A WONDERFUL EXAMPLE OF SCIENCE AND SKILL
In the preceding chapter reference was made to the fact that for the
successful sending of pictures "by wire" one thing was necessary above
all others. That one thing consists in making two machines, perhaps
hundreds of miles apart, start working together, stop together and, when
working, turn at exactly the same speed. Let the reader just picture the
problem to himself, and ask himself how such an arrangement can be
possible. Let him think of a town two hundred miles away and then
meditate on the possibility of making a machine working in his own room
and another in that distant town maintain perfect unanimity in their
movements. The result of such reflection will probably be the assertion
that such a thing is beyond the bounds of possibility. Then he will find
the following description of how it is done extremely interesting.
In the first place it must be understood that each machine is driven by
an electric motor. The motors are designed to run at 3000 revolutions
per minute, and they drive the cylinders of the machines through gearing
so arranged that the latter turn at 50 revolutions per minute.
Now of all machines perhaps the most docile and easily managed is the
direct-current electric motor. Each such machine is made with a view to
its working at a certain speed, but that can be varied within certain
limits, by simply varying the force of the current which drives it. And
that force can be very easily varied by the use of an instrument called
a "rheostat" or variable resistance. We are all familiar with the way in
which the engine-driver regulates the speed of a locomotive, by means of
a valve in the steam-pipe. The opening and closing, more or less, of
the valve enables the speed to be changed at will and adjusted to a
nicety. The rheostat is to the electric current what the valve is to the
steam; it can be opened and closed, more or less, as necessary. By it
the current driving the motor can be made stronger or weaker, and as
that change is made so does the speed of the motor change accordingly.
Thus we see that there is at hand the means of setting a motor to work
at any desired speed.
The difficulty, however, is to tell when the desired speed has been
attained. One can count the revolutions of a machine at two or three
revolutions per minute with a certain amount of accuracy, but fifty
revolutions per minute are more than one could count correctly. Still
less could we count the 3000 revolutions every minute of the motors.
Thus, even if we had the two motors side by side, we should have extreme
difficulty in making them work at the same speed exactly. One might be
doing 3000 while the other did 2990 or 3010 and we should be none the
wiser. And when we separate the two by a distance of many miles, the
task of synchronising them is even worse.
But fortunately there is a simple contrivance by which we can tell very
accurately the speed of a motor. The reader has already been
familiarised, in previous chapters, with the difference between direct
or continuous electric currents and alternating ones. It is the
continuous sort which is used to drive these motors, but a slight
addition to the machine will make it so that while direct current is put
in, to drive it, alternating current can be drawn out of it. Two little
insulated metal rings are fitted on to the spindle of the machine, and
these are connected in certain ways to the wires of the motor; then
against these rings, as they turn, there rub two little metal arms,
called, because of their sweeping action, brushes; and from these
brushes we can draw the alternating current.
For our present purpose the importance of this lies in the fact that the
rate at which that current will alternate depends upon the speed of the
motor. As the motor increases or decreases in speed, so will the rate of
alternation increase or decrease. So that if we can measure the rate at
which the current drawn from the motor is alternating, we shall know
from that the rate at which the machine is working.
This we can do by the aid of a "frequency meter." The working of this is
based upon the acting of a tuning-fork. Everyone knows that a given
tuning-fork always gives out the same note. The note depends upon the
rate at which the fork vibrates, and the reason that one fork always
gives the same note is because it always vibrates at the same rate. That
rate, in turn, depends upon its length. If one were to file a little off
the end of a tuning-fork, its note would be raised, because its rate of
vibration would become faster. Similarly, lengthening the fork would
result in a lower note being given. Thus, a tuning-fork, or any bar of
steel held by one end, and free to vibrate at the other, gives us a
standard of speed which is very reliable. And it so happens that we can
easily use a set of such forks to test the rate of alternation of an
alternating current.
Generally speaking, alternating current is no use for energising a
magnet. The chief reason for that is that the current tends to get
choked up, as it were, in the coil. Alternating current traverses a coil
very reluctantly indeed. It is, however, possible to make an electric
magnet of special design which will work sufficiently well with
alternating current to answer our present purpose. And it will be clear
that just as the alternating current itself consists of a series of
short currents, so the force of the magnet will be intermittent; it will
give not a steady pull, as is usually the case with magnets, but a
succession of little tugs. There will, in fact, be one tug for every
alternation of the current.
A simple form of motor fitted up as just described, and rotating at 3000
revolutions per minute, would give out 100 alternations per second. If,
then, such current were employed to energise a magnet, that magnet would
give 100 tugs per second.
So a small steel bar of the right length to give 100 vibrations per
second can be fixed with its free end nearly touching such a magnet, and
when the current is turned on it will very soon be vibrating vigorously.
For the tugs of the magnet will agree with the natural rate of vibration
of the bar. And just as the two pendulums described in Chapter XII.
responded readily to each other, so the bar responds readily to the
pulls of the magnet. But increase or decrease the rate of alternation
ever so slightly, and that sympathy between magnet and bar is destroyed.
The bar will not then respond. It will only answer when the pulls of the
magnet and the natural rate of vibration of the bar exactly correspond.
So it is usual to place five or six such bars with their ends near the
one magnet. The lengths of the bars vary slightly, so that the rates of
vibration are, say, 98, 99, 100, 101, 102 respectively.
Let us, in imagination, adjust the speed of a supposititious motor until
we get that which corresponds to 100 alternations.
We switch on the current and at first, possibly, we get no response from
any of the vibrating bars. Just a touch to the handle of the rheostat
and we notice that bar 102 shows signs of life. We see then that our
first speed was much too fast, and that reducing it has brought it down
to 102, which is still a little too fast. Just a little more movement of
the handle, and 102 begins to relapse into quiet, while 101 shows
animation. A little more movement and 101 gives place to 100, and then
we know that our motor is working at the desired speed. If our motor had
been too slow to commence with, it would have been 98 which first got
into action, but the method of adjustment would have been precisely the
same.
And thus we see the whole scheme. We regulate the speed by the rheostat,
and meanwhile that tell-tale stream of alternating current comes flowing
out of the motor to indicate to us what the speed is, while the
"frequency meter," with its various vibrating bars, interprets to us
the message which the alternating current brings to us. So by watching
the meter we know when we have got the speed that we desire.
But even that is only half the battle. We have seen how to make a
machine turn at any desired speed, and so we can adjust any two, so that
they revolve at the same speed, but we have not seen how to start and
stop the two machines at the same time.
First of all, it must be understood that in the case of the receiving
machine there is a friction clutch, as it is termed, between the motor
and the cylinder which it is driving. That means that while, under
ordinary circumstances, the motor drives the cylinder round, we can, if
we like, hold the latter still without stopping the motor. When we do
so, the connection between the two simply slips.
So if we fit a catch on the cylinder which is capable of holding it from
rotating, we can still start the motor, and the latter will work. Then,
the moment the catch is released the cylinder will begin to turn too.
The commonest form of "friction drive" is the flat leather belt upon two
pulleys, which everyone has seen at some time or other in a factory. And
it will be quite easy to conceive how, if one of the driven machines
were to stick, the belt might simply slip upon one of the pulleys, yet,
as soon as the machine became free again, it would rotate just as it did
before. It is just the same with what we are considering. The motor
works continuously at its proper speed, but the cylinder can be stopped
when desired by the catch.
Combined with the catch is an electro-magnet, and through its coils
there flows the current of electricity which is engaged in printing the
picture on the cylinder. If a magnet be arranged to attract another
magnet, it will do so only when the energising current flows one way.
When it flows the other way, it does not attract. Therefore it is easy
to arrange matters so that the printing current, though passing through
the coil of the magnet, shall not pull open the catch. But if that
current be _reversed_ in direction for a moment the magnet gives a pull,
open flies the catch, and away goes the cylinder upon its revolution.
Thus, we see, all that is necessary to start the receiving cylinder is
to reverse the current for a moment.
And now let us turn our attention to the sending machine. Upon its
cylinder there is an arrangement which automatically reverses the
current flowing to the main wire once in every revolution. Normally the
current flows to the wire as described in the last chapter, carrying by
means of its variations the details of the picture for reproduction by
the receiving machine at the other end. But for an instant once in every
revolution that current is interrupted and a current sent in the
opposite direction instead. This the sending machine does of itself,
quite automatically.
And now the reader knows of all the apparatus; it remains only to see
how the different parts work in combination.
Standing by the sending machine we first of all turn on the current,
which goes coursing along the wire to the distant station. Then we set
the motor to work and the cylinder begins to rotate. Before it has
completed a single revolution the "reverser" is operated, and just for a
moment the reverse current goes to the wire. On arrival at the other end
that lifts the catch and the receiving cylinder starts. That first
partial revolution of the sending cylinder counts for nothing. Real
business begins when the reverser first acts, and that is the moment
when the receiving cylinder also begins to move. Similarly, when the
sending cylinder stops it sends no more reversed currents, and so the
receiving cylinder is caught by the catch and not released.
So starting and stopping are quite automatic. The same arrangement
enables a continual readjustment of the relative speed of the two
cylinders to take place. With all the best devices, the tuning-forks and
the rest, it is still impossible to attain perfect unanimity, but the
variation in a single revolution cannot be enough to matter; it is only
when the error in one revolution goes on multiplying itself that serious
difference might arise, and that is prevented in the following
beautifully simple way.
The motor which drives the receiving drum is so regulated that it
travels _slightly faster_ than does the other. Thus the receiving
cylinder completes every revolution slightly in advance of the other,
and consequently it is stopped and held by the catch every time. The
catch retains it, of course, until the reverse current arrives and
releases it. Thus not only does the sending cylinder start the other
when the operations first commence, but it does so every revolution.
Every revolution, therefore, the two cylinders start together.
So the two cylinders are set, according to the frequency meter, at as
nearly as possible exactly the correct speeds, and the action of the
reverser, the reverse current and the catch, ensures quite automatically
that at the commencement of every revolution there shall be perfect
agreement between the two. No accumulation of errors can possibly occur,
and the problem, though apparently so difficult, if not insuperable, at
first sight, is surmounted.