Magnetic compass

“ As with other aspects of navigation, technology is rapidly revolutionizing the field of compasses. Amazingly, after at least a millennia of constant use, it is now possible (however advisable it may or may not be aboard any given vessel) to dispense with the traditional magnetic compass.
For much of maritime history the only heading reference for navigators has been the magnetic compass. A great deal of effort and expense has gone into understanding the magnetic compass scientifically and making it as accurate as possible through elaborate compensation techniques. ”

“ The magnetism acquired by soft iron while it is in a magnetic field. Soft iron will lose its induced magnetism when it is removed from a magnetic field. The strength and polarity of the induced magnetism will alter immediately as its magnetic latitude, or its orientation in a magnetic field, is changed. The induced magnetism has an immediate effect upon the magnetic compass as the magnetic latitude or heading of a craft changes. ”

“ Using the Gyrocompass Since a gyrocompass is not influenced by magnetism, it is not subject to variation or deviation. Any error is constant and equal around the horizon, and can often be reduced to less than one degree, thus effectively eliminating it altogether. Unlike a magnetic compass, it can output a signal to repeaters spaced around the vessel at critical positions. ”

“ Compasses used for navigation are equipped with a graduated compass card for direct indication of any horizontal direction. A magnetic compass depends for its directive force upon the attraction of the magnetism of the earth for a magnet free to turn in any horizontal direction. A compass having one or more gyroscopes as the directive element, and tending to indicate true north is called a gyrocompass. ”

“ Most of the work involved with compensating a traditional magnetic compass involves neutralizing magnetic influences other than the earth’s, a complicated and inexact process often involving more art than science. Residual error is almost always present even after compensation. Degaussing complicates the situation immensely.
The electro-mechanical gyrocompass has been the standard steering and navigational compass since the early 20th century, and has provided several generations of mariners a stable and reliable heading and bearing reference. ”

“ Happily this does not hinder us from finding out by what law these forces act, and how they enable us to navigate the ocean. I therefore hope the reader will be interested in a short exposition of the very curious and interesting laws on which the science of magnetism is based, and which are applied in the use of the compass.
The force known as magnetic, on which the compass depends, is different from all other natural forces with which we are familiar. It is very remarkable that iron is the only substance which can become magnetic in any considerable degree. ”

“ New technologies are both refining and replacing the magnetic compass as a heading reference and navigational tool. Although a magnetic compass for backup is certainly advisable, today’s navigator can safely avoid nearly all of the effort and expense associated with the binnaclemounted magnetic compass, its compensation, adjustment, and maintenance. ”

“ Thus the seeming paradox is explained that, as we proceed from the magnetic equator toward its pole, the compass becomes less steady and reliable, while, at the same time, the total intensity of the magnetic field increases.
If a wooden ship with no metal other than copper in her frame were to sail round the globe, her compass, adjusted for dip, would experience only those magnetic phases that have already been described as peculiar to the earth—more or less steadying force and a variation of larger or smaller amount according to location; the ship herself would exert no influence whatever. ”

“ It is however easy to demonstrate by means of the compass that the force is much greater in some parts of the field than in others. Lay the compass upon the cardboard, and observe the rate at which its needle vibrates after being displaced from its position of equilibrium; this will vary greatly in different regions. When the compass is far from the magnet, the vibrations will be comparatively slow; when it is near a pole, they will be exceedingly rapid, the frequency of the vibrations varying as the square root of the magnetic force at the spot. ”

“ To compensate (1) vertical magnets are used; (3) is partly corrected by the soft iron correctors of D; (2) and the remaining part of (3) cannot be conveniently corrected for more than one geographical position at a time. Although a compass may thus be made practically correct for a given time and place, the magnetism of the ship is liable to changes on changing her geographical position, and especially so when steaming at right angles or nearly so to the magnetic meridian, for then sub-permanent magnetism is developed in the hull. ”

“ Effects of Induced Magnetism Induced magnetism varies with the strength of the surrounding field, the mass of metal, and the alignment of the metal in the field. Since the intensity of the Earth’s magnetic field varies over the Earth’s surface, the induced magnetism in a ship will vary with latitude, heading, and heeling angle.
With the ship on an even keel, the resultant vertical induced magnetism, if not directed through the compass itself, will create deviations which plot as a semicircular deviation curve. ”

“ A magnetic force transverse to the rod produces a much feebler magnetization, the effect of which is almost insensible at a distance of a few diameters.
If a long iron rod be placed fore and aft with one end at a distance x from the compass needle, measured towards the ship's head, then, if the section of the rod is A, and its coefficient of magnetization κ, the strength of the pole will be AκX, and, if , the force exerted by this pole on the compass needle will be aX. The rod may be supposed so long that the effect of the other pole on the compass may be neglected. ”

“ N and S represent the Earth’s north and south magnetic poles, and the arrow heads are the north poles of the needles.
Fig. 14.—Diagram to illustrate Magnetic Dip.
Since the Earth is a magnet, we should expect it to be able to induce magnetism in a bar of iron, just as our artificial magnets do, and we can show that this is actually the case. If a steel poker is held pointing to and dipping down towards the north, and struck sharply with a piece of wood while in this position, it acquires magnetic properties which can be tested by means of a small compass needle. ”

“ A carefully made ship’s compass is usually employed, though in some cases the compass card, with its attached magnets, is made reversible, so that the inclination to the zero of the card of the magnetic axis of the system of magnets attached to the card can be eliminated by reversal. ”

“ Gilbert's monumental work, the De Magnete, published in 1600, marks an era in magnetic science. For some four hundred years the employment of the magnetic needle in navigation had been known both in Northern and Southern Europe. While it is possible that the primitive use of the loadstone may be ascribed to the Baltic, it is certain that the employment of a pivotted needle, and the addition of a rose of the winds as a compass card both originated in the Mediterranean. The pivotted needle is described in the Epistle of Peter Peregrinus, written in 1269 ”

“ But I fear it cannot be helped; for if they are moved into any other position, the attraction, though it might not be so great now, would be greater at some future time. It cannot be helped! I am sorry, for it will add nearly a year to my stay in this chamber. You perceive that the needle of that compass points to 1,007.8, or 16 degrees 47.8 minutes. That is the magnetic variation, plus 14.8 minutes for those iron boxes, of this place at the present moment. The magnetic pole is moving slowly toward the west ”

“ The N-seeking pole is sometimes called the marked pole. For convenience, we shall hereafter speak of the N-seeking pole as the N pole, and of the S-seeking pole as the S pole. We shall hereafter speak of the tendency which a bar magnet has to point N and S, as its pointing power. An unmagnetized needle has no pointing power.
26. The Magnetic Needle; The Compass. A small bar magnet, supported upon a pivot, or suspended so that it may freely turn, is called a magnetic needle. When balanced upon a pivot having under it a graduated circle marked N, E, S, W, etc., it is called a compass. ”

“ It is to be remarked that, besides the effect of polar magnetism, there was no doubt of the existence of an effect of induced magnetism requiring correction by other induced magnetism: and experiments for this were made in the Magnetic Observatory. All was ready for trial: and on Aug. 20th I carried my magnets and iron correctors to Deptford, mounted them in the proper places, tried the ship, and the compass, which had been disturbed 50 degrees to the right and 50 degrees to the left, was now sensibly correct. On Aug. 21st I reported this to the Admiralty, and on Aug. ”

“ The intensity (at any point) of the field due to the magnetization may be denoted by Hi, that of the external field by H0, and that of the resultant field by H. In certain cases, as, for instance, in an iron ring wrapped uniformly round with a coil of wire through which a current is passing, the induction is entirely within the metal; there are, consequently, no free poles, and the ring, though magnetized, constitutes a poleless magnet. Magnetization is usually regarded as the direct effect of the resultant magnetic force, which is therefore often termed the magnetizing force. ”

“ Therein he shows, as the result of his customary careful experimental explorations that the magnetism of every known substance (even tissues of the human frame) is manifested in one of two ways. Either the body is, like iron, attracted by the magnet, taking a position coincident with the magnetic forces which he calls paramagnetic (para beside or near, magnetes, magnes, magnet) or bodies—like bismuth, for instance—are repelled by the poles and should therefore be called diamagnetic (dia, across) for they set themselves across, equatorially, or at right angles to the magnetic lines. ”

“ Its magnetism does not produce the slightest influence upon the human body. No one would know that it was magnetic until something containing iron was brought into its immediate neighborhood; then the attraction is set up. The most important principle of magnetic science is that there are two opposite kinds of magnetism, which are, in a certain sense, contrary in their manifestations. The difference is seen in the behavior of the magnet itself. ”

“ In all substances except iron, nickel, and cobalt, the coefficient of magnetization is so small that the induced magnetization of the body produces only a very slight alteration of the forces in the magnetic field. We may therefore assume, as a first approximation, that the actual magnetic force within the body is the same as if the body had not been there. The superficial magnetization of the body is therefore, as a first approximation, , where is the rate of increase of the magnetic potential due to the external magnet along a normal to the surface drawn inwards. ”

“ Within the magnet, running through its substance, existed lines of force of the same nature as those without, exactly equal in amount to those without, and were, indeed, continuous with them. The conclusion must logically be that every line of force is a closed circuit.
Having thus established the exact quantitative laws of magneto-electric induction, he then advanced to make use of the induced current as a means of investigating the presence, direction, and amount of magnetic forces—in other words, to explore and measure magnetic fields. ”

“ Now the magnet when employed in the deflexion experiments is placed with its axis east and west, so that the action of terrestrial magnetism is transverse to the magnet, and does not tend to increase or diminish . When the magnet is made to vibrate, its axis is north and south, so that the action of terrestrial magnetism tends to magnetize it in the direction of the axis, and therefore to increase its magnetic moment by a quantity , where is a coefficient to be found by experiments on the magnet. ”

“ The direction of magnetization of the equivalent magnet may be predicted by remembering that a current travelling round the earth from east to west as the sun appears to do, would be equivalent to that magnetization which the earth actually possesses, and therefore in the reverse direction to that of a magnetic needle when pointing freely. ”

“ Certain bodies, as, for instance, the iron ore called load stone, the earth itself, and pieces of steel which have been subjected to certain treatment, are found to possess the following properties, and are called Magnets.
If, near any part of the earth's surface except the Magnetic Poles, a magnet be suspended so as to turn freely about a vertical axis, it will in general tend to set itself in a certain azimuth, and if disturbed from this position it will oscillate about it. An unmagnetized body has no such tendency, but is in equilibrium in all azimuths alike. ”

“ Electromagnets with iron armatures in contact across their poles will retain, after the current has been cut off, a very large part of their magnetism, even if the cores be of the softest of iron. But so soon as the armature is wrenched off, the magnetism disappears. An air gap in a magnetic circuit always tends to hasten demagnetizing. A magnetic circuit composed of a long air path and a short iron path demagnetizes itself much more rapidly than one composed of a short air path and a long iron path. ”