SOME ADJUNCTS IN THE ENGINE ROOM
Before we deal with the subject of the engines employed in warfare, it
may be interesting to mention two beautiful little inventions which have
been made in connection with them.
Let us take first of all a contrivance which tells almost at a glance
the amount of work which the engines of a ship are doing.
As everyone knows, there is in every ship (except those few which are
propelled by paddles) a long steel shaft, called the tail-shaft, which
runs from the engine situated somewhere near amidships to the propeller
at the stern. Many ships, of course, have several propellers, and then
there are several shafts. Now each of these shafts is a thick strong
steel rod supported at intervals in bearings. If anyone were told that,
in working, that shaft became more or less twisted, he would be tempted
to think he was being made fun of. Yet such is literally the case. The
thick strong massive bar becomes actually twisted by the turning action
of the engine at one end and the resistance of the propeller at the
other. And the amount of that twisting is a measure of the work which
the engine is doing. The puzzle is how to measure it while the engine
is running, for of course the twist comes out of it as soon as the
engine stops.
A space on the shaft is selected, between two bearings, for the fixing
of the apparatus. Near to each bearing there is fitted on to the shaft a
metal disc with a small hole in it. On one of the bearings is fixed a
lamp and on the other a telescope. When the engine is at rest and there
is no twist in the shaft, all these four things--the lamp, the two
holes, and the telescope--are in line. Consequently, on looking through
the telescope the light is visible. But when the engine is at work and
the shaft is more or less twisted one of the holes gets out of line and
it becomes impossible to see the light through the telescope. A slight
adjustment of the telescope, however, brings all four into line again,
which adjustment can be easily made by a screw motion provided for the
purpose. And the amount of adjustment that is found necessary forms a
measure of the amount of the twisting which the shaft suffers and that
again tells the number of horse-power which the engine is putting into
its work.
But it is also necessary to know how fast the engine is working. There
are many devices which will tell this, of which the speedometer on a
motor-car is a familiar example. Most of those work on the centrifugal
principle, the instrument actually measuring not the speed but the
centrifugal force resulting from the speed, which amounts to the same
thing. There is one instrument, however, which operates on quite a
different principle, because of which it is specially interesting. It
consists of a nice-looking wooden box with a glass front. Through the
glass one sees a row of little white knobs. If this be placed somewhere
near the engine while it is at work immediately one of the knobs
commences to move rapidly up and down, so that it looks no longer like a
knob but is elongated into a white band. There is no visible connection
between the instrument and the engine, yet the number over that
particular knob which becomes thus agitated indicates the speed of the
engine.
Let us in imagination open the case and we shall find that the knobs are
attached to the ends of a number of light steel springs set in a row.
The springs are all precisely alike except for their length, in which
respect no two are alike. Indeed, as you proceed from one side of the
instrument to the other each succeeding one is a little longer than the
previous one. Now a spring has a certain speed at which it naturally
vibrates and other things being equal that speed depends upon its
length. You can, of course, force any spring to vibrate at any speed if
you care to take the trouble, but each one has its own natural speed at
which it will vibrate under very slight provocation.
Every engine is, of course, made to run as smoothly as possible. All
revolving or reciprocating parts are for this reason carefully balanced
and in turbines the whole moving part, since it is round and
symmetrical, naturally approaches a condition of perfect balance. Hence
every engine ought to run perfectly smoothly. As a matter of fact,
however, no engine ever does. There are certain limitations to man's
skill and at the high speed of a fast-running engine, such as is to be
found on a destroyer, for example, some little irregularity is sure to
make itself felt by a slight vibration in the floor. It may be hardly
perceptible to the senses, but to a spring whose natural frequency
happens to be just that same speed or nearly so, it will be very
apparent and in a few seconds that spring will be responding quite
vigorously.
It is another example of the principle of resonance, which is employed
so finely in making wireless telegraph apparatus selective. Every
wireless apparatus is made to have a certain natural frequency of its
own and it therefore picks up readily those signals which proceed from
another station having the same frequency while ignoring those from
others. In just the same way a reed or spring in this speed-indicator
picks up and responds to impulses derived from the engine only when they
are of a frequency corresponding with its own natural frequency. Hence,
one spring out of the whole range responds to the vibrations of the
engine while the others remain almost if not entirely unaffected.
In another form, the springs are actuated electrically. A magnet, or a
series of magnets, is arranged so that as the engine turns the magnets
pass successively near to a coil of wire, thereby inducing currents in
that wire. They form, in fact, a small dynamo or generator, generating
one impulse per revolution or two or three or whatever number may be
most convenient. Then the current from this is led round the coil of a
long electro-magnet placed just under the free ends of all the springs.
The magnet therefore gives a series of pulls, at regular intervals, and
the rapidity of those pulls will depend upon the speed of the engine,
while the frequency of them will be registered by the movement of one or
other of the springs.
This instrument can also be employed to determine the speed of aeroplane
motors and, in fact, any kind of engine, especially those whose speed is
very high.