THE GYRO-COMPASS
The magnetic compass has been for ages the mariner's guide over the
trackless waters. In cloudy weather it has been his only means of
knowing the direction in which his craft was heading. Indeed, it is not
too much to say that the maritime commerce of the world was based upon
the behaviour of that little piece of magnetised steel.
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.
Could we entirely eliminate the effects of friction that might be used
as a compass, for it could be set, say with its axle pointing north and
south, at the commencement of the voyage, and it would remain so despite
all the evolutions through which the ship might go.
But there is a better scheme even than that, based upon the peculiar
behaviour of a revolving wheel when it has only two degrees of freedom.
Suppose that we dispense with the ring employed in the previous
arrangement, pivoting the ends of the axle between the prongs of the
fork. The wheel is then free to rotate, and its axle can slew round
through a complete circle by the turning of the fork in its socket, but
there can be no tilting of the axle. Being thus deprived of one of its
movements the gyroscope with three degrees becomes a gyroscope with two
degrees of freedom, and in that form it supplies the need for an
efficient and reliable compass.
The secret of the whole thing is the curious fact that a gyroscope with
two degrees of freedom exhibits a keen desire to place its axis parallel
with the axis of the earth. Owing to the shape of the earth, a device
such as has been described, with its fork standing up vertically, cannot
possibly have its axis really parallel with that of the earth, except on
the Equator. Still it gets as nearly parallel as possible. To be
scientifically accurate, we ought to say that it places it own axis "in
the same plane" as that of the earth.
To understand this we need to realise that all movement is relative. In
ordinary language, when we say a thing is still we mean that it is still
in relation to the surface of the earth, but since the earth is moving
the stillest thing, apparently, is really travelling at enormous speed.
Saint Paul's Cathedral in London, or a tall sky-scraper in New York,
would usually be regarded as supreme instances of immobility. It would
be hard to find better examples of stationariness, as we ordinarily look
at things. Each stands, firm and strong, upon a horizontal base. Yet
each is really turning a somersault every twenty-four hours. The plateau
upon which St Paul's stands, though it seems still and motionless
beneath our feet, is continually tilting; its eastern edge is
continually going downwards and its western edge upwards, as the earth
performs its daily spin. It is only a north and south line which does
not share in some degree this continual tilting action. Every plane,
large or small, so long as it remains horizontal, is being tilted thus,
down at the eastern edge and up at the western. And the plane in which
the axle of a gyroscope with "two degrees" is free to move is a
horizontal plane. Owing to its being held between the prongs of the
fork, while it can swing round to point north, south, east or west, or
towards any point between them, it cannot deviate from the horizontal
plane. Therefore such axle is always being tilted by the motion of the
earth, _except when it happens to be lying exactly north and south_.
Now for a reason which is too complex to go into here a gyroscope
strongly objects to having its axle tilted in this manner. If it be
compelled by superior force to submit to tilting, it tries to wrench
itself round sideways. Anyone who has a gyroscope top and cares to try
the experiment will feel this action quite easily. Hold the spinning-top
in your hand and turn it over so as to tilt the axle, when it will, if
you are not careful, twist itself out of your grasp.
So a gyroscope of the kind we are considering, when the motion of the
earth tilts its axis, turns itself round in its socket until at last it
reaches the north and south position, when the tilting, and therefore
the twisting, ceases. Hence the axle of the gyroscope if left to itself
(the rotation of the wheel being maintained the while) will place itself
in a north and south direction. And, moreover, it will keep in that
direction. It will take some force to slew it round into any other. And
if moved into any other by some extraneous means it will restore itself
to the old position again.
Hence a wheel thus arranged has all the attributes which we need for a
mariner's compass. But unfortunately there are mechanical difficulties
in the way of using such a simple contrivance for that purpose.
Chief of all these is the fact that it is not what engineers call
"dead-beat." That means that it will not go to the proper position and
then remain there quite still. Instead, it will first slightly overshoot
the mark, which being followed by the reverse action, it will come back
and overshoot it just as far in the opposite direction. Instead,
therefore, of a steady pointing, always in the same direction precisely,
it will oscillate more or less, the exact north and south line being the
mean or average position, the centre of the oscillations.
It would of course be possible to damp this, to apply a break as it
were, if the apparatus were to remain stationary. For example, if the
whole concern were immersed in water the resistance of the liquid would
restrain any quick movement of the axle, yet it would not prevent it
from slowly finding its true position. Thus the oscillations would be
reduced to such a small range as to be for practical purposes
negligible. But the drawback to a device of that kind, applied to a
gyroscope on board ship, would be that the axle would be carried round
to some extent every time the ship turned. As she changed direction it
would more or less carry round the water with it; that in turn would
carry the gyroscope, and so the direction of the latter would be for a
time untrue. It would in course of time regain its accuracy, but in the
meantime it would be leading the ship astray.
Consequently the application of this, in itself wonderfully simple,
idea, to this extremely important purpose was accompanied with a
difficulty which was for a long time insuperable.
But all was overcome at last by the genius of Dr Anschutz, of Hamburg,
whose firm were the first to turn out the practicable article. Taking
advantage of another movement of the gyroscope when arranged as has been
described, and using the revolving wheel itself as a centrifugal fan, he
was able to make the wheel blow air "against itself," as it were, when
in any position other than north and south. Thus, if it deviates towards
the east, this jet of air tends to blow it back; if it turns westwards
the jet again comes into operation, tending to bring the erring gyro
back to its proper place; and so the tendency to oscillate is checked.
The finished instrument as it is installed on the latest warships is, of
course, quite different in detail from the simple contrivance which we
have been considering so far, although it is the same precisely in
principle. The essential part is a heavy metal wheel combined with which
is an electric motor which keeps it rotating at a speed of 20,000 or so
times per minute.
The bearings of the wheel are supported upon a metal ring which floats
upon the surface of a trough of mercury. Thus friction is brought down
almost to the irreducible minimum. The only place where the wheel and
its supports touch anything solid is at one delicately made pivot which
serves to keep the floating mechanism in the centre of the mercury
basin, and to prevent it from rubbing against the side of it. The
current which drives the motor reaches it through this pivot and leaves
through the mercury. Thus arranged, although the floating part is of
considerable weight, a very slight force indeed is enough to move it;
while, looking at it the other way, we can see that the ship might turn
rapidly to right or to left, carrying round the mercury bowl with it,
without turning the floating part at all. Thus the gyroscopic action is
very free indeed to exercise its function of keeping the contrivance
pointing always in the one way.
The float has mounted upon it a compass card much like that of the
ordinary magnetic instrument, and the sailor reads it in precisely the
same way. To outward appearance there is little essential difference; in
one case there is a magnet under the card to keep it still, in the other
there is the float with the revolving wheel mounted upon it.
It is customary to have one "master compass" of this kind on a ship,
with an electrical repeater in each of the steering positions. As the
"master" turns in its casing it sends a rapid series of currents to all
the others, causing them to turn in unison with it. The "master" is
fitted in some safe part of the ship where it is least likely to be the
victim of any accidental damage.