WATCHES AND CHRONOMETERS — THE MICROTOME — THE DIVIDING ENGINE — MEASURING MACHINES
Owingto the universal use of watches, resulting from their cheapness, the possessor of a pocket timepiece soon ceases to take a pride in the delicate mechanism which at first added an inch or two to his stature. At night it is wound up mechanically, and thrust under the pillow, to be safe from imaginary burglars and handy when the morning comes. The awakened sleeper feels small gratitude to his faithful little servant, which all night long has been beating out the seconds so that its master may know just where he is with regard to "the enemy" on the morrow. At last a hand is slipped under the feather-bag, and the watch is dragged from its snug hiding-place. "Bother it," says the sleepy owner, "half-past eight; ought to have been up an hour ago!" and out he tumbles. Dressing concluded, the watch passes to its day quarters in a darksome waistcoatpocket, to be hauled out many times for its opinion to be taken.
The real usefulness of a watch is best learnt by being without one for a day or two. There are plenty of clocks about, but not always in sight; and one gradually experiences a mild irritation at having to step round the corner to find out what the hands are doing.
A truly wonderful piece of machinery is a watch—even a cheap one. An expensive, high-class article is worthy of our admiration and respect. Here is one that has been in constant use for fifty years. Twice a second its little balance-wheel revolves on its jewelled bearings. Allowing a few days for repairs, we find by calculation that the watch has made no less than three thousand million movements in the half-century! And still it goes ticking on, ready to do another fifty years' work. How beautifully tempered must be the springs and the steel faces which are constantly rubbing against jewel or metal! How perfectly cut the teeth which have engaged one another times innumerable without showing appreciable wear!
The chief value of a good watch lies in its accuracy as a time-keeper. It is, of course, easy to correct it by standard clocks in the railway stations or public buildings; but one may forget to do this, and in a week or two a loss of a few minutes may lead to one missing a train, or being late for an important engagement. Happy, therefore, is the man who, having set his watch to "London time," can rely on its not varying from accuracy a minute in a week—a feat achieved by many watches.
The old-fashioned watch was a bulky affair, protectedby an outer case of ample proportions. From year to year the size has gradually diminished, until we can now purchase a reliable article no thicker than a five-shilling piece, which will not offend the most fastidious dandy by disarranging the fit of his clothes. Into the space of a small fraction of an inch is crowded all the usual mechanism, reduced to the utmost fineness. Watches have even been constructed small enough to form part of a ring or earring, without losing their time-keeping properties.
For practical purposes, however, it is advantageous to have a timepiece of as large a size as may be convenient, since the difficulties of adjustment and repair increase with decreasing proportions. The ship's chronometer, therefore, though of watch construction, is a big affair as compared with the pocket timepiece; for above all things it must be accurate.
The need for this arises from the fact that nautical reckonings made by the observation of the heavenly bodies include an element oftime. We will suppose a vessel to be at sea out of sight of land. The captain, by referring to the dial of the "mechanical log," towed astern, can reckon pretty accurately howfarthe vessel has travelled since it left port; but owing to winds and currents he is not certain of the position on the globe's surface at which his ship has arrived. To locate this exactly he must learn (a) his longitude,i.e.distance E. or W. of Greenwich, (b) his latitude,i.e.distance N. or S. of the Equator. Therefore, when noon approaches, his chronometers and sextant are got out, and at the moment when the sun crosses the meridian the time is taken. If this moment happens to coincide with four o'clock on the chronometershe is as far west of Greenwich as is represented by four twenty-fourths of the 360° into which the earth's circumference is divided; that is, he is in longitude 60° W. The sextant gives him the angle made by a line drawn to the sun with another drawn to the horizon, and from that he calculates his latitude. Then he adjourns to the chart-room, where, by finding the point at which the lines of longitude and latitude intersect, he establishes his exact position also.
When the ship leaves England the chronometer is set by Greenwich time, and is never touched afterwards except to be wound once a day. In order that any error may be reduced to a minimum a merchant ship carries at least two chronometers, a man-of-war at least three, and a surveying vessel as many as a dozen. The average reading of the chronometers is taken to work by.
Taking the case of a single chronometer, it has often to be relied on for months at a time, and during that period has probably to encounter many changes of temperature. If it gains or loses from day to day, and thatconsistently, it may still be accounted reliable, as the amount of error will be allowed for in all calculations. But should it gain one day and lose another, the accumulated errors would, on a voyage of several months, become so considerable as to imperil seriously the safety of the vessel if navigating dangerous waters.
As long ago as 1714 the English Government recognised the importance of a really reliable chronometer, and in that year passed an Act offering rewards of £10,000, £15,000, and £20,000 to anybody who should produce a chronometer that would fix longitude within sixty, forty,and thirty miles respectively of accuracy. John Harrison, the son of a Yorkshire carpenter, who had already invented the ingenious "gridiron pendulum" for compensating clocks, took up the challenge. By 1761 he had made a chronometer of so perfect a nature that during a voyage to Jamaica that year, and back the next, it lost only 1 min.541/2sec. As this would enable a captain to find his longitude within eighteen miles in the latitude of Greenwich, Harrison claimed, and ultimately received, the maximum reward.
It was not till nearly a century later that Thomas Earnshaw produced the "compensation balance," now generally used on chronometers and high-class watches. In cheap watches the balance is usually a little three-spoked wheel, which at every tick revolves part of a turn and then flies back again. This will not suffice for very accurate work, because the "moment of inertia" varies at different temperatures. To explain this term let us suppose that a man has a pound of metal to make into a wheel. If the wheel be of small diameter, you will be able to turn it first one way and then the other on its axle quite easily. But should it be melted down and remade into a wheel of four times the diameter, with the same amount of metal as before in the rim, the difficulty of suddenly reversing its motion will be much increased. The weight is the same, but the speed of the rim, and consequently its momentum, is greater. It is evident from this that, if a wheel of certain size be driven by a spring of constant strength, its oscillations will be equal in time; but if a rise of temperature should lengthen the spokes the speed would fall, because the spring would have morework to do; and, conversely, with a fall of temperature the speed would rise. Earnshaw's problem was to construct a balance wheel that should be able to keep its "moment of inertia" constant under all circumstances. He therefore used only two spokes to his wheel, and to the outer extremity of each attached an almost complete semicircle of rim, one end being attached to the spoke, the other all but meeting the other spoke. The rim-pieces were built up of an outer strip of brass, and an inner strip of steel welded together. Brass expands more rapidly than steel, with the result that a bar compounded of these two metals would, when heated, bend towards the hollow side. To the rim-pieces were attached sliding weights, adjustable to the position found by experiment to give the best results.
We can now follow the action of the balance wheel. It runs perfectly correctly at, say, a temperature of 60°. Hold it over a candle. The spokes lengthen, and carry the rim-piecesoutwardsat their fixed ends; but, as the pieces themselves bend inwards at their free ends, the balance is restored. If the balance were placed in a refrigerating machine, the spokes would shorten, but the rim-pieces would bend outwards.
As a matter of fact, the "moment of inertia" cannot be kept quite constant by this method, because the variation of expansion is more rapid in cold than in heat; so that, though a balance might be quite reliable between 60° and 100°, it would fail between 30° and 60°. So the makers fit their balances with what is called asecondarycompensation, the effect of which is to act more quickly in high than in low temperatures. This could not wellbe explained without diagrams, so a mere mention must suffice.
Another detail of chronometer making which requires very careful treatment is the method of transmitting power from the main spring to the works. As the spring uncoils, its power must decrease, and this loss must be counterbalanced somehow. This is managed by using the "drum and fusee" action, which may be seen in some clocks and in many old watches. The drum is cylindrical, and contains the spring. The fusee is a tapering shaft, in which a spiral groove has been cut from end to end. A very fine chain connects the two parts. The key is applied to the fusee, and the chain is wound off the drum on to the larger end of the fusee first. By the time that the spring has been fully wound, the chain has reached the fusee's smaller extremity. If the fusee has been turned to the correct taper, the driving power of the spring will remain constant as it unwinds, for it gets least leverage over the fusee when it is strongest, and most when it is weakest, the intermediate stages being properly proportioned. To test this, a weighted lever is attached to the key spindle, with the weight so adjusted that the fully wound spring has just sufficient power to lift it over the topmost point of a revolution. It is then allowed a second turn, but if the weight now proves excessive something must be wrong, and the fusee needs its diameter reducing at that point. So the test goes on from turn to turn, and alterations are made until every revolution is managed with exactly the same ease.
The complete chronometer is sent to Greenwich observatory to be tested against the Standard Clock, which, at10 a.m., flashes the hour to other clocks all over Great Britain. In a special room set apart for the purpose are hundreds of instruments, some hanging up, others lying flat. Assistants make their rounds, noting the errors on each. The temperature test is then applied in special ovens, and finally the article goes back to the maker with a certificate setting forth its performances under different conditions. If the error has been consistent the instrument is sold, the buyer being informed exactly what to allow for each day's error. At the end of the voyage he brings his chronometer to be tested again, and, if necessary, put right.
Here are the actual variations of a chronometer during a nineteen-day test, before being used:—
Day.Gain intenthsof seconds.Day.Gain intenthsof seconds.1st½11th42nd312th33rd413th34th414th45th½15th56th316th27th017th38th018th59th4½19th110th3
An average gain of just over one quarter of a second per diem! Quite extraordinary feats of time-keeping have been recorded of chronometers on long voyages. Thus a chronometer which had been to Australiaviâthe Cape and backviâthe Red Sea was only fifteen seconds "out"; and theEncyclopædia Britannicaquotes theperformance of the three instruments ofs.s.Orellana, which between them accumulated an error of but 2·3 seconds during a sixty-three-day trip.
An instrument which will cut a blood corpuscle into several parts—that's theMicrotome, the "small-cutter," as the name implies.
For the examination of animal tissues it is necessary that they should be sliced very fine before they are subjected to the microscope. Perhaps a tiny muscle is being investigated and cross sections of it are needed. Well, one cannot pick up the muscle and cut slices off it as you would off a German sausage. To begin with, it is difficult even to pick the object up; and even if pieces one-hundredth of an inch long were detached they would still be far too large for examination.
So, as is usually the case when our unaided powers prove unequal to a task, we have recourse to a machine. There are several types of microtomes, each preferable for certain purposes. But as in ordinary laboratory work the Cambridge Rocking Microtome is used, let us give our special attention to this particular instrument. It is mounted on a strong cast-iron bed, a foot or so in length and four to five inches wide. Towards one end rise a couple of supports terminating in knife-edges, which carry a cross-bar, itself provided with knife-edges top and bottom, those on the top supporting a second transverse bar. Both bars have a long leg at right angles, giving them the appearance of two large T's superimposed one on the other; but the top T is converted into a cross by a fourth member—a sliding tube which projects forward towards a frame in which is clamped a razor, edge upwards.
The tail of the lower T terminates in a circular disc, pierced with a hole to accommodate the end of a vertical screw, which has a large circular head with milled edges. The upper T is rocked up and down by a cord and spring, the handle actuating the cord also shifting on the milled screw-head a very small distance every time it is rocked backwards and forwards. As the screw turns, it gradually raises the tail of the lower member, and by giving its cross-bar a tilt brings the tube of the upper member appreciably nearer the razor. The amount of twist given to the screw at each stroke can be easily regulated by a small catch.
When the microscopist wishes to cut sections he first mounts his object in a lump of hard paraffin wax, coated with softer wax. The whole is stuck on to the face of the tube, so as to be just clear of the razor.
The operator then seizes the handle and works it rapidly until the first slice is detached by the razor. Successive slices are stuck together by their soft edges so as to form a continuous ribbon of wax, which can be picked up easily and laid on a glass slide. The slide is then warmed to melt the paraffin, which is dissolved away by alcohol, leaving the atoms of tissue untouched. These, after being stained with some suitable medium, are ready for the microscope.
A skilful user can, under favourable conditions, cut slicesone twenty-five thousandthof an inch thick. To gather some idea of what this means we will imagine that a cucumber one foot long and one and a-half inches in diameter is passed through this wonderful guillotine. It would require no less than 700 dinner-plates nine inchesacross to spread the pieces on! If the slices were one-eighth of an inch thick, the cucumber, to keep a proportionate total size, would be 260 feet long. After considering these figures we shall lose some of the respect we hitherto felt for the men who cut the ham to put inside luncheon-bar sandwiches.
In the preceding pages frequent reference has been made to index screws, exactly graduated to a convenient number of divisions. When such screws have to be manufactured in quantities it would be far too expensive a matter to measure each one separately. Therefore machinery, itself very carefully graduated, is used to enable a workman to transfer measurements to a disc of metal.
If the index-circle of an astronomical telescope—to take an instance—has to be divided, it is centred on a large horizontal disc, the circumference of which has been indented with a large number of teeth. A worm-screw engages these teeth tangentially (i.e.at right angles to a line drawn from the centre of the plate to the point of engagement). On the shaft of the screw is a ratchet pinion, in principle the same as the bicycle free-wheel, which, when turned one way, also twists the screw, but has no effect on it when turned the other way. Stops are put on the screw, so that it shall rotate the large disc only the distance required between any two graduations. The divisions are scribed on the index-circle by a knife attached to a carriage over and parallel to the disc. TheDividing Engineused for the graduation of certain astronomical instruments probably constitutes the most perfect machine ever made. In an address to the Institutionof Mechanical Engineers,[1]the President, Mr. William Henry Maw, used the following words: "The most recently constructed machine of the kind of which I am aware—namely, one made by Messrs. Warner and Swasey, of Cleveland, U.S.A.—is capable of automatically cutting the graduations of a circle with an error in position not exceeding one second of arc. (A second of an arc is approximately the angle subtended by a halfpenny at a distance of three miles.) This means that on a 20-inch circle the error in position of any one graduation shall not exceed1/20,000inch. Now, the finest line which would be of any service for reading purposes on such a circle would probably have a width equal to quite ten seconds of arc; and it follows that the minute V-shaped cut forming this line must be so absolutely symmetrical with its centre line throughout its length, that the position of this centre may be determined within the limit of error just stated by observations of its edges, made by aid of the reading micrometer and microscope. I may say that after the machine just mentioned had been made, it tookover a year's hard workto reduce the maximum error in its graduations from one and a-half to one second of arc."
The same address contains a reference to the great Yerkes telescope, which though irrelevant to our present chapter, affords so interesting an example of modern mechanical perfection that it deserves parenthetic mention.
The diameter of a star of the seventh magnitude as it appears in the focus of this huge telescope is1/2,500inch. The spiders' webs stretched across the object glass areabout1/6,000inch in diameter. "The problem thus is," says Mr. Maw, "to move this twenty-two ton mass (the telescope) with such steadiness in opposition to the motion of the earth, that a star disc1/2,500inch in diameter can be kept threaded, as it were, upon a spider's web1/6,000inch in diameter, carried at a radius of thirty-two feet from the centre of motion. I think that you will agree that this is a problem in mechanical engineering demanding no slight skill to solve; but it has been solved, and with the most satisfactory results." The motions are controlled electrically; and respecting them Professor Barnard, one of the chief observers with this telescope, some time ago wrote as follows: "It is astonishing to see with what perfect instantaneousness the clock takes up the tube. The electric slow motions are controlled from the eye end. So exact are they that a star can be brought from the edge of the field and stopped instantaneously behind the micrometer wire."
Dividing engines are used for ruling parallel lines on glass and metal, to aid in the measurements of microscopical objects or the wave-lengths of light. Adiffraction grating, used for measuring the latter, has the lines so close together that they would be visible only under a powerful microscope. Glass being too brittle, a special alloy of so-calledspeculummetal is fashioned into a highly polished plate, and this is placed in the machine. A delicate screw arrangement gradually feeds the plate forwards under the diamond point, which is automatically drawn across the plate between every two movements. Professor H. A. Rowlands has constructed a parallel dividing engine which has ruled as many as 120,000 lines to the inch.To get a conception of these figures we must once again resort to comparison. Let us therefore take a furrow as a line, and imagine a ploughman going up and down a field 120,000 times. If each furrow be eight inches wide, the field would require a breadth of nearlyfourteen milesto accommodate all the furrows! Again, supposing that a plate six inches square were being ruled, the lines placed end to end would extend for seventy miles!
Professor Rowlands' machine does the finest work of this kind. Another very perfect instrument has been built by Lord Blythswood, and as some particulars of it have been kindly supplied, they may fitly be appended.
If a first-class draughtsman were asked how many parallel straight lines he would rule within the space of one inch, it is doubtful whether he would undertake more than 150 to 200 lines. Lord Blythswood's machine can rule fourteen parallel lines on a space equivalent to theedgeof the finest tissue paper. So delicate are the movements of the machine that it must be protected from variations of temperature, which would contract or expand its parts; so the room in which it stands is kept at an even heat by automatic apparatus, and to make things doubly sure the engine is further sheltered in a large case having double walls inter-packed with cotton wool.
In constructing the machine it was found impossible, with the most scientific tools, to cut a toothed wheel sufficiently accurate to drive the mechanism, but the errors discovered by microscopes were made good by the invention of a small electro-plating brush, which added the thinnest imaginable layer of metal to any tooth found deficient.
During the process of ruling a grating of only a few squareinches area, the machine must be left severely alone in its closed case. The slightest jar would cause unparallelism of a few lines, and the ruin of the whole grating. So for several days the diamond point has its own way, moving backwards and forwards unceasingly over the hard metal, in which it chases tiny grooves. At the end the plate has the appearance of mother-of-pearl, which is, in fact, one of nature's diffraction gratings, breaking up white light into the colours of the spectrum.
You will be able to understand that these mechanical gratings are expensive articles. Sometimes the diamond point breaks half-way through the ruling, and a week's work is spoilt. Also the creation of a reliable machine is a very tedious business. Ten pounds per square inch of grating is a low price to pay.
The greatest difficulty met with in the manufacture of the dividing engine is that of obtaining a mathematically correct screw. Turning on a lathe produces a very rough spiral, judged scientifically. Some threads will be deeper than others, and differently spaced. The screw must, therefore, be ground with emery and oil introduced between it and a long nut which is made in four segments, and provided with collars for tightening it up against the screw. Perhaps a fortnight may be expended over the grinding. Then the screw must undergo rigid tests, a nut must be made for it, and it has to be mounted in proper bearings. The explanation of the method of eliminating errors being very technical, it is omitted; but an idea of the care required may be gleaned from Professor Rowlands' statement that an uncorrected error of1/300,000of an inch is quite sufficient to ruin a grating!
In the Houses of Parliament there is kept at an even temperature a bronze rod, thirty-eight inches long and an inch square in section. Near the ends are two wells, rather more than half an inch deep, and at the bottom of the wells are gold studs, each engraved with a delicate cross line on their polished surfaces. The distance between the lines is the imperial yard of thirty-six inches.
The bar was made in 1844 to replace the Standard destroyed in 1834, when both Houses of Parliament were burned. The original Standard was the work of Bird, who produced it in 1760. In June, 1824, an Act had been passed legalising this Standard. It says:—
"The same Straight Line or Distance between the Centers of the said Two Points in the said Gold Studs in the said Brass Rod, the Brass being at the temperature of Sixty-two Degrees by Fahrenheit's Thermometer, shall be and is hereby denominated the 'Imperial Standard Yard.'"
To provide for accidents to the bar, the Act continues: "And whereas it is expedient that the said Standard Yard, if lost, destroyed, defaced, or otherwise injured, should be restored to the same Length by reference to some invariable natural Standard: And whereas it has been ascertained by the Commissioners appointed by His Majesty to inquire into the subject of Weights and Measures, that the Yard hereby declared to be the Imperial Standard Yard, when compared with a Pendulum vibrating Seconds of Mean Time in the Latitude of London in a Vacuum at the Level of the Sea, is in the proportion of Thirty-six Inches to Thirty-nine Inches and one thousand three hundred and ninety-three ten-thousandth Parts of an Inch."
The new bar was made, however, not by this method, but by comparing several copies of the original and striking their average length. Four accurate duplicates of the new standard were secured, one of which is kept in the Mint, one in the charge of the Royal Society, one at Westminster Palace, and the fourth at the Royal Observatory, Greenwich. In addition, forty copies were distributed among the various foreign governments, all of the same metal as the original.
The French metre has also been standardised, being equal to one ten-millionth part of a quadrant of the earth's meridian (i.e.of the distance from the Equator to either of the Poles), that is, to 39·370788 inches. Professor A. A. Michelson has shown that any standard of length may be restored by reference to the measurement of wave lengths of light, with an error not exceeding one ten-millionth part of the whole.
It might be asked "Why should standards of such great accuracy be required?" In rough work, such as carpentry, it does not, indeed, matter if measurements are the hundredth of an inch or so out. But when we have to deal with scientific instruments, telescopes, measuring machines, engines for dividing distances on a scale, or even with metal turning, the utmost accuracy becomes needful; and a number of instruments will be much more alike in all dimensions if compared individually with a common standard than if they were only compared with one another. Supposing, for instance, a bar of exact diameter is copied; the copy itself copied; and so on a dozen times; the last will probably vary considerably from the correct measurements.
Hence it became necessary to standardise the foot and the inch by accurate subdivisions of the yard. This was accomplished by Sir Joseph Whitworth, who in 1834 obtained two standard yards in the form of measure bars, and by the aid of microscopes transferred the distance between the engraved lines to a rectangularend-measure bar,i.e.one of which the end faces are exactly a yard apart.
He next constructed his famous machine which is capable of detecting length differences ofone millionthof an inch. Two bars are advanced towards each other by screw gearing: one by a screw having twenty threads to the inch, and carrying a graduated hand-wheel with 250 divisions on its rim; the other by a similar screw, itself driven by a worm-screw, working on the rim, which carries 200 teeth. The worm-screw has a hand-wheel with a micrometer graduation into 250 divisions of its circumference. So that, if this be turned one division, the second screw is turned only1/250×1/200of a division, and the bar it drives advances only1/20×1/200×1/250=1/1,000,000of an inch. The screw at the other end of the machine (which in appearance somewhat resembles a metal lathe) is used for rapid adjustment only.
DELICATE MEASURING MACHINESDELICATE MEASURING MACHINESThe upper illustration shows a Pratt-Whitney Measuring Machine in operation to decide the thickness of a cigarette paper, which is one-thousandth of an inch thick. This machine will measure variations of length or thickness as minute as one hundredth-thousandth of an inch. The lower illustration shows a Whitworth Measuring Machine which is sensitive to variations of one-millionth of an inch.
DELICATE MEASURING MACHINESThe upper illustration shows a Pratt-Whitney Measuring Machine in operation to decide the thickness of a cigarette paper, which is one-thousandth of an inch thick. This machine will measure variations of length or thickness as minute as one hundredth-thousandth of an inch. The lower illustration shows a Whitworth Measuring Machine which is sensitive to variations of one-millionth of an inch.
DELICATE MEASURING MACHINES
The upper illustration shows a Pratt-Whitney Measuring Machine in operation to decide the thickness of a cigarette paper, which is one-thousandth of an inch thick. This machine will measure variations of length or thickness as minute as one hundredth-thousandth of an inch. The lower illustration shows a Whitworth Measuring Machine which is sensitive to variations of one-millionth of an inch.
"He (Sir J. Whitworth) obtained the subdivision of the yard by making three foot pieces as nearly alike as was possible, and working these foot pieces down until each was equal to the others, and placing them end to end in his millionth measuring machine; the total length of the three foot pieces was then compared with a standard end-measure yard. These three foot pieces were ground until they were exactly equal to each other, and the threeadded together are equal to the standard yard. The subdivision of the foot into inch pieces was made in the same way."[2]
A doubt may have arisen in the reader's mind as to the possibility of determining whether the measuring machine is screwed up to the exacttightness. Would the measuring bars not compress a body a little before it appeared tight? Workmen, when measuring a bar with callipers, often judge by the sense of touch whether the jaws of the callipers pass the bar with the proper amount of resistance; but when one has to deal with millionths of an inch, such a method would not suffice. So Sir Joseph Whitworth introduced afeeling-piece, orgravity-piece. Mr. T. M. Goodeve thus describes it inThe Elements of Mechanism: The gravity-piece consists of a small plate of steel with parallel plane sides, and having slender arms, one for its partial support, and the other for resting on the finger of the observer. One arm of the piece rests on a part of the bed of the machine, and the other arm is tilted up by the forefinger of the operator. The plane surfaces are then brought together, one on each side of the feeling-piece, until the pressure of contact is sufficient to hold it supported just as it remained when one end rested on the finger. This degree of tightness is perfectly definite, and depends on the weight of the gravity-piece, but not on the estimation of the observer.
In this way the expansion due to heat when a 36-inch bar has been touched for an instant with the finger-nail may be detected.
One of the most beautiful measuring machines commercially used comes from the factories of the Pratt-Whitney Co., Hartford, Connecticut, the well-known makers of machine tools and gauges of all kinds. It is made in different sizes, the largest admitting an 80-inch bar. Variations of1/100,000of an inch are readily determined by the use of this machine. It therefore serves for originating gauge sizes, or for duplicating existing standards. The adjusting screw has fifty threads to the inch, and its index-wheel is graduated to 400 divisions, giving an advance of1/20,000inch for each division: while by estimation this may be further subdivided to indicate one-half or even one-quarter of this small amount. Delicacy of contact between the measuring faces is obtained by the use of auxiliary jaws holding a small cylindrical gauge by the pressure of a light helical spring which operates the sliding spindle to which one of these auxiliary jaws is attached.
On one side of the "head" of the machine is a vertical microscope directed downwards on to a bar on the bed-plate, in which are a number of polished steel plugs graved with very fine central cross lines, each exactly an inch distant from either of its neighbours. A cross wire in the microscope tells when it is accurately abreast of the line below it. Supposing, then, that a standard bar three inches in diameter has to be tested. The "head" is slid along until the microscope is exactly over the "zero" plug line, and the divided index-wheel is turned until the two jaws press each other with the minimum force that will hold up the feeling-piece. Then the head is moved back and centred on the 3-inch line, and thebar to be tested is passed between the jaws. If the feeling-piece drops out it is too large, and the wheel is turned back until the jaws have been opened enough to let the bar through without making the feeling-piece fall. An examination of the index-wheel shows in hundred-thousandths of an inch what the excess diameter is.
On the other hand, if the bar were too small, the jaws would need to be closed a trifle: this amount being similarly reckoned.
We have now got into a region of very "practical politics," namely, the subject ofgauges. All large engineering works which turn out machinery with interchangeable parts,e.g.screws and nuts, must keep their dimensions very constant if purchasers are not to be disgusted and disappointed. The small motor machinery so much in evidence to-day demands that errors should be kept within the ten-thousandth of an inch. An engineer therefore possesses a set of standard gauges to test the diameter and pitch of his screw threads and nuts; the size of tubes, wires; the circumference of wheels, etc.
Great inconvenience having been experienced by American railroad-car builders on account of the varying sizes of the screws and bolts which were used on the different tracks—though all were supposed to be of standard dimensions—the masters determined to put things right; and accordingly Professors Roger and Bond and the Pratt-Whitney Co. were engaged to work in collaboration in connection with the manufacture of tools for minute measurements, viz. to1/50,000inch. "To give an idea of what is implied by this, let it be supposedthat a person should take a pair of dividing compasses and lay off 50,000 prick-marks1/8inch apart in a straight line. To do this the line would require to be over 520 feet, or nearly a tenth of a mile long. Imagine that many prick-marks compressed into the space of an inch, and you have an imperfect idea of the minuteness of the measurements which can now be made by the Pratt and Whitney Co."[3]
The standard taps and dies were supplied to tool-makers and engineers, who could thus determine whether articles supplied to them were of the proper dimensions. Nothing more was then heard of nuts being a "trifle small" or bolts "a leetle large." And so beautifully tempered were the dies made from the standards that one manufacturer claimed to have cut 18,800 cold-pressed nuts without any difference being perceptible in their sizes.
To appreciate what the difference of a thousandth of an inch makes in a true fit, you should handle a set of plug and ring gauges; the ring a true half-inch internally, the plugs half-inch, half an inch less one ten-thousandth of an inch, and half an inch less one-thousandth, in diameter.
The true half-inch plug needs to be forcibly driven into the ring on account of the friction between the surfaces. The next, if oiled, will slide in quite easily, but if left stationary a moment will "seize," and have to be driven out. The third will wobble very perceptibly, and would be at once discarded by a good workman as a bad fit.
For extremely accurate measurements of rods, calliper gauges, shaped somewhat like the letter Y, are used, the horns terminating in polished parallel jaws. Such a gauge will detect a difference of1/20,000inch quite easily.
So accurately can plug gauges be made by reference to a measuring machine, that a gold leaf1/30,000inch thick would be three times too thick to insert between the gauge and the jaws of the machine!
You must remember that in high-class workmanship these gauges are constantly being used. As time goes on, the "limit of error" allowed in many classes of machine parts is gradually lessened, which shows the simultaneous improvement of all machinery used in the handling of metal. James Watt was terribly hampered, when developing his steam-engine, by the difficulty of procuring a true cylinder for his pistons to work in with any approach to steam-tightness. His first cylinder was made by a smith of hammered iron soldered together. The next was cast and bored, but stuffing it with paper, cork, putty, pasteboard, and "old hat" proved useless to stem the leakage of steam. No wonder, considering that the finished cylinder was one-eighth of an inch larger in diameter at one end than at the other. Watt was in advance of his time. Neither machinery nor workmanship had progressed sufficiently to meet the requirements of the steam-engine. To-day an engineer would confidently undertake to bore a cylinder five feet in diameter with a variation from truth of not more than one five-hundredth of an inch.
Before passing from the subject of measuring machines, which play so important a part in modern mechanism, wemay just glance at the electrical method of Dr. P. E. Shaw. He discovered recently that two clean metal surfaces can, by means of an electric current, feel one another on touching with a delicacy that far transcends that of the purely mechanical machine. The mechanism he employs is thus devised: A finely cut vertical screw having fifty threads to the inch has a disc graduated into 500 parts. The screw can be turned by means of a pulley string from a distance, and it is thus possible to give the top end of the screw a movement of1/25,000inch, when a movement corresponding to one graduation is made.
This small movement is reduced by a train of six levers, the long arm of each bearing on the short arm of the one before it. The movement of the last lever of the train is thus reduced to1/4,000of that of the screw point, so a movement of1/4,000×1/25,000× =1/1,00,000,000inch is obtained!
How can such a movement be judged? A telephone and voltaic cell are joined to the last lever of the train and to the object whose movement is under examination. If they touch, the telephone sounds. An observer listens in the telephone, and if the object moves for any reason he can find out how much it moves by turning the screw until contact is made again.
Out of the many applications of this apparatus three may be given.
(1) A short bar of iron when magnetised elongates about1/1,000,000of its length. If further magnetised it contracts. These changes can readily be measured with the instrument.
(2) The smallest sound audible in the telephone is due to a movement of the diaphragm of the telephone by about1/50,000,000of an inch. This has been actually measured by Dr. Shaw and is by far the smallest distance ever directly recorded. It is about twice the diameter of the molecules of matter.
(3) Dispensing with levers, the screw alone is used for rougher work. Dr. Shaw has shown that one hundred-thousandth of an inch is the smallest dimension visible under a microscope. By fitting an electric measuring apparatus to the microscope carriage it becomes quite easy to measure minute distances. The microscope contains a cross wire which, when the object has been laid on the microscope stage, is centred on one side of the object. The electric contact screw is then advanced till it makes contact with the stage and a sound arises in the telephone. A reading of the screw disc having been taken, the screw is drawn in and the microscope stage is traversed sufficiently to bring the wire in line with the other side of the object. Once more the operator makes electrical contact and gets a second reading, the difference between the two being the diameter of the object. In this manner the bacillus of tuberculosis has been proved to have an average diameter of31/250,000of an inch.
The same method is employed to gauge the distance between the lines on a diffraction grating.