Inventors at Work, with Chapters on Discovery

It was a residual effect which led to the discovery of the planet Neptune. The orbit of Uranus being exactly defined, it was noticed by Adams and Leverrier that after making due allowance for perturbations by all known bodies, there remained a small disturbance which they believed could be accounted for only by the existence of a planet as yet unobserved. That planet was forthwith sought, and soon afterward discovered, proving in mass and path to be capable of just the effect which had required explanation.

Measurements Refined: the Interferometer.

In the measurement of length or motion a most refined instrument is the interferometer, devised by Professor A. A. Michelson, of the University of Chicago. It enables an observer to detect a movement through one five-millionth of an inch. The principle involved is illustrated in a simple experiment. If by dropping a pebble at each of two centres, say a yard apart, in a still pond, we send out two systems of waves, each system will ripple out in a series of concentric circles. If, when the waves meet, the crests from one set of waves coincide with the depressions from the other set, the water in that particular spot becomes smooth because one set of waves destroys the other. In this case we may say that the waves interfere. If, on the other hand, the crests of waves from two sources should coincide, they would rise to twice their original height. Light-waves sent out in a similar mode from two points may in like manner either interfere, and produce darkness, or unite to produce light of double brilliancy. These alternate dark and bright bands are called interference fringes. When one of the two sources of light is moved through a very small space, the interference fringes at a distance move through a space so much larger as to be easily observed and measured, enabling an observer to compute the short path through which a light-source has moved. In the simplest form of interferometer, light from any chosen source, S, is rendered approximately parallel in its rays by a double convex lens at L. The light falling upon the glass plate A is divided into two beams, one of which passes to the mirror M, while the other is reflected to M¹. The rays reflected from M¹, which pass through A, and those returned from M reflected at d, are reunited, and may be observed at E. In order to produce optical symmetry of the two luminous paths, a plate C exactly like A is introduced between A and M. When the distance from d to M and to M¹ are the same the observer sees with white light a central black spot surrounded with colored rings. When the mirror M¹ is moved parallel to itself either further from or nearer to A, the fringes of interference move across the field of view at E. A displacement of one fringe corresponds to a movement of half a wave-length of light by the mirror M¹. By counting the number of fringes corresponding to a motion of M¹ we are able to express the displacement in terms of a wave-length of light. Where by other means this distance is measurable, the length of the light-wave may be deduced. With intense light from a mercury tube 790,000 fringes have been counted, amounting to a difference in path of about one-fourth of a metre.

Many diverse applications of the interferometer have been developed, as, for example, in thermometry. The warmth of a hand held near a pencil of light is enough to cause a wavering of the fringes. A lighted match shows contortions as here illustrated. When the air is heated its density and refractive power diminish: it follows that if this experiment is tried under conditions which show a regular and measurable displacement of the fringes, their movement will indicate the temperature of the air. This method has been applied to ascertain very high temperatures, such as those of the blast furnace. Most metals expand one or two parts in 100,000 for a rise in temperature of one degree centigrade. When a small specimen is examined the whole change to be measured may be only about ¹⁄_10,000 inch, a space requiring a good microscope to perceive, but readily measured by an interferometer. It means a displacement amounting to several fringes, and this may be measured to within ¹⁄₅₀ of a fringe or less; so that the whole displacement may be measured to within a fraction of one per cent. Of course, with long bars the accuracy attainable is much greater.

Application to Weighing.

The interferometer has much refined the indications of the balance. In a noteworthy experiment Professor Michelson found the amount of attraction which a sphere of lead exerted on a small sphere hung on an arm of a delicate balance. The amount of this attraction when two such spheres touch is proportional to the diameter of the large sphere, which in this case was about eight inches. The attraction on the small ball on the end of the balance was thus the same fraction of its weight as the diameter of the large ball was of the diameter of the earth,—something like one twenty-millionth. So the force to be measured was one twenty-millionth of the weight of this small ball. In the interferometer the approach of the small ball to the large one produced a displacement of seven whole fringes.

In order that this instrument may yield the best results, great care must be exercised in its construction. The runways of the frame are straightened with exactitude by a method due to Mr. F. L. O. Wadsworth. The optical surfaces of the planes and mirrors in the original designs were from the master hand of Mr. John A. Brashear of Allegheny, Pennsylvania. Each mirror is free from any irregularity greater than ¹⁄_880,000 inch, and the opposite faces of the mirrors must be parallel within one second of arc, or ¹⁄_1,296,000 part of a circle.[27]

A Light-Wave as an Unvarying Unit of Length.

Now for a word as to Professor Michelson’s suggestion that an unvarying unit of measurement may be found in a certain light-wave, as observed in the interferometer. Everybody knows that each chemical element burns with colors of its own. When we see red fire bursting from a rocket we know that strontium is ablaze; when the tint is green it tells us that copper is on fire, as when a trolley-wheel jumps from its electric wire. When these sources of light are looked at through an accurate prism of glass in a spectroscope they form characteristic spectra, and these spectra in their peculiarities of color reveal what elements are aflame. In most cases the rays from an element form a highly complicated series; to this rule cadmium, a metal resembling zinc, is an exception. It emits a red, a green, and a blue ray; the wave-lengths of these rays Professor Michelson proposes as a basis of reference for the metallic standards of length adopted by the nations of Europe and America. He says: “We have in the interferometer a means of comparing the fundamental standard of length with a natural unit—the length of a light-wave—with about the same order of accuracy as is at present possible in the comparison of two metre-bars, that is, to one part in twenty millions. The unit depends on the properties of the vibrating atoms of the radiating substance, and of the luminiferous ether, and is probably one of the least changeable qualities in the material universe. If therefore the metre and all its copies were destroyed, they could be replaced by new ones, which would not differ among themselves. While such a simultaneous disaster is practically impossible, it is by no means sure that notwithstanding the elaborate precautions that have been taken to ensure permanency, there may not be slow molecular changes going on in all the standards, changes which it would be impossible to detect except by some such method as that here presented.”

Thus, by dint of mechanical refinements such as the world never saw before, some of the smallest units revealed to the eye become the basis of all measurement whatever, reaching at last those cosmical diameters across which light itself is the sole messenger. In the early days of spectroscopy many doubters said, What good is all this? Since then a full reply has been rendered to their question and, at this unexpected point, the spectroscopic examination of an unimportant metal may afford a measuring unit of ideal stability. Cases like this suggest the query, Is any knowledge whatever quite worthless?

CHAPTER XVII
MEASUREMENT—Continued

Weight, Time, Heat, Light, Electricity measured with new precision . . . Exact measurement means interchangeable designs, and points the way to utmost economies . . . The Bureau of Standards at Washington . . . Measurement in expert planning and reform.

The Balance in Measurement.

Our grandfathers supposed that trade began in barter; we have been able to go one step further back in history to find that barter followed upon the custom of exchanging presents. This custom, among shrewd and self-respecting people, came at length to a degree of fairness, and led to rough and ready modes of weighing, gradually improved. In the British Museum, in a papyrus of Hunnafer, who lived in Egypt thirty-three centuries ago, we have pictured a well-constructed balance of equal arms, in which a feather is outweighing a human soul. In its successive improvements the balance registers the progress of many arts and sciences, and in its turn has promoted them all. It must be built of a metal, or an alloy, hard, durable, and not easily corroded. Its centre of motion should be a little above its centre of gravity; its knife edge should have an angle of about 60 degrees. Appliances must render it easy to lift the weighing apparatus when out of use, so that unnecessary wear of the knife edge may be avoided, as well as needless strain throughout the structure. Air currents should be kept off by a suitable case, or, better still, the instrument should be enclosed in a receiver exhausted of air altogether. The weights, made with scrupulous care of standard metal or alloy, should be guarded from tampering, abrasion, and corrosion, from dirt or other accretions. A weighing should be slowly performed, the weights placed in the center of one pan, the object weighed in the center of the other pan; to eliminate errors due to inequality in the length of arms, the article weighed and the weights are then made to exchange places. The platform should be of the utmost strength and rigidity, so as precisely to maintain its level at all times.

As long ago as 1798 a balance was erected having an accuracy of one part in 1,600,000; fifty years later ten-fold greater accuracy had been attained; to-day results much more astonishing are achieved. A precision balance manufactured by Messrs. Albert Rueprecht & Son, Vienna, is shown on page 220, as furnished in 1902 to the International Bureau of Weights and Measures at Sevres, France. It is provided with means for applying the smallest weights of platinum from a distance of three to four metres, so as to guard against perturbations due to the warmth of an operator’s body. The weights may be shifted from one pan to the other, and the oscillations observed through a telescope, at a distance of four metres. This balance will detect the ¹⁄₅₀₀ of a milligram when weighing a mass of 500 grams, or one part in 250,000,000. Such balances, and those of Paul Bunge, of Hamburg, require ten to twenty months of skilled labor for their completion. The International Bureau of Weights and Measures has a balance of extraordinary sensitiveness at the Pavillon de Breteuil, Sevres, where the work of the Bureau goes forward. This instrument measures the difference in the attraction of the earth for a mass of one kilogram when that weight is moved nearer to or farther from the centre of the earth by as little as one centimetre. Thus placing two weights, of common shape, each a kilogram, one on top of the other, and two other weights in the other pan beside one another, would introduce a noteworthy difference in a comparison.

Measurement of Time.

At the very dawn of civilization, the day, however crudely, was divided into parts. These parts, long afterward, probably in Babylonia, became the twenty-four hours which have descended to us. The means of time-keeping came first, in all likelihood, from measuring the simple shadow of a stick, the gnomon, still set up as a sun-dial in our gardens. Next came an hour-glass with its falling sand; the clepsydra, with its water dropping from a jar; the burning of candles definite in length. At last came the supreme discovery that a pendulum, of given length, if kept in one place oscillates in an unvarying period, be its arc of motion long or short. Tradition has it that in Arabia, about the year 1000 A. D., the pendulum was used in time-keeping. Granting this to be true, we must nevertheless give Galileo credit for his independent discovery as he observed the swaying lamp of the cathedral at Pisa, early in the seventeenth century. In 1657 Huygens employed a pendulum in the construction of a clock which, of course, displayed a new approach to accuracy. In 1792 Borda and Cassini had improved their time-pieces so as to be correct within one part in 375,000, that is to one second in 104 hours. For the sake of portability, clocks were gradually reduced in size until they became watches. Instead of a pendulum they were furnished with its equivalent, a balance wheel, Pierre Le Roy having discovered that there is in every spring a certain length where all the vibrations, great or small, are performed in approximately the same period. For actuation, watches were provided with mainsprings which have steadily undergone improvement in quality and in placing.

Time-Pieces Improved.

Many refinements have brought the time-keeper for the ship, the observatory, the railroad, to virtual perfection. Its wheels, pinions, balance-staffs are manufactured automatically, as at Waltham, Massachusetts, to an accuracy of ¹⁄₅₀₀₀ inch or even less, thanks to that great inventor, Mr. Duane H. Church. In modern watch-making the most durable materials are used, magnetic perturbations are avoided by employing alloys insensitive to magnetism, and the effects of fluctuating temperatures are withstood by Earnshaw’s compensated balance wheel. This wheel is in halves, each nearly semicircular and attached at one end to a stout diameter. Its outer rim, being made of brass, when warmed expands more than its inner rim of steel. Thus, in a rising temperature the wheel curves inward with its duly placed weights, so that the reduction in elasticity of the hair-spring caused by heat is compensated. Experiments are afoot which look toward a marked improvement in the making of time-pieces, by using invar, a nickel-steel with practically no expansibility by heat. This alloy is already employed for pendulums with satisfactory results, both at the Naval Observatory and at the Bureau of Standards, in Washington. It has been described on page 169.

The Best Clocks in the World.

At the Paris Observatory the standard clock, by Winnerl, is in a vault twenty-seven metres underground. At that depth the temperature changes are less than one fifth of a degree during the year, yet the effect of barometric changes on the rate of the clock have proved to be serious. This difficulty is avoided in the Naval Observatory at Washington, by enclosing the standard clock in an air-tight case within which the air is reduced to a pressure lower than that ever shown by a barometer at that level. To avoid risks of air leaking through this case were it to be pierced by a moving axle, this clock is actuated by weights lifted electrically by a small primary battery. The slight electric current required has no perturbing effect on the clock. This time-piece, provided with an escapement of great excellence, was manufactured by Clemens Riefler of Munich.

At the Observatory of the Case School of Applied Science, Cleveland, Ohio, another Riefler clock has a mean error of but .015 second per day. This means that in a year the total error is not more than 5.475 seconds, or one part in 5,760,000 of the 365 days. Such errors, minute as they are, give a good deal of trouble when they are irregular, that is, when the clock is sometimes slow, sometimes fast, in a fashion apparently lawless. When the divergences are fairly constant they can usually be traced to their source, making it feasible to apply a remedy.

Ascertaining the Force of Gravity.

A pendulum which swings once in a second at the base of a tall tower will require for the same travel a little more than a second when borne to the top of the tower, because then further from the centre of the earth. Still greater will be the difference in its periods as it swings first at the base of a mountain and next at its summit. A pendulum, therefore, is a means of learning the force of gravity at a given place, and without sacrifice of accuracy it is well that it should be as small as possible. In 1890, Professor T. C. Mendenhall, then superintendent of the United States Coast and Geodetic Survey, designed a pendulum one fourth the length of those previously used, and of admirable precision. Afterward pendulums were built of dimensions further reduced to about two and one half inches in length, with periods of oscillation of one fourth of a second. Such pendulums are easily carried to stations difficult of access, and have been employed on the summits of high mountains, including Pike’s Peak: their indications agree well with those of the larger and somewhat cumbersome apparatus previously used.

Heat Measured.

Much the most convenient means of measuring temperature is the common glass tube filled with mercury. This metal is chosen because a liquid, and because it varies extremely in bulk when warmed or cooled. Materials of parallel susceptibility are adopted for instruments which measure the intensity of magnetism or of electricity, the working core of the instrument being made of a substance highly responsive to magnetism or to electricity.

A mercurial thermometer, for all its convenience, has its accuracy assailed on more sides than one. When the barometric pressure rises, the bulb is compressed; when the barometer falls, the bulb enlarges by virtue of the diminution in atmospheric pressure. Further, when its graduated tube is upright the mercury exerts a distending pressure which introduces error. At all temperatures the metal is giving off a vapor which has tension, in its upper ranges entailing marked inaccuracies. The glass itself of which the instrument is made, when of ordinary composition, spontaneously undergoes changes of volume. While this is a minor source of error it may be almost completely avoided by using a boro-silicate glass from the factory of Schott & Genossen, at Jena. Other substances than mercury are employed in thermometers with gratifying results. Hydrogen gas is found very suitable within the interval from -30° to 200° Centigrade. Pentane serves in temperatures reaching down to -180°.

But it is in alliances with electricity that the measurement of heat has its broadest scope and utmost exactitude. It was long ago remarked that heating a metallic conductor increases its resistance to the flow of an electric current; to measure that resistance in a platinum wire serves, therefore, to measure its temperature. An instrument on this principle is the bolometer of the late Professor S. P. Langley, of Washington. Through a strip of platinum barely ¹⁄₅₀₀ inch in width, and less than ¹⁄₅₀₀₀ inch in thickness, a current of electricity flows continuously. When radiation, visible or invisible, on occasion from a star, falls upon it, the strip when warmed by as little as one millionth of a degree duly records the fact. An instrument, modified from the Crookes radiometer by Professor E. F. Nichols of Columbia University, New York, is more sensitive still. An exhausted hollow metal block has a window of fluorite, a mineral transparent to ether vibrations of a long range of frequencies. Suspended inside the block is a fine quartz fibre supporting a horizontal bar, at the ends of which are attached thin plates of mica, blackened on one side. Rays passing through the fluorite window strike the blackened side of the mica, which is parallel and opposite to it. The resulting rise in temperature causes the vane to revolve against the torsion of the quartz fibre. The angle of torsion when thermal equilibrium is reached, measures the intensity of the incident radiation.

Another principle is adopted in the electrical instruments which expose to heat a junction of two different materials, usually metallic, giving rise to an electric current, easily measured. Experience shows that the most satisfactory couples for temperatures between 300° C. (570° F.) and 1600° C. (2900° F.) are those devised by M. Le Chatelier, one half consisting of pure platinum, the other half an alloy of ten per cent. rhodium and ninety per cent. platinum. Such instruments are indispensable in the arts which employ high temperatures. In producing chlorine by the Deacon process, or in the baking of porcelain, an undue variation of temperature of only twenty degrees may cause a complete failure of the operation.

The Measurement of Light.

It is probable that about one half the electricity from the dynamos of America is sent into lamps, and this is but part of the whole outlay for light, still chiefly produced by petroleum and gas. Hence the importance of measuring the light from lamps, jets, and mantles of various kinds, and testing the efficiency of shades and reflectors. First of all comes the decision as to a standard for comparison. Great Britain has adopted the Harcourt lamp, consuming pentane, as a standard for ten candle-power, referring to the old time candle of spermaceti. Germany employs the amylacetate lamp introduced by Von Hefner Alteneck, as a standard for its Hefner unit of illumination. Both lamps share in a difficulty which attends all combustion: atmospheric conditions which vary from hour to hour, from place to place, greatly affect the intensity of a flame. Hence incandescent lamps, which have been compared with these fundamental standards, are used as working standards. They can be operated by a uniform current of specified voltage, and after a hundred hours’ use their constancy of radiation for a considerable period is remarkable.

Having settled upon a standard candle or lamp the measurement of light demands extreme care, and, at the best, can never approach the accuracy of other laboratory measurements. Many photometers have been invented, some of them highly elaborate, but the type oftenest used remains in essence the simple instrument long ago devised by Bunsen. On a frame supported by a stand, S, is stretched a sheet of white paper in the centre of which is a grease spot. This spot allows more light to pass through it and consequently reflects less than the unmarked portion of the paper. If the sheet is more strongly lighted from behind than from in front, it appears bright on a dark ground. If it is illuminated more strongly in front than at the back it will seem dark upon a bright ground. When equal lights fall on both sides, the spot becomes invisible, since it can then appear neither darker nor brighter than the surrounding paper. In its simplest use the screen is placed between a standard candle or lamp at A and the light to be measured at B: the screen is moved along its graduated slide until the grease spot vanishes. If the screen is twice as far from B as from A when the spot disappears, then B is four times as intense as A in light; if the screen were thrice as far from B as from A, then B would be nine-fold as bright as A, the intensity of light diminishing as the square of the distance of its source.

An open-arc lamp, without a reflector, sends to the ground a fairly wide ring of brilliant rays; on both sides of that ring the illumination is feeble. Other sources of light also vary a good deal in the brilliancy of the beams which they emit in various planes. It is therefore usual to measure the light from a lamp as sent forth in all planes, or at least in its principal planes. When incandescent lamps are brought to a photometer they are as a rule placed on a spindle turning so swiftly that their mean horizontal candle-power may be read at once. For measuring the mean spherical intensity a photometer devised by Professor Matthews of Purdue University is employed. This apparatus has a series of mirrors arranged in a semicircle around a lamp, reflecting all the received light upon a single surface.

Light may have great brilliancy and yet be undesirable from its color; we are all familiar with the havoc that gas light may play with hues of blossom and leaf that in sunshine are beautiful. Through ages untold the human eye has been seeing by rays from the sun, and from immemorial habit is best served by light of similar quality. A simple instrument, the spectrometer, casts upon a screen the spectrum from a mercury tube, a Nernst lamp, a Welsbach mantle, or other illuminant, and enables us to compare it with the spectrum of sunshine. Then, as in placing a light pink shade over a Welsbach mantle, we act on the intimations of analysis greatly to the relief of the eye.

An incandescent bulb or mantle may be satisfactory both in brilliancy and color, but a further question is, How long will the filament or the mantle last, and at what point in deterioration should it be discarded? Tests during the first, the fiftieth, the hundredth, and other successive hours will tell us how much the intensity falls off. Just when a bulb or a mantle should be dismissed from service depends partly on the rate of deterioration, and partly on the prices of bulbs and current, of mantles and gas.

Hardly less important than testing sources of light is the investigation of their reflectors and shades. As a rule our lamps are too brilliant, and in many cases they send their light in wasteful directions. It is a general and absurd practice to buy a dollar’s worth of light and then kill sixty cents’ worth of it with a thick opal or cut-glass shade. Examination with the photometer has revealed that many popular patterns of reflectors and shades are most ineffective, while those of the Holophane make, when kept scrupulously clean, send the light just where it does most good and at the lowest possible expenditure of energy. This theme has attention on page 78.[28]

The Sky as a Field for Measurement.

The sky has been the supreme field for measurements more refined from age to age. Professor William Stanley Jevons, in “Principles of Science,” says: “At Greenwich Observatory in the present day, the hundredth part of a second is not thought an inconsiderable portion of time. The ancient Chaldeans recorded an eclipse to the nearest hour, and even the early Alexandrian astronomers thought it superfluous to distinguish between the edge and centre of the sun. By the introduction of the astrolabe, Ptolemy and the later Alexandrian astronomers could determine the places of the heavenly bodies within about ten minutes of arc. But little progress then ensued for thirteen centuries, until Tycho Brahe made the first great step toward accuracy, not only by employing better instruments, but even more by ceasing to regard an instrument as correct. Tycho, in fact, determined the errors of his instruments, and corrected his observations. He also took notice of the effects of atmospheric refraction, and succeeded in attaining an accuracy often sixty times as great as that of Ptolemy.

“Yet Tycho and Hevelius often erred several minutes in the determination of a star’s place, and it was a great achievement of Roemer and Flamsteed to reduce this error to seconds. Bradley, the modern Hipparchus, carried on the improvement, his errors in right ascension being under one second of time, and those of declination under four seconds of arc according to Bessel. In the present day the average error of a single observation is probably reduced to the half or quarter of what it was in Bradley’s time; and further extreme accuracy is attained by the multiplication of observations, and their skilful combination according to the method of least squares. Some of the more important constants, for instance that of nutation, have been determined within the tenth part of a second of arc.

“It would be a matter of great interest to trace out the dependence of this vast progress upon the introduction of new instruments. The astrolabe of Ptolemy, the telescope of Galileo, the pendulum of Galileo and Huygens, the micrometer of Horrocks, and the telescopic sights and micrometer of Gascoyne and Picard, Roemer’s transit instrument, Newton’s and Hadley’s quadrant, Dollond’s achromatic lenses, Harrison’s chronometer, and Ramsden’s dividing engine—such were some of the principal additions to astronomical apparatus. The result is that we now take note of quantities ¹⁄_300,000 or ¹⁄_400,000 the size of the smallest observable in the time of the Chaldeans.”

Electricity Measured.

As important as the measurements of the astronomer are those of the electrician. It was as recently as 1819 that Oersted, a Danish physicist, published a discovery which became a foundation stone of electrical engineering, and upon which rises the art of electrical measurement. He observed that when an electric current is passing through a wire, a nearby magnetic needle tends to place itself at right angles to the wire, the deflection varying with the strength of the current. When instead of a wire, a coil, duly insulated, is employed to carry the current, effects much more decided are displayed. At first current-measurers, or galvanometers, employed simple compass needles; these proved to be unsatisfactory. They were affected by the variations which occur in the intensity of the earth’s magnetism; and no matter how carefully a needle was made, it varied in strength from week to week, from year to year; again, a current might be so strong as to create magnetism overwhelming in comparison with that of the earth, and quite beyond the measuring power of a compass needle. A galvanometer on a plan due to Professor James Clerk Maxwell, employs a permanent magnet, or an electro-magnet, which is stationary, between the poles of which may freely turn a coil bearing the current to be measured. This current in the case of an ocean cable is so weak that no other means of indication will serve. Lord Kelvin’s recording apparatus for such a cable is a galvanometer on this principle. In order to concentrate the lines of magnetic force on the vertical sides of the coil, a piece of soft iron, D, is fixed between the poles of the magnet. This iron becomes magnetized by induction, so as to produce a very powerful field of force, in the minute spaces between it and the two magnetic poles, through which spaces the vertical sides of the coil are free to move. Instruments of this kind, developed by D’Arsonval, are known by his name.

Weston Instruments.

Instruments for electrical measurement, with stationary magnets and moving coils, of great excellence, are manufactured by the Weston Company, Waverly Park, New Jersey. Their accuracy rests upon several important discoveries by Dr. Edward Weston: first, a method of making a magnet which is really permanent, retaining its original strength for a long time: second, by the preparation of a remarkable group of alloys which under ordinary variations of temperature manifest scarcely any change in conductivity, and which set up but little thermo-electric action as they touch other metals in an instrument. Let us see how a Weston voltmeter, or measurer of electric pressure, is constructed.

A light rectangular coil of copper wire, C, is wound on an aluminium frame pivoted in jeweled bearings so as to be free to rotate in the ring-like space between an inner cylindrical soft iron core, K, and the pole pieces P and P of the permanent magnet, M. A light aluminium pointer, p, is attached to the coil and is free to move across the scale, D. The current enters the coil through the two spiral springs S and S, which serve also to control the movement of the coil. When a current passes through the coil the dynamic action between the current and the magnetic field tends to rotate the coil, and the position of equilibrium between this force and the torsion of the springs, indicated by the pointer, measures the current passing through the coil. Because the magnetic field is practically unvarying throughout, and the torsion of the springs is proportionate to their deflection, the scale is virtually uniform. This is not assumed in their manufacture, however, for each instrument is calibrated by direct reference to standards. As the aluminium frame moves through the magnetic field, slight currents are generated within the metal; these serve to dampen vibrations so that the pointer comes to rest almost instantly without friction. That the magnetic field may have the utmost strength, the air gap in which the coil rotates is made as narrow as possible; this is ensured by workmanship of the highest skill, and by tools specially designed. The hardened steel pivots are ground and centered as in the best watch-making: the coil is balanced by means of adjustable weights so that none but electrical forces may come into play. In a Weston voltmeter of regular type, the maximum current required for a full scale-deflection is only 0.01 ampere. Instruments of much higher sensibility are constructed for measuring insulation, requiring but 0.0006 ampere for the same deflection. So much for the task of measuring electrical pressure.

For measuring electrical currents, which differ from pressures as the quantity of water flowing in a pipe differs from the pressure of that water as shown in a common gauge, a Weston ammeter, or ampere-meter, may be employed. It is similar to the voltmeter just described, being in fact a milli-voltmeter actuated by the difference in electrical potential, or pressure, between the terminals of a standard resistance, the shunt, through which a definite fraction of the current passes. It is as if a known part of the flow of a river being measured, the volume of the whole stream is learned.

The two principal alloys discovered by Dr. Weston, and used in his instruments, are manganin and nickelin. Manganin has about twenty-five times the resistance of copper, and increases in resistance about 0.00001 for each degree Centigrade through which its temperature rises. Nickelin has about twenty-nine times the resistance of copper, and decreases in resistance about 0.00004 for each degree Centigrade through which its temperature rises. These and other alloys used in construction are carefully worked and annealed according to methods perfected in years of experience. After a wire for an instrument is drawn, its fibres, being in a state of unequal strain, undergo an artificial aging process so that their resistance shall remain unchanged after adjustment. The Weston instruments are based on the international volt and ampere adopted by the National Bureau of Standards at Washington. Instruments of the regular portable type have a guaranteed accuracy of one part in 400, while the laboratory standard semi-portable instruments are guaranteed to one part in 1000. Weston voltmeters and ammeters are constantly being checked after years of active service, and are found correct within the guaranteed limits of accuracy.

This remarkable success testifies to the importance of asking, What properties are needed in the material of which an instrument is to be built? That question duly answered, it becomes a task for research to provide these materials, that skill may put them together in compact and convenient form.[29]

The Bureau of Standards at Washington.

Whether in the laboratory of the chemist or the physicist, in the machine shop or the engine-room, every means of measurement must be based on standards created with the highest skill and guarded with the utmost care. For the United States these ultimate standards, in full variety, are brought together at the Bureau of Standards at Washington, of which Dr. S. W. Stratton is director. Here are safeguarded copies of the international metre and the kilogram adopted by Executive Order in 1893 as fundamental units of length and mass; here, too, are standard yards and pounds, bearing fixed legal relations to the international metre and kilogram. The Bureau is prepared to determine the length of any standard up to fifty metres, to calibrate its subdivisions, and to determine its coefficient of expansion for ordinary temperatures. To the credit of American workmanship be it said that at times the micrometers received from leading manufacturers, for use in workshops of the best class, are so refined in their measurements as to tax to the utmost the resources of the Bureau. Its precision balances, by Rueprecht of Vienna, and Stuckrath of Berlin, weigh a kilogram within ¹⁄₂₀₀ part of a milligram, that is, within one two-hundred-millionth part of its load.

In the department of electricity a resistance may be measured all the way from ¹⁄_100,000 of an ohm to 100,000 ohms. Here are voltmeters, and wattmeters of the best types. Magnetism, as swiftly summoned or dismissed in the cores of dynamos and motors, is here measured with the utmost exactitude. In some of the instruments fused quartz has been used as a means of suspension because its high elasticity and great strength allow it to be drawn as extremely fine threads. Dr. K. E. Guthe, now of the University of Iowa, while at the head of the section of magnetic measurements, found that fibres equally serviceable may be drawn from steatite, or soapstone, such as forms a common kind of gas-burner. Thick quartz threads break easily when bent, those of steatite do not.

In thermometry, a section in charge of Dr. Waidner, much work goes forward in testing clinical and other thermometers for manufacturers. The whole range of heat measurement is covered by instruments adapted to recording the highest attainable temperatures until we reach apparatus by which, through observation of its light, the absolute temperature of the electric arc has been found to be 3720° C. Measurements of light proceed in another section. Here a photometer designed by Mr. Edward P. Hyde, of the Bureau staff, has reached the hitherto unexampled accuracy of one part in 200. The Bureau has an extensive workshop where new designs for improved apparatus are constantly in hand. For services on behalf of the national or any state government the Bureau makes no charge; moderate fees are required from firms and individuals. In its new and adequate quarters the Bureau is doing work as authoritative as that of any similar institution in the world.

Refined Measurement Improves Machinery.

In manufacturing modern tools and machinery, the thousandth of an inch is the usual limit of allowable error. A micrometer caliper measuring to this limit is here shown. The pitch of its screw is 40 to the inch, and the beveled edge of the screw-thimble is divided into 25 parts, so that motion from one division to the next takes the screw ¹⁄₂₅ of ¹⁄₄₀ of an inch, or ¹⁄₁₀₀₀. By carrying refinement a step farther, ¹⁄_10,000 of an inch can be detected. The production of a screw such as this was simply impossible by the lathe as used almost up to the close of the eighteenth century, its operator holding in his hand a gouge or chisel. Of inestimable importance was Henry Maudslay’s invention of the slide-rest which firmly holds the tool, moving it automatically along the wood or metal being cut. See illustration on page 96. James Watt, as he endeavored to improve the steam engine, before the slide-rest was invented, was sorely vexed and thwarted by the ill-shaped containers for steam which served him as cylinders. Perhaps the chief task accomplished by the lathe has been its own improvement, so that to-day surfaces are readily cut by its tools accurately to within a thousandth part of an inch. Vastly beyond this feat was Professor H. A. Rowland’s production of a virtually perfect screw, which enabled him to rule on concave gratings 5.9 inches square, 110,000 lines with such precision that the error between any two of the lines is probably less than ¹⁄_3,000,000 of an inch. These gratings brought to view spectra much more extended and clear than those observable in a spectroscope, however powerful. The concave plates employed by Professor Rowland were made by Mr. John A. Brashear of Allegheny, Pennsylvania.

Measurement is greatly indebted to accurate means of enlarging the images of objects as viewed in the telescope or the microscope. Glass grinding tools are to-day so exquisitely contoured that a lens forty-two inches in breadth shows the image of a star as an immeasurable dot. It was in pressing together two lenses of very large and known radius that Newton measured the lengths of light-waves. With homogeneous rays, such as those of yellow light, the successive rings of light and darkness marked the points at which the intervals between his lenses were equal to half a light-vibration or any multiple thereof. Measuring these intervals, by noting their distances from the common centre of his lenses, he found the wave-length of the particular light he was studying.