Marvels of Scientific Invention Part 16

Thus as the pen in the artist's hand draws his sketch, so does the automatic hand at the other place, it may be at a great distance, repeat faithfully his work, and the sketch grows line by line simultaneously at both ends.

There is not s.p.a.ce here to detail how, by another current superposed upon those referred to already, the receiving-pen is made to dip itself periodically into the inkwell at the will of the sender. By a cunning use of alternating current this is done without in any way interfering with the action of the cranks as described above.

But of course there is a severe limitation to the usefulness of this machine, inasmuch as the drawing has to be made at the time of transmission, and it can only be "put on the wire" by the hand of the artist himself.

CHAPTER XIV

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 a.s.sertion 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 supposit.i.tious 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 circ.u.mstances, 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 pa.s.sing 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 acc.u.mulation of errors can possibly occur, and the problem, though apparently so difficult, if not insuperable, at first sight, is surmounted.

CHAPTER XV

SCIENTIFIC TESTING AND MEASURING

Science, whether it be of the pure variety, that which is pursued for its own sake--for the mere greed for knowledge--or applied science, the purpose of which is to a.s.sist manufacture, is based entirely upon accurate testing and measuring. It is only by discovering and investigating small differences in size, weight or strength that some of the most important facts can be brought to light. There are some problems, too, that defy theory, since they are too complicated; they involve too many theories all at once, and such can only be solved by accurate tests. And all these necessitate the use of very ingenious and often costly devices.

Electrical measuring instruments were of sufficient importance and interest to warrant a chapter of their own, but there are many others of great value, and not without interest to the general reader.

For example, some years ago there was a collision in the Solent, just off Cowes, between the cruiser _Hawke_ and the giant liner _Olympic_.

The cause of this was a subject of dispute and of litigation; the theorists theorised; some reached the conclusion that the _Hawke_ was to blame, and others the _Olympic_; and where doctors disagree who shall decide? It was wisely decreed that tests should be made to settle the question.

The main point was this. The officers of the _Hawke_, by far the smaller vessel, averred that they were drawn out of their course by suction caused by the movement of so large a ship as the _Olympic_ in the comparatively narrow and shallow waters of the Solent; in other words, that the _Olympic_ in moving through the water caused a swirling, eddying motion in the water, tending to draw a lighter vessel towards itself. And that is just one of those problems with which theory is unable to deal. So it was transferred to the National Physical Laboratory at Teddington, near London, for investigation by experiment.

At this inst.i.tution, which is a semi-national one, there is a tank constructed for purposes such as this. The word tank leads us to underestimate its size somewhat, for it is 494 feet long and 30 feet wide. It is solidly constructed of concrete, with a miniature set of docks at one end, and a sloping beach at the other.

On either side are rails upon which run trollys which support the ends of a bridge which spans the whole. This bridge can be propelled along, by means of electric motors operating the wheels of the trollys, from one end of the tank to the other, at any desired speed, within, of course, reasonable limits, and from it may be towed any model which it is desired to test.

The models used are usually made of wax, by means of a machine specially designed for the purpose. It should be explained that the plans of a ship consist of a series of curves, each of which represents the contour of the vessel at one particular height. For example, if you can imagine a ship cut horizontally into slices of uniform thickness, then each slice could be shown on the drawing (the "shear plan," as it is termed) by a curved line. Near the keel the lines would, of course, be almost straight, but they would bulge more and more as they occur higher up.

And what this machine is required to do is to make, quickly and economically, a wax model which shall be an exact reproduction, on a small scale, of the vessel under discussion. It may be--it most often is--a ship as yet unbuilt, the behaviour of which it is desired to test.

Or it may be an existing vessel, as it was in the case mentioned just now. However that may be, the model is made from the drawings.

A block of wax rests upon a table, while the drawing is spread upon a board near by. A pointer is moved by hand along one of the lines, and its movement is repeated by a rapidly revolving cutter which cuts away the wax to a similar curve. By suitable adjustments the cutter can be made to magnify or reduce the size, so as to produce any desired scale.

Thus every line is gone over and a similar curve cut in the wax at the correct height. Of course this only produces a lump of wax shaped _in steps_, as it were, but it is then quite easy to trim it down by hand, so as to produce a smooth model of the ship, perfectly accurate in its shape, and a copy on a small scale of the vessel portrayed on the drawing.