THE ELIMINATION OF THE QUADRANTAL ERROR
From our description of the cause of the quadrantal error it should
be clear that it is of a variable erratic nature, or at least that,
unlike the latitude and north steaming errors, its magnitude cannot
be forecast from a knowledge of the ship’s speed, course, latitude,
or other factors. Its direction is determined by the direction of
the ship’s course, but its amount is settled by the violence of the
rolling and pitching, and cannot therefore be calculated and tabulated
in a practically useful way. It follows, therefore, that to get rid of
the upsetting influence of the quadrantal error we cannot resort to
“correcting” the compass readings for it, but must entirely eliminate
it.
In the early (1910) Anschütz compass there were no means of eliminating
the quadrantal error, for the existence of the error, or at least the
importance to be attached to it, was not at first recognised. The
compass, we believe, showed errors from this cause of as much as 20
deg. to 40 deg. As a result, within a very short time the design was
discarded, and what amounted to an entirely different compass was
substituted for it. The 1912 Anschütz compass will be briefly described
later on, but here it may be said that the quadrantal error in it is
eliminated by adding two more gyro-wheels to the sensitive element. The
theory of this compass is not very easy to understand, so that it will
be best if we explain first how the quadrantal error is eliminated in
the Sperry and the Brown compasses.
The early Sperry, like the early Anschütz compass, was open to the full
action of the quadrantal error. In the form now in use, however, it is
prevented from taking effect by controlling automatically the position
of the excentric pin connecting the weight or “bail” to the gyro
casing. In the Sperry compass, as we know, the weight S (Fig. 35) is
not hung from the inner supporting ring or its equivalent, but from the
follow-up or phantom ring, its pendulous effect being communicated to
the casing and thence to the spinning wheel through the excentric pin
A. The kicks of the weight at the out positions during the rolling of
the ship are also transmitted through the pin to the casing and wheel
inside it.
[Illustration: FIG. 35. Sperry Compass on N.W. course.]
Let us suppose, as shown in Fig. 35, that at the port out position
of the compass swing the excentric pin is _east_ of the axis H J by
the amount required just to bring it vertically below the centre of
the spinning wheel. Then the southward kick of the weight when the
vessel rolls on a north-west course is transmitted to the casing as a
southward force applied at the point A. This force, acting about the
axis E F as before, produces precession in the direction K, and, as
before, we may resolve this movement into the components N M. The force
at A has, however, in addition, a turning moment about the axis H J,
for it is applied to the casing at a point lying at a distance A D from
this axis, and not, as in Fig. 33, at a point virtually on it. This
moment about H J would tend to make the end B of the axle turn in the
direction of F, and therefore results, in accordance with the rule of
the gyroscope, in an actual movement about the axis E F, the end B of
the axle precessing downwards towards J, that is, in the direction R.
We may resolve the precession R into two components T U.
Similarly, at the starboard out position, let us suppose that the
excentric pin is this time _westwards_ of the axis H J by exactly the
amount required again to bring it vertically below the centre of the
wheel. As before, in the case shown in Fig. 33, the northward kick of
the weight acting about E F results in a precession L, which may be
resolved into the components P Q. The kick, however, as at the port out
position, also has a moment about the axis H J. The kick is reversed
in direction, but the point of its application to the casing is now on
the other side of H J. Consequently the kick tends to rotate the casing
about H J--and therefore produces precession about E F--in the same
direction as does the kick at the port out position. The precession
produced is represented by V, and can be resolved into the components
W X.
Considering now the four pairs of component precessions, we see that
they cancel out in pairs. Thus N and Q cancel, U cancels X, T wipes out
M, and W does the same to P. The two components M P, which were the
cause of the quadrantal error, are just balanced by the two additional
components T W. The axle therefore does not rise vertically, but
remains horizontal, and as a result no quadrantal error can arise.
It will be seen that the Sperry method of eliminating the quadrantal
error requires the excentric pin to be movable relatively to the
bail and casing. To be more precise, while the casing, the bail, the
phantom ring, and all the other parts of the compass may swing round
the axle of the spinning wheel, or the external gimbal axis coincident
or parallel with the axle, matters have to be arranged in such a way
that when the ship rolls the excentric pin shall not partake of this
motion, but remain constantly in the vertical below the centre of the
spinning wheel. We have to remember, however, that the Sperry method of
damping the horizontal vibrations of the axle hangs upon the pin being
displaced towards the east of the axis H J when the compass is in the
even keel position. The two requirements are met by so controlling the
position of the pin relatively to the bail and casing that when the
bail, casing, etc., swing sideways under the influence of the rolling
of the ship, the pin is maintained at a fixed distance eastwards of the
true vertical through the centre of the spinning wheel at all points in
the oscillation of the bail, casing, etc.
[Illustration: FIG. 36. Sperry Ballistic Gyro.]
The stabilisation of the excentric pin in this manner is effected
gyroscopically by means of the attachment shown in Fig. 36. This device
consists of a small high-speed electrically driven gyroscope running
inside a casing which is mounted rotatably on a vertical axis inside a
stirrup frame. This frame, as shown in Fig. 37, is hung pendulum-wise
on the north side of the main gyro casing, the axis of its suspension
being collinear with the axle of the main gyro-wheel. The axle of the
small gyro-wheel is thus aligned in the east and west direction. The
stirrup bracket is turned up horizontally below the casing of the
small gyro, and at its end is fitted with guides, carrying a pair of
rollers. These rollers constitute the excentric pin and, as shown in
Fig. 37, engage within two curved channel-sectioned tracks attached one
to the bail and one to the main gyro casing. If when the bail, casing,
etc., swing under the influence of the ship’s rolling the excentric
pin should attempt to follow suit--either by reason of friction at the
axis of suspension of the stirrup bracket or at the track rollers--the
wheel and casing of the small gyro will start processing round the
vertical axis inside the stirrup, for the attempt is equivalent to an
endeavour to tilt the small gyro-axle in an east and west vertical
plane, and therefore calls forth the usual gyroscopic reaction. The
precession of the small gyro on its vertical axis is made to react on
the stirrup bracket by means of a spring connection between the bracket
and the casing, as shown in Fig. 36. The direction of spin of the small
gyro-wheel is such that the force thus applied to the stirrup bracket
opposes and just balances the frictional or other force trying to make
it swing with the bail, casing, etc., of the main gyro. In this way the
excentric pin as the vessel rolls is caused to maintain its original
distance from the vertical line through the centre of the spinning
wheel.
[Illustration: FIG. 37. Stabilized Excentric Pin (Sperry Compass).]
The suppression, or rather the avoidance of the quadrantal error in
the Brown compass, is achieved in a manner which is mechanically very
distinct from that adopted for the same purpose in the Sperry compass.
[Illustration: FIG. 38. Diagram of Brown Compass.]
An elementary diagram of the Brown compass is given in Fig. 38. In this
sketch A is the casing, inside which the gyro-wheel mounted on the
axle B (C) rotates in the direction of the dotted arrow--B, as before,
representing the north-seeking end of the axle. The casing is supported
on an east and west horizontal axis E F inside a vertical ring, this
ring being journalled at H J inside a frame D, the equivalent of the
square frame in our simple model. To obtain complete freedom for the
gyro the frame D is mounted on an athwartship axis G K, which is
itself carried inside a ring journalled within the binnacle on an axis
L parallel with the ship’s longitudinal centre line. The pendulum
weight S is fixed to the lowest point of the frame D. If the weight
S and frame D be set swinging on the axis G K, the swinging movement
will, of course, be directly communicated through the journals H J to
the vertical supporting ring, but, as the trunnions E F of the casing
are really supported on knife edges, the swinging movement of the frame
D cannot be communicated from the vertical ring through the trunnions
E F to the casing A, and thence to the wheel. Yet it is essential that
the weight S should be able to act pendulum-wise on the casing and
wheel, for otherwise, as we know, the system would be without directive
force.
[Illustration: FIG. 39. Oil Control Bottles (Brown Compass).]
The connection between the weight and the casing is not a mechanical
one, but is effected by making use of the air blast created by the
fan-like action of the wheel inside the casing. As we have explained
in connection with the damping system adopted in this compass, the
trunnion F is hollow, and delivers the air blast through a nozzle M,
fixed relatively to the vertical supporting ring, into a divided box N.
From this box pipes are led to two oil bottles--one of which is shown
at P--fixed to the casing on the east side of the axle, one bottle
being on the north face of the casing and the other on the south. The
pressure of the air blast acting unequally upon the oil in these two
bottles when the casing tilts about the axis E F results, as we have
already explained, in any horizontal oscillation of the sensitive
element about the axis H J being damped. In a similar way two bottles
Q R half-filled with oil are fixed to the north and south faces of
the casing on the west side of the axle of the spinning wheel. These
bottles are also connected to the box N, but the connecting pipes are
crossed so as to join each bottle Q R to the division of the box
remote from it, and not to the adjacent division, as in the case of
the damping bottles P. When, therefore, the weight S and frame D are
swung slowly pendulum-wise on the axis G K, as shown in the first view
in Fig. 39, the nozzle M, being fixed to the vertical ring, delivers
more air into one division of the box N than the other, and therefore
a greater air pressure is exerted inside one bottle than inside the
other. Consequently oil flows through the connecting pipe T from the
former bottle to the latter until the columns of oil are sufficiently
unequal to balance the difference of pressure of the air inside the
bottles. It will be noticed that the crossing of the pipes connecting
the bottles with the box N results in the oil being accumulated in
that bottle, which lies away from the side to which the weight S has
been swung. Hence, although there is no mechanical connection between
the weight and the casing of the spinning wheel, the effect when the
weight S is swung slowly is thus substantially the same as it would be
if there were, for the weight of the extra oil forced into the bottle
R exerts a turning moment on the casing about the axis E F, and tends
therefore to make the casing follow the deflection of the weight S and
frame D.
Similarly, should the axle B C dip, as shown in the second view in Fig.
39, extra oil will accumulate in the bottle which has been elevated by
the dipping, and as a result a restoring moment about the axis E F will
be applied to the casing, just as it would be if the weight S had been
directly connected to the casing and had been deflected by the dipping
movement.
The second view in Fig. 39 illustrates the generation of the directive
force in the Brown compass. Let us suppose that the compass is at the
equator, and that by some agency the axle is turned so that its end B
points due west. The rotation of the earth will cause the axle to dip
slowly into some such position as that shown. During this slow dipping
movement oil will flow slowly from the bottle Q into the bottle R. The
unbalanced weight of oil will apply a turning moment to the sensitive
element about the axis E F and will therefore produce actual motion
about the axis H J, which will precess the end B of the axle towards
the north. Any tendency for the axle to vibrate about the north and
south line as a result of the momentum acquired by the sensitive
element while coming up from the west will be damped by the air blast
acting upon the oil in the two other bottles P fixed, as shown in Fig.
38, on the east side of the axle.
[Illustration: FIG. 40. Brown Compass on West Course.]
The point we require here to notice especially about this action is
that, as the tilting of the axle produced by the rotation of the earth
takes place very slowly--it cannot exceed the speed of rotation of the
earth on its polar axis, namely, 0.0007 of a revolution per minute--the
flow of oil from the bottle Q to the bottle R takes place very slowly
also. The oil therefore acquires practically no momentum, and rises in
the bottle R in strict accordance with the tilt acquired by the axle.
If the tilting motion be stopped or reversed, the oil would remain in
the bottle R at just the level it had reached, or would immediately
begin to flow back again, for its momentum being negligible, it has not
sufficient kinetic energy to rise higher in the bottle R after the
upward motion of the bottle has ceased.
On the other hand, when the ship rolls, say, on a due west course,
as represented in Fig. 40, the outer gimbal ring supporting the
athwartship axis G K, the frame D carrying the weight S, and the
vertical supporting ring will acquire an oscillation about the fore
and aft axis L in tune with the rolls of the ship. The air blast
will thus be directed into the two divisions of the box alternately,
and therefore oil will flow from one bottle to the other. It is to
be noticed, however, that the rolling motion of the ship induces in
this manner a flow of oil from one bottle to the other at a very much
greater rate than does the tilting action of the earth’s rotation dealt
with above.
A ship rolling through 45 deg., out to out, in a complete period of
10 seconds rotates about its rolling centre with an average velocity
equivalent to 1.5 revolutions per minute--or over 2000 times as fast as
the speed of rotation of the earth on its polar axis--and at the mid
point of its roll it will move with an actual velocity of about twice
the average. The momentum acquired by the oil in flowing from bottle to
bottle is therefore in this case not negligible. In fact, when the ship
reaches one of its out positions and starts to return, the oil does not
immediately begin to flow back into the other bottle, but is carried
by its kinetic energy to a still higher level in the bottle in which
it has been rising. As a result, the oscillation of the oil between
the two bottles lags behind the oscillation of the pendulum weight S,
and therefore that of the ship itself. The lag acquired is such that
the oil is just level in the two bottles when the ship is at either of
its out positions, and when the ship is at the mid point, or even keel
position of its roll, although the air blast is for the moment evenly
distributed between the two compartments of the divided box, the oil
is standing at its maximum level in the bottle on that side of the
wheel from which the ship is recovering herself. The action of the oil
in the bottles during a rapid oscillation of the compass system is,
in fact, quite analogous to that of the water in the Frahm system of
anti-rolling tanks.
It will thus be seen that the net effect of transmitting the “kicks”
derived during rolling from the pendulous weight of the wheel through
the Brown oil bottle arrangement is simply to delay the application of
the kicks to the gyro-wheel by a constant amount, namely, by the time
taken for the ship to roll from either out position to the mid position
or a quarter of a complete period. Thus as the ship sailing due west
rolls through the mid position from starboard to port the compass
system experiences the turning moment about the axis E F, which with a
rigidly fixed pendulum it would receive at the starboard out position
from the northwards kick of the weight. Similarly, the equivalent of
the southwards kick of the weight at the port out position is felt by
the wheel when the compass is passing through the even keel condition
on the subsequent roll from port to starboard. It is clear that with
the ship sailing due west--or east--this delaying of the kick does
not affect the result established previously for a rigidly connected
pendulous weight. Any tendency for the axle to precess towards the
west when the compass is passing through the even keel position in one
direction is completely annulled by the tendency to precess towards the
east at the succeeding passage through the even keel position in the
opposite direction.
On quadrantal courses, however, the delay in the application of the
kick is most important, for when the ship rolls it results in the
elimination of the quadrantal error. That it does so can easily be
understood by reference to Fig. 33. The kicks, being received when
the compass is passing through the even keel position, and not at
the out positions, precess the wheel about the axis H J at a time
when this axis is truly vertical, and not when it is inclined. The
precessional movements have therefore no vertical components M P. They
are represented completely by the horizontal components N Q. The axle
thus does not depart from the horizontal plane, and any movements in
this plane arising from the tendency to precess in the directions N Q
cancel each other at successive passages of the compass through the mid
position.
The early form of Anschütz compass was followed by the 1912 pattern in
which the quadrantal error was successfully eliminated. An example of
the 1910 form was obtained by Messrs. Elliott Brothers, of Lewisham,
from Anschütz and Co., of Kiel, and was, we believe, fitted on board
H.M.S. _New Zealand_. Its defects becoming apparent, its manufacture
in this country was not proceeded with, but upon the appearance of the
improved type in 1912, Messrs. Elliott took up its construction and
supplied several to the Admiralty. The Sperry compass, however, secured
the preference in the British Navy, and with the outbreak of the war
Messrs. Elliott ceased practically to make Anschütz compasses. On the
other hand, the German Navy continued to use the 1912 type of Anschütz
compass, with very little alteration or addition, right throughout the
war. In view of the fact that every German submarine was fitted with a
compass of this form, and bearing in mind the high degree of excellence
attained in the navigation of the enemy’s underwater craft, there can
be no doubt that the modern Anschütz compass is a very satisfactory
device.
In Fig. 41 we give a purely diagrammatic representation of the compass.
The outer square frame may, as usual, be regarded as mounted within
external gimbal rings, providing a fore-and-aft axis and an athwartship
axis. The square frame contains a vertical ring free to turn about the
axis H J and itself containing a horizontal ring mounted on an east
and west axis F E. The pendulum weight S is attached, as usual, to the
inner horizontal ring. The essential difference between the 1912 and
the 1910 forms of Anschütz compass lies in the fact that, as shown in
our diagram, the inner ring, or its equivalent, does not surround a
single gyro-wheel, but has attached to it the casings of three distinct
gyros.
[Illustration: FIG. 41. Diagram of Anschütz (1912) Compass.]
As shown in the first plan--in Fig. 42--the three gyros are situated
at the corners of an equilateral triangle. One gyro K is placed at the
south end of the meridional diameter of the horizontal ring, with its
axle pointing towards the centre of the ring. The two other gyros L M
are placed at 60 deg. east and 60 deg. west of north, with their axes
pointing towards the centre of the gyro K--_not_ towards the centre
of the ring. The wheels of all three gyros rotate anti-clockwise, as
seen from the north, looking south--that is, they rotate in the same
direction as do the wheels of all single-gyro compasses as seen from
the same standpoint.
[Illustration: FIG. 42. Plans of Gyros.]
If we neglect the two gyros L M for the moment, imagining them to be
replaced by simple deadweights to counterbalance the weight of the gyro
K, it will be seen that this system differs from that of the early
Anschütz compass only in the fact that the gyro-wheel has been reduced
in size, and has been displaced from the centre of the horizontal ring
to the southern edge. The displacement of the wheel in this manner in
no way affects the essential working of the compass. In a single-gyro
compass we might with some advantage similarly displace the spinning
wheel, for by so doing and by counterbalancing the displaced weight
we should, at least partially, get rid of the concentration of mass
about the east-west axis, which, as we shall see, introduces, if not
corrected, an additional source of error into the compass readings when
the ship rolls on quadrantal courses.
The reduction in the size of the gyro-wheel would, of course, reduce
the magnitude of the directive force. In the 1912 design the wheels of
all three gyros run at 20,000 revolutions per minute--that is, at the
same speed as the wheel in the 1910 design. But each wheel is only 5
in. in diameter instead of 6 in., and weighs, with its axle, 5 lb. 2
oz., or only half as much as the wheel in the earlier form. The gyro K
considered alone would therefore supply a directive force of but half
the former amount.[6]
It follows, obviously, that if a second gyro of the same speed and size
as that at K be fixed in accurate alignment at the north end of the
meridional diameter of the horizontal ring the directive force at any
angle of horizontal deflection away from the north will be doubled,
that is, will be made equal to that developed in the 1910 design. Such
a compass might be constructed, but it would exhibit the quadrantal
error--or at least the inertia force portion of that error--just as
badly as did the early Anschütz.
The essence of the 1912 design lies in the fact that not one but two
gyros are attached to the north side of the horizontal ring, and in the
additional fact that the axles of these two gyros are not parallel with
but are inclined to the axle of the south gyro K.
When the sensitive element of this compass is in the north resting
position, the axle of the gyro K is aligned with the meridian and
therefore the rotation of the earth--the compass being supposed at the
equator--merely moves the axle parallel with itself. On the other hand,
with the sensitive element in the north resting position, the axles of
the gyros L M are inclined to the meridian at 30 deg. The gyro L is
virtually in the condition of a single-gyro compass, with the north end
of its axle partially turned towards the east. Under this condition, as
we know, the rotation of the earth will tend to make the north end B of
the axle rise above the horizontal plane. Conversely, the gyro M is in
the condition of a single-gyro compass, with the north end of its axle
partially turned towards the west. The earth’s rotation in this case
tends to make the north end of the axle dip below the horizontal plane.
Thus, in the 1912 Anschütz compass, when the sensitive element is in
the north resting position, the gyro K under the rotation of the earth
is without effect on the pendulous weight S, the gyro L is striving
to swing it towards the north, and the gyro M is trying with an equal
effort to swing it towards the south. The weight, therefore, remains
in the plumb line and applies no turning moment to any of the gyros.
As there is no turning moment, there is no precessional tendency. The
sensitive element remains directed towards the north. This alignment,
as in a single-gyro system, is the true resting position, and under it
no directive force is applied to the sensitive element.
To study how the three gyros act together to restore the sensitive
element to the north resting position should it be deflected
therefrom, let us suppose that the deflection suffered is one of 30
deg. towards the east. As shown in the second plan in Fig. 42, the
effect of such a deflection is to place the three gyros K L M in the
condition respectively of a single-gyro system when the axle is (_k_)
deflected 30 deg. to the east, (_l_) deflected 60 deg. to the east,
and (_m_) aligned on the north. At this deflection, then, the gyro M
contributes no restoring force. The directive force contributed by the
gyro K is half that contributed by the single gyro of the 1910 design
when the deflection is 30 deg., for the mass--or moment of inertia--of
the wheel is half the earlier value. The directive force contributed
by the gyro L is something greater than that contributed by the gyro
K, for the virtual deflection eastwards is 60 deg. instead of 30 deg.,
and, as we know, the directive force increases with the deflection.
Thus, the three gyros taken together supply a directive force, when
the sensitive element is deflected through 30 deg., which is somewhat
greater than that supplied by the single double-sized wheel of the
1910 design.[7] This result is a general one. Whatever the deflection
may be, the directive force supplied by the three-gyro compass is
always about one-third greater than the directive force of the 1910
design at the same deflection. This increased force is developed even
if the deflection be less than 30 deg. In such cases the gyro M will,
of course, supply a force, a non-restoring force, to the sensitive
element. Only when the deflection exceeds 30 deg. eastwards does the
gyro M assist the gyros K and L to restore the sensitive element to
the north resting position. If the deflection be to the west, however,
the gyro M is the chief assistant of the gyro K, a laggard’s part being
played by the gyro L until 30 deg. of westerly deflection is reached.
The manner in which this three-gyro system avoids the quadrantal error
can now be discussed. The quadrantal error, it may be recalled, arises
when the ship rolls on an intercardinal course, and is primarily
caused by the fact that the whole compass system can swing on its
external gimbal axis in tune with the rolls of the ship. If we could
so arrange matters that during the roll of the ship from side to side
the compass system would swing on the external gimbals, no more and no
less than the amount required just to keep the axis H J truly vertical
at all points of the roll, then the north and south “kicks” received
on the weight S at the out positions of the roll would cause the
sensitive element to precess first one way, then the other, but always
in a horizontal plane. There would be no vertical component in the
precession, the cumulative effect of which, as we have seen, produces
the quadrantal error.
In the Brown compass, the quadrantal error is eliminated by delaying
the effect of the “kicks” on the weight S until the axis H J is truly
vertical--that is to say, the “kicks” on the weight are not transmitted
to the spinning wheel until the compass system is passing through the
even keel position. In the Sperry compass the weight S is virtually
shifted back and forth from east to west of the axis H J in tune
with the rolling of the ship in such a way as to introduce a second
component of vertical precession, which in amount and direction is just
sufficient to nullify the vertical component causing the quadrantal
error. In the 1912 Anschütz compass, the object aimed at is the
maintenance of the axis H J truly vertical at all times during the
rolling condition.
This object is achieved, as we shall now show, by setting the axles
of the gyros L M--Fig. 41--inclined to the axle of the gyro K. In a
single-gyro compass, or in the 1912 Anschütz compass with the gyros
L M suppressed, the system is very stiff against vibrations on the
east-west axis E F, but is quite easily set vibrating about the
north-south axis provided by the external gimbal mounting. The reason
is, of course, that any vibration about the axis E F is met by the
resistance of the gyro-wheel, for the vibration causes the axle to
alter its direction, whereas no gyroscopic resistance is called into
play by the vibration on the north and south axis, for this axis is
coincident or parallel with the gyro-axle. Thus, in the 1910 Anschütz
compass the period of vibration about the east-west axis was something
like 70 min., whereas the period about the north and south axis was
only one or two seconds. The smallness of the latter period readily
resulted in the compass system getting into a swing in tune with the
period of rolling of the ship--from five to twelve seconds or so. Were
the axles of the gyros L M in the 1912 form set parallel with that
of the gyro K, there would still be no gyroscopic resistance exerted
against vibration on the north-south axis. As it is, the inclination of
the axles of these two gyros, taken together, has virtually the same
effect as would be obtained by the addition to the sensitive element of
a separate single gyro, with its axle aligned at right angles to the
axle of the gyro K.
The gyroscopic stiffness against vibration about the east-west axis
E F is hardly affected by the inclination of the gyros L M at the
angle chosen by the designers. The resistance, instead of being that
contributed by three gyros, is equivalent to that supplied by about
2¾ gyros. The period of vibration on this axis E F is arranged to be
the standard 85 min. About the north and south axis we have virtually
the resistance of one gyro. The period of vibration about this axis,
instead of being only one or two seconds, is about 80 sec. With this
lengthened period of vibration there is practically no chance of the
compass system getting into a swing in tune with the rolling of the
ship, for every 2½ to 6 seconds the vibrating influence is reversed
in direction, and in this interval of time the system cannot acquire
any substantial degree of swinging motion. Thus, as the compass,
following the rolls of the ship, moves from side to side, it suffers
a pure translational movement. The axis H J remains truly vertical at
all instants during the roll, and therefore the north and south kicks
of the pendulous weight at the out positions of the roll precess the
gyro-axle about H J in purely a horizontal plane, the precessional
tendency at one out position cancelling that at the other. There is no
vertical component in the precession, and therefore there can be no
quadrantal error.
It is of interest and of some amusement to note that during the war
the Germans applied a fourth gyro to the Anschütz compass, and that
this additional gyro was carefully removed from every compass before
they surrendered their submarines to us. The deceit was, however,
of no avail, for the application of the fourth gyro was known to
us long before the war ended, a complete compass so fitted having
been recovered from a sunken submarine and repaired and carefully
studied. The fourth gyro was applied to the external gimbal rings.
These rings, when the submarine rolled, were found to acquire at times
a violent oscillation of their own, for, of course, they received
no stabilisation from the gyroscopic elements of the compass. On
board submarines the violence with which the rings vibrated would
occasionally threaten to wreck the compass. By adding a gyroscope to
the gimbal rings, the period of vibration of the rings was lengthened
to about 16 sec., so that little or no opportunity to swing was left
to them. With the exception of this addition, the Anschütz 1912
compass was used by the Germans throughout the war practically without
alteration.