The Magnetic Compass

Since the earliest sailors set off in their tiny craft on long distance voyages, some method of establishing direction on the Earth's surface has been a necessity. It seems fairly certain that the early pioneers, such as the Phoenicians in Europe and the Polynesians in the Pacific, used their knowledge of the sun and the stars, the prevailing winds and currents to guide them. They had no instruments, but generations of accumulated experience enabled them to cover immense distances and eventually return from whence they came. By modern Stan­dards, however, such methods as they used were bound to be crude and chancy, particularly in bad weather.
The discovery of a natural ferrous rock, lodestone, led to the first instrument that would give a visual indication of direction. Lodestone is magnetic and could therefore be seen to attract pieces of iron. Soon, no doubt, it was discovered that stroking a piece of iron with the rock would induce the iron in its turn to become magnetic. Then someone found out that a piece of iron or lodestone, suitably shaped and suspended, would point in a fixed direction approximately North or South. No-one knows, for certain, in which country a primitive magnetic compass was first used, but it is definitely recorded that compasses were used in both Europe and China between
A.D. 1000-1100
The earliest compasses probably consisted of a needle or double-bow shaped piece of iron or lodestone floated on a piece of wood in a water-filled vessel; alternatively a thin magnetised needle was pricked through a piece of straw which kept it afloat on the water surface. Whichever method was used, it must have been appreciated that the North-seeking force on the needle was weak and therefore an almost friction-free method of suspension was needed.


The Earth's Magnetism
The cause of the phenomenon which showed that a piece of freely suspended iron would point North remained a mystery to the early astronomers and explorers. At one time it was thought that the North end might be attracted by the Pole Star (Polaris) which lay close to the Earth's axis of rotation and was therefore known to give a good general indication of North. There were other theories, but matters became even more difficult to explain when it was discovered that a properly balanced needle not only indicated a different North in different areas of the world but also tended to dip its North end down from the horizontal as it travelled into higher Northerly latitudes.
In about the seventeenth century, it was suggested, correctly as we now know, that the Earth itself was a vast magnet whose poles attracted one end of a magnetised needle (in accordance` with the normal laws of magnetism, a magnet always has two poles; the North-seeking/Red end of one will always attract the South-seeking/Blue end of the other; or unlike poles attract: like poles repel) .
At that time, however, there was no explanation for the apparent difference between Magnetic North (as defined by compass) and True North, the direction of the geographical North Pole. Also, in different places the angle between the two was known to vary. In fact, in the middle of the sixteenth century, some European compass makers deliberately mis­aligned the needle when it was fixed to a graduated card so that when the needle pointed at Magnetic North the card indicated True North. This was all very satisfactory whilst the ship remained in home waters, but as soon as it ventured away considerable inaccuracies became apparent. It slowly became obvious that the magnetic poles could not be in the same place as the geographical poles, and furthermore that the magnetic poles were not stationary but changing their positions slowly over long periods of time. Not until 1831 was it definitely established that the Earth's Magnetic Pole was over i000 miles from the Geographical North Pole in the approximate position 700 North 97° West. The South Mag­netic Pole has since been accurately located in one of the most inaccessible places in the world on the bleak snow plateau of Antarctica (about 720 30' South x55° East).

This concept of the Earth having its own Magnetic Poles explained the problems that had been worrying scientists over the years. As with an ordinary bar magnet, magnetic `lines of force' emanate more or less vertically out from the North Magnetic Pole, bend over the Earth's surface, and then descend vertically into the South Magnetic Pole. At any point on the Earth's surface, therefore, a simple magnet (or compass needle) suitably suspended will align itself to these lines of force. If it were held over the North Magnetic Pole, the North-seeking end of the needle would point vertically downwards, at the Magnetic Equator the needle would be horizontal and parallel to the Earth's surface, and at the South Pole its South-seeking end would point downwards (see Figure x).
At intermediate points North or South of the Equator, the needle tilts down at a certain angle. The Earth's magnetic field, therefore, has two components; a HORIZONTAL force (H) and a VERTICAL force (Z), the vertical force being maximum at the Poles and zero at the Equator; the horizontal force zero at the Poles and maximum at the Equator. If a magnet is suspended so that one end can tilt up and down—towards or away from the Earth's surface—it will (except at the Magnetic Equator) take up a certain angle from the horizontal called `the Angle of Dip'. It used to be thought that this could be used to measure Latitude (the angular distance North or South of the Geographical Equator) before the difference between the positions of the Magnetic and True Poles was known, but in any case it would be extremely difficult to observe this angle at sea in a rolling boat.
Variation—The Angle between Magnetic North and True North
The Horizontal component of the Earth's magnetic field is of greater importance as far as the magnetic compass is con­cerned. If a magnet or compass needle is suspended so that it will only rotate in the horizontal plane, it can only react to the earth's Horizontal magnetic force (H). As has been seen, this force will be a maximum at the Magnetic Equator and zero at the Poles; for this reason, a magnetic compass cannot be used at or near the Magnetic Poles. In a modern compass, the needles are carefully balanced and pivoted to gain the maxi­mum effect from the horizontal force and to nullify the effects of the vertical component of the Earth's field. In normal latitudes, with one exception which will be mentioned later, the Earth's vertical force (Z) can be ignored.
Due partly to the physical difference in position between the Magnetic and Geographical Poles, at most places on the Earth's surface there is an angular difference in direction between the two as seen from the observer's position. This angle, between TRUE NORTH and MAGNETIC NORTH, is known as the ANGLE OF VARIATION.



In Figure 2, Position I the angle or Variation is zero, as the direction of the Geographical and Magnetic Poles is coincident. However, in Positions 2 & 3, the two Poles lie in different directions from the observer and the angle between these directions in each case is the Angle of Variation.
The early navigators had tried to associate Dip with Lati­tude; they also tried to associate Variation with Longitude, i.e. the angular distance East or West of an arbitrary meridian.*
* A Meridian is a Great Circle passing through both geographical poles. '1'hc Greenwich Meridian is that which passes through Greenwich, and is historically the one from which Longitude is measured.



It was established, for example, that as the Atlantic was crossed the variation first reduced to zero and then increased again in the opposite sense. They inferred that charts could be drawn joining places of equal variation and assumed that the distribution of these places would be regular, and therefore the
V lines joining them symmetrical. As time went on, more and more information on the variation in different parts of the world became available; it became very apparent that the construction of a Variation chart was no simple task, as not only was the distribution irregular, but the variation in any place was changing over the years. A number of variation charts were published in the eighteenth century, but the first Admiralty Variation chart was not printed until 1858. These charts enabled ships to obtain a very approximate longitude by observing variation, but when the chronometer was de­veloped longitude could be more accurately defined by astronomical means. The variation charts then began to be used as a means of correcting the compass heading—the purpose for which they are used today.
Modern techniques, and the vast store of information garnered over the years, now enable Variation to be predicted accurately for any given place and time. The earth's magnetic field, in other words, has been precisely plotted and its yearly change established. At the present time, the variation in Britain is changing annually by about 8 minutes of arc. There are also small seasonal and daily changes in variation although these are not of significance to the practical navigator. The peculiar distribution of places having equal variation has been charted and places where local magnetic effects are strong (due to a large amount of ferrous rock near the surface) discovered.
To the modern seaman, variation is no mystery but allow­ance must still be made for it if an accurate TRUE COURSE is to be steered at sea. Remember it is the angle between TRUE NORTH and MAGNETIC NORTH and is defined as WESTERLY VARIATION if Magnetic North is to the West of True, and EASTERLY if Magnetic is East of True North. (See Figures 3A and 3B).
The value of the variatio#in any particular place is shown on the chart of the area; on ocean charts `isogonic' lines, that is to say lines joining places of equal variation, are drawn at




intervals (usually of one degree) . On coastal charts of larger scale, the variation is usually shown inside the `compass rose', together with the amount by which it is increasing or decreasing annually. The Compass Rose on a chart consists of two con­centric circles graduated in degrees with the aid of which courses and bearings can be laid off on the chart. The North point of the outer (True) circle is aligned with the meridian and therefore indicates True North on the chart; courses and bearings laid off from this circle will be True. The North point of the inner (Magnetic) circle is aligned to Magnetic North at the date the chart was printed. Magnetic courses and bearings can therefore be laid off directly from the inner circle but it must be remembered that, owing to the annual change in variation, this will only give a reasonably accurate answer for a few years; if the chart was bought a long time ago, fairly large errors can result. Therefore it is usually preferable to look at the variation value written inside the compass rose, correct it for the annual change since the chart was published—the date and yearly change is shown next to the value—and then apply it arithmetically to obtain a True course or bearing. The method of doing this (with examples) is described in detail later in this chapter.

The Modern Magnetic Compass
There are many makes of magnetic compass now on the market, and it is not possible to describe them all here. The most common type is probably the liquid-filled compass with a horizontal compass card (designed to be viewed from above) suspended on a needle pivot within a sealed compass bowl. Another common small craft compass is contained in a transparent hemispherical bowl and has a vertically read card; the figures are engraved on the thick edge of the card and viewed from eye level.
In selecting a compass for a boat, the pocket will probably play a big part in the choice. However, some of the qualities to look for (or against) are listed below:
(a) It should preferably be gimballed, i.e. suspended in two rings which enable the bowl to keep level and comparatively still however much the craft pitches or rolls. If a compass is not gimballed, the card may foul the bowl as the boat moves and thus give an inaccurate and erratic heading. In a gimballed compass, the clearance between the card and the bowl allows a tilt of about 10 degrees which is quite adequate to cope with a sudden, sharp movement. Some compasses used in boats are of the un-gimballed type, similar to (or indeed may be) aircraft compasses. Most simple aircraft compasses are un­gimballed, but sprung in some fashion underneath by felt pads, springs, rubber cushions and the like. They also have a greater clearance between the card and the cover glass; normally about 200. This is sufficient in an aircraft for as the aircraft banks steeply (say 90°), centrifugal force causes the compass card to take up a false horizontal, dipping one side down in the direction of bank. However in a boat, the motion is different and unless there is adequate clearance, the card may foul. An analogy can be made with a bucket of water; if this is swung round and round by the handle (aircraft turning) the water will remain in the bucket: put the bucket on a swing, however, where the direction of motion is reversed (boat rolling), and it will slop over.
(b) Dry Card or Liquid Compass
Dry Card. In this type, a graduated card made of paper or mica, with magnets attached beneath it, is suspended by a cap and pivot bearing. The lighter the card, the less friction and wear is imposed on the pivot. There is a limit to the amount the weight can be reduced, as the magnets must be strong enough to move the card physically. Secondly, more than one magnet must always be used, as a single needle, suspended at its centre, would tend to align itself in the direction of any motion imposed on it; for example, if the boat rolled, a single bar would try to swing in the direction of roll due to its moment of inertia.
Apart from the inevitable wear on the pivot, the main disadvantage of a dry card compass is its comparative in­stability, particularly in a small craft with a short period of roll. This wandering of the compass can be reduced to some extent by making the compass have a long period of oscil­lation—by attaching an aluminium or other alloy ring to the perimeter of the card, or by surrounding it with a copper bowl in which magnetic eddy currents set up by the magnets tend to oppose movements of the card.

20 SMALL BOAT NAVIGATION THE MAGNETIC COMPASS 2t
The main advantages of a Dry Card compass are its cheap­ness and simplicity.
Liquid Compass. If the bowl containing the compass card is made watertight and filled with a liquid, some immediate benefits accrue. The liquid has a frictional damping effect on the card thus reducing any unwanted quick oscillations. The card and magnets can also be made just sufficiently buoyant to reduce the weight and friction on the pivot to a minimum. These are overriding advantages, and it was for this reason that many large ships went over to liquid compasses in about 1906.
There are, however, certain difficulties which must be overcome in the design of a good liquid compass. Obviously the liquid must not be allowed to evaporate or freeze. The liquid normally used is a mixture of water and alcohol (about 47% alcohol). To. prevent evaporation, the bowl must be completely sealed, but must allow the liquid room to expand as the temperature rises and then contract when the temper­ature falls without any bubbles forming. In good compasses, these factors are allowed for by the fitting of expansion chambers or bellows, usually made of copper, which expand and contract as the pressure changes inside the bowl.
Another problem which the designers had to face was that of liquid `swirl'. As the boat alters course, some of the liquid is drawn round the bowl by friction. The effect is mainly felt by the liquid near the edge of the bowl. To prevent this swirl affecting the card, a good liquid compass has a card of con­siderably smaller diameter than the bowl. This also reduces the effect of another undesirable feature which could be encountered in bad weather. With a heavy roll or pitch, the card becomes tilted relative to the bowl (particularly in a compass with no gimbals) and liquid tries to pass from below the card to above it. If the gap between the bowl and card is adequate, this liquid movement will have no serious effect. Some compasses have domed, transparent top glasses and this shape of case tends to reduce the swirl effects, whilst at the same time providing some magnification of the card,
Occasionally, if a compass is old and its joints perished, or perhaps it has been damaged in some way, a bubble will appear in the liquid. It may be that the compass is still usable in this condition, but there is always a danger that it may readincorrectly, particularly if the bubble is large and has been allowed to remain in the compass for some time. There is no doubt that the best solution is to take the compass to a reput­able compass maker for the removal of the bubble. However in emergency, the bellows cover or the top of some compasses can be removed by undoing the securing screws, lifting the top glass and rubber seal, and then refilling the bowl with alcohol and water. It is not easy to do this without trapping a certain amount of air. If alcohol cannot be had, distilled water is probably the best alternative additive, but dilution will, of course, raise the freezing point of the liquid. Neat alcohol, on the other hand, should not be used as too strong a solution will probably damage or raise the paints in the bowl or on the card.
Compass Graduations
Nowadays most compass cards are graduated from o degrees (North) right around clockwise to 859 degrees—the 360 degrees of a circle. Thus o° is North—ogo° East-18o° South-2700 West—and back again to North or 3600. But, partly as a tradition from sailing ship days, and partly because it is difficult to steer a boat to within i degree, many cards are still marked with some of the old compass points.
The old system was to divide the angles between the Cardinal points (North—East—South—West) first into quadrants and then further into points. A point was of i ii degrees, and there were therefore 32 points in the full circle. Each point was named—Boxing the Compass'—as shown in Figure 4.
Even on a modern compass, the Cardinal and Quadrantal points (NE—SE—SW—NW) are usually marked on the card inside the degree graduations. To set a particular course, the appropriate graduation on the card is brought opposite the Fore-and-Aft mark on the compass bowl—this is called the 'Lubber's Line'. For example, if North East is the course required, NE (0450) must be brought opposite the Lubber's Line by use of the helm.
Alignment of the Compass
Great care must be taken when the compass is first installed in a boat to ensure that the Lubber's Line is truly aligned fore and aft, i.e. parallel to the boat's keel. This is a comparatively
Even more difficult a problem is caused by the magnetisation of iron that is neither hard nor soft. This is sometimes called `Intermediate Iron', and the magnetic effect associated with it
SUB-PERMANENT magnetism. A piece of iron of this nature can
be magnetised quite easily, but only retains this magnetism over a limited period.
Deviation
The modern boat owner is therefore faced with the prospect that a good deal of iron, of one sort or another, may be within spitting distance of the compass. In a well designed craft, this factor will have been taken into account and the best possible position chosen for the compass, non-ferrous metals used near it, etc. However, there is a practical limit on what can be done, particularly in a craft with a metal hull. Tables, called `Safe Distance' tables, exist which list a wide number of objects—instruments, echo sounders, radio sets etc.—against their minimum safe distances from the compass bowl. These tables are principally of assistance to the boat builder, but may be of interest to the owner should he wish to add additional equip­ment or stow portable/potable objects on board.
Any magnetic material not outside these safe distances may cause a significant deflection or deviation of the compass needle. The amount of deviation caused will vary on different courses depending on the relative position of the compass and the object, and of the type of iron of which the latter is made.
The resulting `Angle of Deviation' on a particular heading is defined as the angle between Compass North and Magn1~tic North. An angle of Deviation described as 2 degrees West means that Compass North is 20 W of Magnetic North. This element of error is therefore defined in a similar way to Vari­ation, although its cause is different and (unlike Variation) its value will vary as the boat alters course.
The TOTAL COMPASS CORRECTION is a combined allowance for the errors caused by VARIATION and DEVIATION. Examples
are shown in Figures 3C, 3D, 3E & 3F.
Deviation and `Swinging'
The best magnetic position for a compass in a boat would probably be on top of the mast, but this would be a little
THE MAGNETIC COMPASS 25
inconvenient for the owner. The boat builder therefore chooses the best practical position for the compass consistent with ease of use at sea. Boats vary a great deal in their magnetic properties, but even in the best circles some magnetic influences are likely to be present. The Deviation of the compass on ALL headings must therefore be found.
This is normally done by `Swinging Ship', i.e. swinging the boat slowly through North—East—South—West (but not necessarily in that order) through the whole 360° whilst checking the compass against a known True bearing.
Qualified Swinging Officers are employed for checking the magnetic compasses of big ships, but part of the job is well within the capabilities of the average boat owner. It is not difficult to find the deviation of a compass although a bit more knowledge of the subject is needed to remove the errors once they have been found. (The latter process, Compass Correction, will be described in Chapter 2). Once established, the Deviation values can be used to correct the compass heading so that a true course can be steered.
WHEN—Ideally, a boat should be swung to observe compass Deviation:
(a) On first delivery to the owner.
(b) At the beginning of a new sailing season.
(c) On a large change in Magnetic Latitude (say io°).
(d) If any new fittings or equipment are put into the boat ... remember that new outboard motor.
(e) If you have any cause to mistrust it!
(f) Finally, and most important—if any of the correctors have been changed or moved. (Note small son's move­ments) .
WHAT STATE—Before a craft is swung, it is as well to have everything on board in its normal seagoing position-the anchor stowed, engine (if outboard) in its normal place, Mum's knitting needles firmly entrenched in her knitting and not near the compass—in fact all portable articles such as knives, keys, cans of beer kept well away from the scene of operations. Similarly, keep the boat away from other iron
S SMALL BOAT NAVIGATION THE MAGNETIC COMPASS 29
How to take the Bearing
Whatever method is used, an accurate compass bearing of the charted distant object, transit etc. must be taken. The larger craft are often fitted with a compass from which a bearing can be taken directly; some form of sighting attachment is fitted on top of the compass for this purpose. The earliest designs took the form of a notched sight, not unlike the back-sight on a rifle. Nowadays, these bearing devices, normally called azimuth circles, have a prism (and sometimes a telescope) mounted on a ring fitted over the compass card. The object is viewed through the `V' notch on top of the prism, and at the same time the reflected image of the compass card is visible beneath the `V'. A hairline running down the face of the prism from the bottom of the notch appears to cut the card image at the bearing obtained. The advantage of the prismatic arrange- ment over the old open sight is that the observer's eye need not be exactly aligned with the centre of the card when the bearing is taken. Providing the object is seen through the notch, and the reflected image of the card in the prism, the bearing will be accurate.
If the compass in your boat is fitted with an azimuth circle, well and good. The process of taking bearings for ordinary navigation as well as swinging for compass adjustment is straightforward. However, an azimuth circle is not much use if the compass itself is not placed in the boat so that a good all round view of the horizon can be obtained fi 6m it. Many compasses, such as those used for steering alone, are sited well down below the gunwale. The point here is that if the boat has one compass with good horizon visibility, bearings can be taken with its azimuth circle and any other compasses com­pared with it. If the boat has no compass with a view, or azimuth circle, then life is more difficult. In this event, a bearing plate should be used for swinging. Briefly, this consists of a graduated non-magnetic card which can be turned manually by means of a knob on its face-plate, over which is fitted an azimuth circle. The instrument is placed high up on its tripod on the centre-line of the boat, and its Lubber's Line accurately aligned with ship's head (by taking a sight on the bow). The next boat's compass heading to be checked–e.g. North East (o45°—is pre-set on the bearing plate opposite the
Lubber's Line. As the boat reaches this heading (ship's head 045° by the compass being checked) the shore bearing is taken from the bearing plate. The bearing obtained at that instant is directly equivalent to that which would have been obtained, were it possible, from the compass direct.
Perhaps hand bearing compasses ought to be mentioned here, as they provide an alternative method of taking bearings, albeit a fairly unreliable method in this context. They are simple hand held magnetic contpasses with some form of sighting device for taking bearings. The marine variety are normally fitted on top of a wooden handle. The snag with them is that, being portable, they suffer a varying and unknown amount from any local magnetic influences in the boat and are therefore not really reliable as checking instruments. The hand bearing compass could suffer a worse deviation than the fixed compass, and certainly its deviation will alter as it is moved from place to place in the boat; it will always be an unknown quantity. However, if these limitations are fully appreciated, the hand bearing compass can be an invaluable aid to yachts­ment for day to day use, and could be used, at a pinch, for checking another compass. When using it for this purpose, select a place in the boat where it is least likely to feel any magnetic influences. In a new boat, this will be largely a matter of guesswork, but in a known craft it will soon become apparent—from compass checks on passage and practical fixing-in which position the hand bearing compass can best be used and how good its results have been. Obviously, if it is known to perform accurately on board, there is nothing against using it for checking another compass.
The Swing
The business of swinging the boat round from one heading to the next should be done as slowly as possible. Ideally, one complete circle should take about an hour, but patience, or the call of less mundane things, may not permit this. Do not, however, ` be tempted to swing too fast as this may have an adverse effect due to certain induction influences on the compass. Forty minutes should be the minimum. This process has only to be done once, if it is done well, and you will sleep the sounder for it later on.
C
With a well placed compass, the taking of bearings should also be fairly simple. All you need is Qne bearing on every two points, or better still, every ten degrees; thus sixteen or thirty-six headings and their related bearings.
You may find that on a particular heading the shore object is `wooded', i.e. obscured from the compass by some part of the boat's structure. Therefore, if possible, have up your sleeve the true bearing of an alternative object. You will also find, with experience, that using the modern azimuth circle it is quite easy to take a good bearing `through' some obstruction, providing this does not have too large a spread across the field of view.
Procedure. In calm weather, and with the use of an engine, it is possible to carry out a swing alone and unaided. However, it is much simpler if there is another person on board to handle the boat whilst the `swinger' takes the bearings and notes down results. Before starting the swing, prepare a notebook listing in one column the sixteen (or thirty-six) ship's heads and in the other leave spaces for the compass bearing of the object against each ship's head. If using more than one distant object or interpolating between transits, another column will be needed for true bearings. It does not matter on which heading you start, nor does it matter in which direction, clockwise or counter-clockwise, the boat is swung. Steady the boat therefore on the nearest `s convenient ` heading and take an accurate bearing of your chosen object. Write this bearing down opposite the ship's head concerned. Then swing slowly on to the next heading and repeat the process, noting each time the compass bearing (and the true bearing if using more than one object), Continue the swing until all headings have been checked.
A few tips—you must take great care in getting accurate bearings but it is not vital that the ship's head be exactly on the chosen heading when the bearing is taken. A couple of degrees either way will not matter. Secondly, if you decide to use transits, and interpolate between them whenever necessary, it is as well to draw a small diagram in your notebook showing the true bearings of the transits so that you can interpolate between them on the spot and not have to keep referring to a chart.
Tabulating the Results. Having completed the swing, the results
THE MAGNETIC COMPASSy 31
must be tabulated for future use. An example is given below, followed by explanatory notes:
A B C D E
-- ,~~°raw)
True ~r
Bearing of ` Distant Magnetic Compass
Ship's Head Object/ Bearing of Bearing of
by Compass Transit Object Object Deviation/`

Deg. (°) Deg. (°) Deg. (0) Deg. (°) Deg. (°)`
N 0000 223° 228° 2320 40 W
NNE 0221 223 228 2331 51 W
NE 045 223 229 234 6 W
ENE o671 223 228 233 5 W
E 090 *279 284 z87 3 W
ESE II21 223 228 23o 2 W
SE 135 223 228 229 I W
SSE 1571 *2791k 284 282 2 E
S 18o 223 228 223 5, E
SSW 2021 223 228 22I 7 E
SW 225 223 228 223 5 E ,..
WSW 2471 223 228 224 4 E
W 270 223 228 225 3 E
WNW 2922 223 228 226 2 E
NW 315 223 228 228 Nil
NNW 3371 223 228 230 2 W
(I) * Denotes second distant object used, as the first was `wooded'
on these bearings.
(2) Variation—at place of swing-5° West.

Explanation of this table
COLUMN `A'—SHIP'S HEAD BY COMPASS
The successive Ship's heads—by the compass being checked—
are written in this column.
COLUMN `B'—TRUE BEARING OF OBJECT
Two distant objects have been used in the course of this swing. The True bearings of the objects from the swinging position .. are taken off the chart and entered in this column.
COLUMN `C'—MAGNETIC BEARING OF OBJECT
This is the bearing of the object after the Variation has been applied to the True Bearing. The Variation in this locality
LZ `_3