
4. Screw the stud/screw back into place.
5. Gently let the bowl return to upright.

In some modern compasses, small bubbles may be removed as follows;

• Invert the bowl gently. This would cause the bubble to enter a bubble trap provided for this purposes.
• Gently return the bowl to upright.

          The bubble should have disappeared. If the bubble is large. It would have to be removed as described in (1) to (5) above.