THE ANSCHÜTZ (1910) COMPASS
Some few words of description fall to be made regarding certain
mechanical and electrical features of the principal types of
gyro-compass--features, that is, which, not being primarily connected
with the gyroscopic behaviour of the compass, have not so far been
mentioned in our discussion of the general theory of the device.
A sectional diagrammatic view of the early form of Anschütz compass is
given in Fig. 44. This compass, although it is now obsolete by reason
of the fact that no provision was made in its design for the quadrantal
error, is still, we think, of interest from other than the historical
point of view.
[Illustration: FIG. 44. The Anschütz (1910) Compass.]
The gyro-wheel is shown at A. The axle is carried in bearings inside
the casing B. This casing is attached to a tubular stalk C, which is
fixed to a head D carrying the compass card R. This system of parts
as a whole is floated in mercury Q contained inside a circular steel
bowl K, the flotation being secured by means of a hollow steel ring S
joined to the head D by a dome-shaped member E, which is perforated
all over for the sake of lightness. As the float S is completely
immersed in the mercury, the exact level to which it will sink can be
delicately settled by controlling the form of the bore inside the head
D. The central position of the float inside the bowl is controlled
by means of a steel stem T fixed at its upper end to the centre of
the glass G covering the card and pointed at its lower end to dip
into a cup containing mercury which is fixed centrally inside the
stalk C. The stem T is insulated, and is enclosed within a tube, the
upper end of which is also attached to the glass G and the lower end
of which is splayed out to dip into a second mercury cup surrounding
but insulated from the first. Through this tube and its mercury cup
one phase of a three-phase current is led to the motor driving the
gyro-wheel from the terminal F. The second phase reaches the motor
from the terminal H by way of the stem T, while the third phase is
transmitted through the bowl, the mercury Q, and the float. The bowl is
provided with knife-edge bearings at L, whereby the whole system shown
is mounted within two gimbal rings providing the external longitudinal
and athwartship axes. The outer gimbal ring is supported inside
the binnacle by means of springs, the attachments of these springs,
as well as the gimbal supports, being insulated on account of the
method adopted for transmitting the third phase of the current to the
gyro-motor. The gyro-axle runs on ball bearings. The driving motor
comprises a stator carrying the windings and fixed inside the casing B
and a rotor forming a rigid part of the gyro-wheel itself.
It will be seen that this design of compass does not explicitly
reproduce the horizontal axis E F, nor the pendulous weight S of our
simple model. The float is, however, free to tilt within the bowl
in any direction, so that the system of suspension is practically
equivalent to the provision of an infinite number of horizontal axes
E F. As regards the apparent absence of the pendulous weight, it is to
be noted that the centre of gravity of the floating system is below
its metacentre, and therefore that the pendulum effect is exactly
reproduced. The damping details are not shown in the engraving, but
they are substantially as we have described them previously.
In view of the high speed, 20,000 revolutions per minute, at which the
gyro-wheel runs, giving a stress in the rim of about 10 tons per square
inch and a peripheral velocity of 340 miles an hour, it is of interest
to note that during a test to destruction the wheel did not fail until
it was being driven at a speed involving the supply to it of five times
the normal driving power. A special motor generator had to be built
to enable this test to be carried out. Even the normal motor driving
the gyro-wheel had to be specially designed, for no motor capable of
running at 20,000 revolutions was at the time commercially available,
while the small space within which it had to be accommodated rendered
the question of temperature rise a very serious one. It was found,
it may be added, that in designing the motor the usually accepted
magnetic constants for the iron in the motor did not hold good with a
periodicity as high as 333 cycles per second.
The flotation of the sensitive element in mercury is a simple and
convenient method of obtaining a practically frictionless support for
the vertical axis--the axis H J of our elementary model. The absence
of friction arises from the facts that the drag of the mercury on
the parts of the sensitive element with which it is in contact is
proportional to the velocity with which the parts move through it,
and that this velocity is always extremely slow. If the mercury is
freshly distilled, there is, it would appear, substantial absence of
frictional drag. But in course of time dust, oil, etc., may collect on
the surface of the mercury, and may introduce a drag on the sensitive
element of sufficient amount to interfere with the accuracy of the
compass readings. The suspension of the vertical axis by flotation
has, however, the advantage that no external gimbal rings need be
provided to give the sensitive element three degrees of freedom in all
configurations. The sensitive element, as we have said, is virtually
provided with an infinite number of horizontal axes, so that within
the bowl itself there is always a longitudinal and an athwartship axis
about which the sensitive element may turn. The external gimbal axes
provided in this compass permit the bowl to remain horizontal when the
ship rolls or pitches; they are not essential gyroscopically. A similar
remark applies to the later form of Anschütz compass in which the
sensitive element is also floated in mercury.