French Brass Vertical Dip Circle Compass.

An extremely accurate instrument which measures the magnetic dip generated by the lines of force of the Earth's magnetic field. This dip circle is made of a brass cylinder with two glass flat end-windows, containing a centrally mounted magnetic needle free to rotate about its supporting axis in a vertical plane. The angle of dip is 0° at the equator and 90° at the poles. At the magnetic north and south poles, the needle dips vertically and the angle of dip is 90°. This precision instrument needs to be kept immobile and sometimes even enclosed in a box so that the needle will not be disturbed by electrostatic forces or breeze. One of its most important uses in travel was to tell the difference between the geographic North Pole and the North Magnetic Pole. The inside of the dip circle is marked Made in France. The instrument comes in a beautiful wood box with purpel felt lining.