Chapter 1: Delicate Instruments

WATCHES AND CHRONOMETERS--THE MICROTOME--THE DIVIDING ENGINE--MEASURING MACHINES

Owing to 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 waistcoat pocket, 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, protected by 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 of time. 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 how far the 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 chronometers he 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 that consistently, 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. 54-1/2 sec. 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 more work 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-pieces outwards at 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 a secondary compensation, the effect of which is to act more quickly in high than in low temperatures. This could not well be 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, at 10 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:--

Gain in tenths Day. of seconds. 1st 1/2 2nd 3 3rd 4 4th 4 5th 1/2 6th 3 7th 0 8th 0 9th 4-1/2 10th 3 11th 4 12th 3 13th 3 14th 4 15th 5 16th 2 17th 3 18th 5 19th 1

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 Australia viâ the Cape and back viâ the Red Sea was only fifteen seconds “out”; and the Encyclopædia Britannica quotes the performance of the three instruments of S.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 the MICROTOME, 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 slices _one twenty-five thousandth_ of 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 inches across 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. The DIVIDING ENGINE used for the graduation of certain astronomical instruments probably constitutes the most perfect machine ever made. In an address to the Institution of 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 exceed 1/20,000 inch. 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 took over a year’s hard work to 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 is 1/2,500 inch. The spiders’ webs stretched across the object glass are about 1/6,000 inch 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 disc 1/2,500 inch in diameter can be kept threaded, as it were, upon a spider’s web 1/6,000 inch 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. A diffraction 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-called speculum metal 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 nearly fourteen miles to 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!