Types of Magnetic compass :
- Dry card compass.
- Wet card compass.
In olden days there used to be a dry card compass on the upper bridge for use in navigation. It is now part of history.
- Made by rice paper.
- Diameter 254mm (10 inches)
- Cover aluminum ring.
- Silk thread used to be attached.
- The weight of the card is usually around is grams.
Wet card compass:
- Made of mica
- Diameter 15cm
- Bowl diameter 23cm
The bowl is filled with a mixture of distilled water and pure ethyl alcohol so that the mixture has the following properties.
- Low freezing point about -300C
- Small coefficient of expansion
- Does not discolor the card.
- 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.