It has always, however, been subject to certain faults. To commence with, it points, not to the geographical north, but to the "magnetic pole," a point some distance from the geographical pole, and one, moreover, which is not quite permanent. The fact that the magnetic pole varies its position is impressively shown by the fact that a special department at Greenwich Observatory is continually employed, by the aid of delicate self-recording instruments, watching and setting down its fluctuations. And the premier observatory of the world, it should be remembered, exists primarily, not in the interests of pure science, but as a department of the British Admiralty in order to study matters of interest to navigation. Thus we have testimony to the importance of these little vagaries on the part of the magnetic compass.
But in addition to these inherent faults there is a new source of error in the magnetic compass which man has introduced himself by making his ships of iron instead of wood. Every ship of the present day is a huge magnet. A piece of iron left in the same position for a length of time becomes polarised, which is to say that it acquires the properties of a magnet; and two magnets always exert an influence upon each other. Consequently the ship, after lying for perhaps a year in one position, during the period of building, becomes itself magnetic and interferes with its own compass.
Then, again, our methods of ship construction aggravate this trouble. It is believed that every molecule of iron is itself a minute magnet with a north and south pole of its own. These lying in confusion in the mass of unmagnetised iron neutralise each other, so that the mass, taken as a whole, does not exhibit any magnetic power. But if by some means the whole of the millions of millions of molecules can be set the same way—with all their north poles in one direction, and their south poles in the opposite direction—then they will all act together. Instead of neutralising each other they will then help each other, and under those conditions the mass of iron will possess that peculiar power which is distinctive of a magnet. So long as a piece of iron is left in the same position the magnetism of the earth is thus acting upon the molecules. Just as it tends to place the compass needle north and south, so it does with every molecule in the iron mass. And if, while lying still, the iron be hammered, the shaking of the molecules due to the hammering loosens them as it were and assists the earth's power in pulling them into position.
One has only, then, to watch the riveting up of a ship, and to see the vigorous way in which the riveters wield their hammers, to realise that when the thousands or even millions of rivets have all been finished the material of that ship will have had the very best possible chance of becoming magnetic.
To make matters worse still, ships are often loaded with great weights of iron among their cargo. That, too, may affect the compass. On warships there are the heavy guns, each weighing, with its turret, hundreds of tons, and they move, so that their effect upon the compass is not always the same, but may vary from time to time. And finally one may mention the electrical machinery in a modern ship consisting largely of powerful magnets.
Altogether, then, it is not surprising that the old magnetic compass is somewhat unreliable. It has to be coaxed into doing its duty. Pieces of iron and magnets have to be disposed about it to counteract these disturbing influences with which it is surrounded. Before a voyage experts have to come on board to adjust the compasses, and even then there is reason to believe that the instrument sometimes plays the ship false.
It is not to be wondered at, then, that the naval authorities in particular throughout the world have welcomed the advent of a new compass which appears to possess none of these drawbacks. It points to the geographical north, to the actual pivot, if one may so speak, upon which the earth turns. It is non-magnetic, so that the presence of iron or magnets even in its immediate neighbourhood has little or no effect upon it. On the other hand, it has to be driven by a current of electricity, and it seems just possible that in some great crisis it might fail, although every provision is made for alternative sources of supply in case of one failing, and there is always the possibility of falling back upon the old magnetic compass should the new one go wrong.
In principle the improved compass is, like its older brother, simplicity itself. The latter is but a small piece of iron magnetised; the former is nothing more than a spinning-top.
It is rather strange that although the spinning object has been a familiar toy for years, and that, moreover, its behaviour has been the subject of investigation by some very eminent scientific men, it is only of recent years that its principles have been put to practical use.
Everyone is familiar with the fact that a round block of wood will support itself upon a comparatively tall peg so long as it is rapidly rotating. And that is but one of the curious things which a rotating body will do. For example, imagine a wheel mounted upon an axle the ends of which are supported inside a ring, while the ring again is supported on pivots between the two prongs of a fork, the fork being free to swivel round in a socket. The wheel is then free to move in any direction. Technically, it is said to have "three degrees of freedom." It can spin round, its axle can turn over and over with the pivoted ring inside which it is fixed, while it can also swing round and round as the fork turns in its socket. Assuming that the joints are all perfectly free, that the pivots move in their sockets with perfect freedom—which, of course, they do not—then a wheel so mounted could move in any direction under the influence of any force that might act upon it. Now a wheel so mounted if left alone remains in precisely the same position so long as it goes on rotating. If it be turning sufficiently quickly its tendency to remain will be strong enough to overcome the friction of any ordinarily well-made instrument. Consequently a wheel of that description has been used to demonstrate the rotation of the earth, it remaining still (except, of course, for its rotating movement) while the earth has moved under it.