The Telewriter This remarkable instrument transmits actual writing and drawings, the receiving pen copying precisely the movements of the sending penThe TelewriterThis remarkable instrument transmits actual writing and drawings, the receiving pen copying precisely the movements of the sending pen
In the Korn transmitter for photographs selenium is employed as follows:—A transparent photograph is made, on a celluloid or gelatine film, and this is fixed upon a glass cylinder mounted as already described. A pencil of light falls upon this in much the same way as in the case of the receiver just described, and, as the cylinder revolves, describes a fine spiral line all round and round it.
Moreover, the light passes right through the photograph and falls upon a mirror inside, off which it is reflected on to a selenium cell. At every moment, then, the light is falling upon some small part of the photograph, and the amount of it which gets through and ultimately reaches the selenium depends upon the density of that part.
Current, meanwhile, is flowing from a battery through the selenium, and thence over the main wire to the distant station. As the light pencil traces its spiral path over the rolled up photograph every variation in the density of the picture is reproduced as a variation in the current through the selenium. This, at the remote end, operates the Einthoven galvanometer, the movements of which vary the shade of the spiral line being drawn upon the photographic paper.
This process takes place with remarkable celerity, so that in a few minutes the innumerable variations constituting a complete photograph can be transmitted and faithfully recorded at the distant end of the wire.
But perhaps the most successful of these methods is that known as the telectrograph. It is surprisingly like the scheme of Caselli in principle, and forms another example of the fact that good ideas often fail through lack of the proper means to carry them out. Mr Thorne-Baker, the inventor of the telectrograph, has had at his disposal accumulated stores of knowledge and skill which did not exist in Caselli's time. Consequently the former has made a brilliant success where his predecessor produced only an interesting but somewhat ineffective attempt.
Reference has been made already to the half-tone blocks wherein a host of small dots of varying sizes make up apicture. Now instead of parallel rows of dots parallel lines of varying thickness will give very much the same result. The former are made by photographing the picture through a sheet of glass ruled with two sets of lines at right angles to each other. The latter can be made by using a screen with lines one way only instead of two ways. It is therefore quite easy for a blockmaker to produce a "process block" wherein lines are used instead of dots. For this particular purpose, however, it is not an ordinary block that is needed, although it is in essentials very similar. The picture to be transmitted is photographed through a screen as if a half-tone block were to be made. The negative so obtained is then printed by the gum process on to a sheet of soft lead and, after washing, the picture remains upon the lead in the form of lines of insoluble gum on a background of bare lead. A squeeze in a press drives the gum into the lead, and so gives the whole sheet a smooth surface over which a stylus will ride easily, but which is, nevertheless, made up of conductive parts and non-conductive parts, the latter forming the picture.
The lead sheet is then put upon a revolving cylinder and turned under a moving stylus in the manner with which we are now familiar. The sheet is placed with the lines lengthwise of the cylinder so that current passes to the stylus except as it passes over the breadth of the lines, and so similar lines are built up at the distant end.
The receiving mechanism is of the electro-chemical type which Caselli used. The current passes from the receiving stylus to the paper, and there makes its mark in a way that will be understood from the description of the earlier apparatus.
The supreme advantage of this method of working, over that of Professor Korn, is that the operator can see what he is doing. To obtain good results, a number of electrical adjustments have to be made, and whether he has got them right or wrong can be seen as soon as the picture begins to grow upon the receiving paper. If a little readjustment be needed the operator sees it and can set things right before the reallyimportant part of the picture begins to appear, whereas with the Korn apparatus he does not know what is happening at all, since he can see nothing until the picture is finished and the photographic paper has been developed.
It will be apparent, too, to anyone who has carefully considered the wireless telegraphy chapters, that it ought to be possible to make the sending stylus or its equivalent control a wireless transmitter and a wireless receiver to operate the receiving stylus, so as to be able to send pictures by "wireless." Experiments to this end have been made with some measure of success, and sooner or later we are almost sure to hear that the difficulties, which are by no means small, have been overcome.
But we cannot conclude this chapter without a fuller reference to that marvellous invention, the telewriter.
In this a man makes a sketch with a pen on a piece of paper, or maybe he writes a message, and simultaneously a pen, hundreds of miles away if need be, does precisely the same thing. The receiving instrument draws the sketch line by line, or it transcribes the message in the actual handwriting of the sender. A little touch, almost weird in its naturalness, is that every now and then the receiving pen leaves the paper and dips itself into a bottle of ink, after which it resumes its work at the very spot where it left off.
Now how the complicated lines and curves, the strokes and dots which make up a written language, even the little shakes and defects which give each man's writing a personality of its own, how all these can be sent over a wire is at first sight very difficult to understand. The inventor of this apparatus has discovered an extremely simple way of doing it.
But even he does not attempt to do it with one wire, it should be said, for he uses two. This is no drawback when, as is often the case, it is used in conjunction with a telephone, for the latter, to be effective, also requires two wires. Years ago single wires were employed for telephones as for telegraphs, the circuit being completed through the earth. But the difficulty arose that every wire through which currentsflow is apt to induce currents in neighbouring wires—the induction coil is based upon that fact—and so messages in one wire were overheard on others, or, what was perhaps more annoying still, the dots and dashes passing in a telegraph wire would produce loud noises in a telephone wire that happened to be near. The use of two wires, however, entirely removes that trouble, for the neighbouring current then induces two currents instead of one, one in each, and it so happens that these are opposed to each other, so that they neutralise each other. So every telephone wire now is double and therefore is ready, as it were, to have the telewriter fitted to it.
But even with two wires the difficulty seems insuperable until we remember that the most complex of curves can be resolved into two simple movements.
The sending pen, with which the original writing or drawing is done, is attached to the junction of two light rods. The farther end of each rod is attached to the end of a light crank fixed so that it can rotate or oscillate, after the manner of cranks, in the plane of the desk upon which the paper lies. All the joints mentioned are of the hinge nature, so that as the pen is moved about the rods turn, more or less, one way or the other, the two cranks. This simple mechanism, it will be observed, carries out very effectively the principle just mentioned, for it resolves the motion of the pen, no matter how complicated it may be, into a simple rotating motion of the two cranks.
So the cranks turn this way or that as the draughtsman makes his picture, and it is very easy to arrange that their movement shall vary the strength of two electric currents, whereby we obtain electric currents varying in accordance with the movement of the cranks.
This is done by making each crank operate a variable resistance or rheostat. When in its extreme position on one side the crank permits current to flow freely, but as it moves over to the other extreme position the resistance in the path of the current is increased. Such an arrangement is a common feature in electrical apparatus.
So current from a battery flows to the two wires leading to the distant station, each passing through the rheostat connected to one of the cranks. We may think of the rheostats as taps which can be turned on or off by the action of the cranks. Let us imagine that crankais in the position when the current flows freely—when the electrical "tap" is fully open; then a strong current will flow along wirea, returning to the sending battery via the earth. As that crank is moved the current will gradually be reduced, until, if it be moved right over to the other extreme, the current will be at its feeblest.
Fig. 15.—A Message received by Telewriter.Fig. 15.—A Message received by Telewriter.
Arrived at the other end, this current passes to a device which we may describe simply as a magnet so arranged that its action pulls round a crank against the restraining action of a spring.
Now the stronger the current the more does that magnet pull and the farther does the receiving crank turn. The sending crank varies the resistance, the resistance varies the current, the current varies the strength of the receivingmagnet, and the magnet varies the position of the receiving crank. Properly adjusted, then, the motion of the crank at the one end is communicated through that long chain of causes and effects, until at last it is repeatedexactlyby the movement of the crank at the other end.
The same thing occurs simultaneously over each of the two wires, crankaat the sending end communicating over wireato crankaat the other end, while crankbcommunicates its motion over wirebto the other crankb. Each sending crank is closely imitated in its every action by the corresponding one at the distant station.
The two receiving cranks are connected by light rods to the receiving pen in precisely the same way that the sending pen is connected. Consequently, not only are the separate movements of the two cranks repeated at the remote station but the complex movements of the sending pen, which gave rise to the actions of the cranks, are also conveyed to, and repeated by, the recording pen. The movements of the first pen are resolved into rotating motions by the two cranks, these are transferred to the other cranks, and their movements are in turn converted back into the written curves.
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 space 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.
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 alternatedepends 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 "frequencymeter," 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 ifthat current bereversedin 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 isonly 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 travelsslightly fasterthan 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.
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 assist 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 cruiserHawkeand the giant linerOlympic. The cause of this was a subject of dispute and of litigation; the theorists theorised; some reached the conclusion that theHawkewas to blame, and others theOlympic; 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 theHawke, 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 theOlympicin the comparatively narrow and shallow waters of the Solent; in other words, that theOlympicin 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 institution, 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 isspread 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 shapedin 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.
It can also be hollowed out, ballasted with weights inside, and so made to sink to any desired level, thereby representing the vessel when fully loaded, half loaded and so on. All sorts of unequal loading can be produced if needed, indeed every condition of the real ship can be imitated in the model.
It can then be towed to and fro in the tank by the travelling carriage described above. The speed of towing can be varied by changing the speed of the motors which drive it. The force needed to pull the model through the water is measured by means of a dynamometer which registers the pull on the towing apparatus.
A matter very often needing investigation is the shape and size of the wave thrown up by the bow of the vessel, and of that left behind her, known as the "bow wave" and the "stern wave" respectively. These waves represent wasted energy, for they are no use and are produced actually by the power of the engines of the ship as they drive her along. The ideal ship would cause no waves, but since that is a degree of perfection impossible even to hope for, the shipbuilder has to content himself by so designing his ships that these waves shall be as small as possible.
The waves are recorded photographically, in some cases by the kinematograph.
Some of the large shipbuilders have their own tanks, andso have the naval authorities of the great naval Powers. The one at Teddington was established through the munificence of a famous British shipbuilder, Mr Yarrow, who not only defrayed the cost of construction, but gave an endowment to assist in its upkeep. It is intended to serve the needs of the smaller builders who have not tanks of their own, and also for the investigation of matters of general interest to shipbuilders, and for such tests as that relating to theHawkeandOlympic. In this last-named case, of course, two models were made, one to represent each ship, and they were towed along in such a way as to imitate very closely the movements of the ships at the time when they collided. It was as the result of these tests that theOlympicwas ordered to pay damages to the Admiralty, it being held that she was the cause of the accident.
A very interesting investigation of this kind was recently carried out in the tank at the United States Navy Yard. The port of New York consists very largely of jetties projecting out from the banks of the river. With the growth of the Atlantic liner the old jetties had become too short, and questions arose as to the elongation of them. If it were done, how would it effect the current in the river, and the handling of shipping generally? If, on the other hand, it were not done, what would be the effect of the ships lying with their ends projecting out into the stream unprotected by a jetty.
To determine these points the experimental tank was converted into a model of the New York Harbour, or at all events of that part in connection with which these questions arose.
A false floor was put in, so as to make the depth exactly right in proportion to the width. Little model jetties were arranged to represent exactly the real ones, while against them were moored model vessels, so that the effect upon them could be observed as the model of the large vessel was towed past.
In addition to this, special appliances were arranged forfinding out what the disturbance might be which the movement of a giant liner produces under the surface as well as above it. For this purpose buoyant balls were employed, moored at various distances below the surface, from which thin rods projected upwards, the movement of which rendered visible the movements of the submerged balls and therefore the effects of the under-water currents.
All these things had to be observed at one and the same time—the moving model itself, the models alongside the jetties, the commotion on the surface, the swayings to and fro of the rods attached to the submerged floats—all, or most of which, at all events, it was impossible to make self-recording. Yet, seeing that it was of the utmost importance that the relations between all these things should be observed, and recorded from time to time as the model was towed along, it is evident that something must be done, and a cunning use of the kinematograph solved the problem quite easily. At various points commanding a good view of the model harbour and its shipping these machines were placed, and so several series of photographs were obtained, by the study of which all the different movements could be seen and compared. A large dial too was rigged up upon the travelling carriage by which the model was towed, a finger on which denoted the distance which the carriage had travelled at any moment. This large dial came into each photograph, of course, and so each picture bore upon itself a clear record of that particular moment in the voyage of the model to which it referred.
Thus we see an instance of how the very latest and most up-to-date methods of amusement are sometimes applied to serve very practical purposes.
Akin to the experiments upon ships are aerial experiments to determine matters connected with the navigation of the air. At Barrow-in-Furness the great firm of Vickers, shipbuilders and armament manufacturers, and latterly builders of aerial craft for the British Admiralty, have erected a machine for testing the efficiency of aerial propellersand other things of a kindred nature. Upon the top of a tall tower there is pivoted a long arm of light iron framework. To the end of this a propeller can be fixed, so that as the arm revolves there is produced almost exactly the same conditions as those which prevail when a propeller drives an aeroplane or steerable balloon.
By means of suitable mechanism the propeller can be turned at any desired speed, with the result that it drives the arm round and round upon its pivot on the top of the tower. The force which the propeller thus exerts can easily be measured, and so can be determined such questions as the most efficient speed for each type of propeller, the power which any particular one can develop, the best form for each particular need, and so on.
Materials, too, require the most careful testing, in order that they may be put to the best possible use in modern machinery and structures. For example, anyone can measure the strength of a spring, but what do we know as to its lasting power? Springs often have to form part of a machine in which they are stretched and compressed millions of times, and the question arises as to what is the best shape and material for the purpose. It may be that the spring which works best a few times will be the first to become "weary," for with repeated strain such things as steel get tired, just as the human frame does. Now that is a matter which will yield to no calculation, the only way to determine it is actual test. So a mechanism has to be employed which will extend and compress the spring over and over again, just as it will be in actual use, with a counter of the nature of a cyclometer to count how many times it has been subjected to this distortion. Then the apparatus is set going and left to itself for hours, or even for days, during which time it may work the spring millions of times. This may go on until it breaks, or else it may be done a prearranged number of times, and then the spring taken out and tested by other means to see how its strength has been affected.
Metal bars are often subjected to sudden blows, light in themselves but oft repeated. The point to be determined then is how many times the blow may fall before permanent injury is done to the bar. To investigate such matters we have the "repeated-impact" machine. The bar is held in a suitable holder, under a hammer which gives it a blow, the force of which can be easily regulated, at regular intervals, the number of blows being counted by a suitable recording mechanism. Ultimately the bar breaks, under a blow the like of which it can endure singly without any apparent strain at all. The machine, by the way, can be caused to turn the bar round to some degree after each blow, so that it is struck from all directions in succession.
The microscope, too, has established its place in the testing laboratory. It is a very valuable adjunct to chemical and mechanical tests.
Suppose, for example, that a bar of steel is being investigated; it can be put into a machine and pulled until it breaks in two. The machine registers the amount of the pull which was applied. Or a small piece can be put under a press and compressed to any desired degree. It can also be tested by impact or even pulled apart by a sudden blow, as described inMechanical Inventions of To-day. The bar can be supported by its ends and loaded or pulled down in the centre, so that its power of resisting bending can be determined. It can be judged, too, from its chemical composition. Steel, in particular, depends for its properties very largely upon its chemical composition. The difference between cast-iron, wrought-iron and steel, also the differences between the innumerable varieties of steel, are due almost entirely to the admixture of a certain percentage of carbon with the metal. This can be ascertained by chemical analysis. This form of inquiry has the advantage over the more purely mechanical methods in that the latter, for the most part, have to be applied to the bar as a whole, whereas the quality may vary in different parts, the surface in particular being liable to differ from the interior. In suchcases, one analysis can be made of a piece cut from the surface and another of a piece from the centre.
And it is here, too, that microscopical analysis comes in. For this purpose a piece is sawn off the bar, and the end ground perfectly smooth. This is then washed in a suitable chemical, such as a mild acid, which acts differently upon the different materials of which the "metal" is built up, thereby rendering them visible one from another. A photograph taken through a microscope then shows the structure of the metal; how the different constituents are built together.
This is known as metallographic testing, and its advantage as compared with chemical analysis is that the latter shows, as we might say, what are the bricks of which the thing is built, while the former shows how the bricks are arranged. Indeed it is hardly correct to speak of the advantage or superiority of one over the other, since each is the complement of the other, supplying the information which the other fails to give.
And there are other mechanical tests which have not yet been mentioned. There are machines which twist a bar so as to discover its power to resist torsion, there are others which apply a downward pressure on one part of the bar and an upward one on an adjacent part, so as to show its capabilities in withstanding shearing strain.
Moreover, many of these tests are nowadays, in a well-equipped testing-house, carried out in conjunction with the use of heat. It stands to reason that a part of a machine which will have to work under considerable heat may have to be of different material from a part which works under a normal temperature. In some cases the bar is surrounded by a spiral wire through which electric current is passing, and by the regulation of this current any desired temperature can be set up in the bar. Or it may be placed in a bath of hot oil in such a way that the bar shall be raised to any temperature required, without interfering with the machinery which exerts the tension or pressure, or whatever it be.
Years ago such elaborate tests as these were never thought of. There are certain well-known figures, to be found in all engineering text-books, which give what stresses different materials ought to be able to stand, and these were, and are still, to a large extent, relied upon, it being taken for granted that the material used will be up to the average standard. In large and important works, however, the testing has been developed upon scientific lines, so that it is known from actual experiment what each particular thing is capable of. This not only means security but economy, for it is sometimes found that a substance is stronger than it is thought to be, and so things made of it can be designed to give the requisite strength lighter and cheaper than they would have been otherwise.
Some of the machines employed are of enormous strength, capable of exerting a pull or a compression of, it may be, 100 tons or more. They are often made, too, with self-recording appliances, whereby the course of the test is set down automatically upon a chart. For example, when a bar is being tested for tension, it is desirable to know not only the actual pull under which it came in two, but the behaviour of the test piece during the period before that. It begins to stretch as soon as the tension is applied, theoretically at all events, and if the metal were perfectly ductile it would stretch continuously as the load increases, until at last the breaking stress is reached. But in actual practice it probably stretches somewhat by fits and starts, and a record of that fact will be of great value in estimating the strength of the material in actual work. For such, an automatically made record, which can be studied at leisure, is of the utmost importance.
But perhaps the finest instance of scientific methods in manufacture is to be found in the methods by which standard parts of machines are measured, so as to ensure that they shall be interchangeable.
It may surprise the casual reader to be told that an absolutely exact measurement is an impossibility. It issafe to say that out of a million similar articles—articles made with the intention that they shall be exactly alike—there are no two which are, in fact, absolutely similar. They may be made with the same machines and the same tools, handled by the same man, but machines and tools wear or get out of adjustment, while man's liability to err is proverbial. Astronomers are the greatest experts in the art of measurement, and they recognise the possibility, nay, the probability, of error so frankly as to make every measurement several times over; if it be an important one they make it, if possible, a great many times over, and then take the average of the results. By this means they eliminate, to a certain extent at any rate, the error which cannot be avoided. That process is to allow for errors on the part of their instruments, for the most part. To deal with personal errors another method is used as well, for it is known that some observers have a natural tendency to err on one side more or less, while others tend to make mistakes in some degree on the other side. This tendency to err is known as the "personal equation" of the observer, and there are machines and tests by which the personal equation of each man can be determined, or perhaps it would be more correct to say estimated, so that in all observations made by him the proper allowance can be added or deducted.
But of course it would be extremely difficult to apply such methods in a workshop. It would never do to have to measure everything several times over, hoping that the average would come out in such manner as to indicate that the thing being measured was the size required. Instead, therefore, of wasting time seeking an accuracy which is known to be unattainable, the manufacturing engineer adopts a scientific system of measurement wherein a certain amount of inaccuracy is determined upon as permissible, and then simple appliances are used to see that it does, in fact, fall within those limits. For instance, a round bar is to be made, say, an inch in diameter. Now we know from what has just been said that, when made, we have no meansof telling whether the bar is really and truly an inch in diameter or not. We consider, then, what it is for, and decide, say, that it will be near enough so long as we are sure that it is not larger than one inch plus one thousandth, nor less than one inch minus one thousandth. So long as it does not exceed or fall short of its reputed size by more than one thousandth of an inch, then we know that it will answer its purpose.
Now, having come to that decision, we can build up a system upon which any intelligent workman can proceed, with the result that all the inch bars which he makes will be the same size within the limits of1⁄1000over or under, so that the greatest possible difference between any two will be1⁄500.
This system involves the use of two gauges for every size. The man employed upon making one-inch bars has a plate with a hole in it 11⁄1000inches in diameter and another hole999⁄1000of an inch in diameter. One of these is the "go in" gauge; the other is the "not go in." So that all he has to do, in order to be quite sure that his work is right, is to see that it can be poked through one of these holes, but not through the other. No trouble at all, it will be observed, adjusting fine measuring appliances, simply a plate with two holes in it, and the workman can be sure that he is turning out articles every one of which is practically correct, with no variation beyond a slight inequality too small to matter.
And probably at some other part of the factory there is a man making articles each of which has a hole in it, into which this bar must fit. How does he manage? He is provided with a gauge somewhat the shape of a dumb-bell, one end of which is slightly larger than the other. One is the "go in" end, the other the "not go in" end. If the hole which he makes will permit the former to enter, but will refuse admittance to the latter, then he knows that that hole is sufficiently near its reputed size to answer its purpose.