Magnetic compass

A magnetic compass gets its directive force from the Earth's magnetic field. The force acts on a magnetized needle, or several needles, affixed to a card. The needles rotate freely horizontally. Until the mid 19th century, compasses were dry; that is, the compass card rotated in air on a pivot pin. Edward S. Ritchie developed the first liquid magnetic compass (or wet card compass) in 1862, which solved problems of instability in dry card compasses. Wet card compasses have the card suspended in a liquid, usually a light oil, which has a float in the center to take most of the weight of the card off the pivot. The oil damps or smooths the card motion in a rough sea.