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

1. Low freezing point about  -30⁰C
2. Small coefficient of expansion
3. Does not discolor the card.
4. Low relative density about 0.93

By immersing the card in a liquid, oscillations caused by vibration, rolling & pitching are damped, without loss of accuracy. The card therefore has a dead beat movement which is convenient.

Errors  of magnetic compass:-

1. VARIATION

The true North Pole and the magnetic north pole are not located at the same spot. This variation causes a magnetic compass needle to point more or less away from true north. The amount the needle is offset is called variation because the amount varies at different points on Earth’s surface. Even in the same locality variation usually does not remain constant, but increases or decreases at a certain known rate annually.

The variation for any given locality, together with the amount of annual increase or decrease, is shown on the compass rose of the chart for that particular locality.

Remember:  If the annual variation is an increase, you add; if it is a decrease, you subtract