care. A quantity of the best steel wire, as thick as a knitting-needle, is selected and cut into lengths to fit the tubes; bundles of these wires large enough to enter the tubes are made up and tempered to the degree that experience has proved best for holding captive the magnetic charge. This is imparted to each bundle between the poles of a powerful electro-magnet. Of course, every one of the many slender wires that compose a bundle is itself a magnet; they lie together with their north poles in contact, and likewise their south poles: hence repulsion—a mutually deteriorating influence—is the result; and indeed oft-times a bundle of wires loses much of its magnetic strength because the steel is not of a quality and temper to resist the destructive force.
Each bundle is weighed and its magnetic strength tested, and, in placing them on a card, due care is had to the equal distribution of weight and force on each side of the center, for the characteristic of symmetry is ever kept in view. The magnets, except one, are rigidly set in the tubes, and the latter sealed; the movable magnet has screws at one end for the purpose of adjusting the magnetic axis of the whole system to that diameter of the card which passes through the north and south points: it would be the imaginary dotted line (Z . . . Z) shown in Fig. 6.
The card as above described, with tubes, magnets, bulb, rim, brass cap, and jewel all in place, weighs many ounces—a heavy weight for the delicate force of terrestrial magnetism to turn about on a pivot, how highly soever both this and the jewel may be polished.
The essential principle observed in the manufacture of the instrument is to reduce the friction on the pivot to a minimum and increase the moving power—the strength of the magnets—to a maximum; and this object is greatly furthered by the introduction of the liquid: its buoyant effect upon the card reduces the pressure of several ounces to that of a few grains.
The liquid has another advantage—it steadies the card, prevents all those small oscillatory movements that characterize a dry or air compass, while at the same time enabling the magnetic power to cope more efficiently with its burden. The liquid must fill the bowl completely, otherwise an air-bubble would gather and impede the free motion of the card.
The same compass may guide a ship into all climates—polar seas and tropical oceans; but as every change of temperature causes a varying expansibility of the copper bowl and the liquid in it, the former, when filled to complete fullness, would soon burst, were no provision made for expansion. To prevent this, an air-tight case of thin flexible metal is placed in the bottom of the bowl, which contracts or expands with every changing press-