Popular Science Monthly/Volume 4/January 1874/Quicker Than Lightning
THE Faithful have a tradition that Mohammed, on one occasion, in starting for heaven, upset a pitcher with his foot: he had ninety thousand interviews with the Most High, and, when he returned, the water was not yet spilled from the pitcher. It may be admitted that this was quick work, and that Mohammed was undoubtedly smart; but, when it comes to "interviewing," the Arabs must yield to the Yankees. In the laboratory of Columbia College, Prof. Rood has had interviews with one of the messengers of the Most High at a rate that leaves the prophet nowhere. Besides, with all respect to the hundred million believers, the Mussulman story is but a piece of Oriental fancy, while the Christian reports not only what he has actually seen, but can also make others see. Our optics are none of the best, but we have seen the professor run down his ethereal game, and can attest that it was more exciting than a horse-race. Let us consider this "descent of man" into the regions of infinitesimal time.
Of all the curious things that science has revealed, none are so confounding to the ordinary reason as what has been learned respecting the order of Nature in its extremest aspect of minuteness. Objects fade away from the customary range of the senses, and we habitually think, what was long believed to be the fact, that there remains nothing more; or, that we find the edge and final termination of things but little beyond what is familiarly recognized. But we now understand that Nature is fathomless below as well as boundless above, and that, beneath the grasp of unaided sense, there are an inexhaustible wealth of wonders, a fixedness of relations, a definite play of interacting forces, and a sharp exactness in the working of law, which we could never infer from the coarser processes of the world of common experience.
As we are to speak of the briefest known duration of luminous effects, it will be proper first to recall how much is involved in the act of sight. When the man of experiment talks to us about what occurs in the thousandth of a second, he is, of course, dealing with something recognized, or which has affected both his body and his mind in that short space of time, and this is necessarily an illustration of how quickly his composite machinery can work. Then the agency which acts upon him must be taken into account, and also the cause of that agency, for they both belong to the same order of activities. When we look upon a source of illumination, as a candle or a star, we are affected by something that is done at those points. The light originates in the vibration of the molecules of matter. These vibrations are communicated to some medium which can convey the impulses at a demonstrated velocity of nearly 200,000 miles per second. The luminous waves strike the retina of the eye, and they are again translated into the molecular vibrations of nervous matter, and the physical influence is turned into a sensation by the organ of consciousness. The act of seeing thus involves the constitution and action of the visible object, the mode of movement of the force, the operation of the organ of vision, the changes of the nerve-line, and the cerebral act of recognition. There is a dynamic chain connecting thought and the object seen through a nether world of minuteness, but where all is correlated in a common scale of relations; and, whenever we see any thing, this whole train of transformations is implicated in the effect. The molecular tremors of Sirius, the ethereal thrills of space, and the rhythmic swing of the nervous elements, are but parts of a unified system of subsensible dynamics. Bearing in mind, then, what is involved in a single act of vision, let us now trace the course of experiment which has led to the latest results regarding its duration.
Phosphorus, the light-bearer, as its name implies, has the property, long supposed to be peculiar to it, of faintly shining in the dark. But, if a diamond is exposed to sunshine, and then withdrawn into darkness, it continues feebly luminous for a considerable time, and is, therefore, said to be phosphorescent. Other substances, as sulphuret of calcium, and sulphuret of barium, have also been long noted for this property, and recent researches have shown that, so far from being any thing peculiar, the same property is manifested in a much lower degree by a vast number of substances. The differences are in the time the phosphorescence continued after withdrawal from the sun's rays. It was found, in most instances, extremely short, only the small fraction of a second, and it became necessary to devise some means of measuring the time in different cases. A contrivance was necessary which should expose an object to the sun, and then jerk it quickly into total darkness, where it could be seen by the observer if it dragged any light along with it, for even the thousandth of a second.
A contrivance for this purpose was made by Edmund Becquerel, and called the phosphoroscope. It consisted of a train of wheels and pinions (Fig. 1) for producing rapid revolving motion. There was a hollow barrel or case at the top of the machine, pierced with an opening, within which, as seen in the figure, the object to be experimented with is attached to a fixed stand. On the opposite side of the case there is another opening in a corresponding position, not shown in the figure. The outer case does not revolve, but within it there is a pair of disks (Fig. 2) rigidly connected upon a spindle which is turned by the machinery. Each of these disks has four openings, those of the one being not opposite, but midway between those of the other. Of course, then, when these disks are inside the case, it is impossible to see through. The arrangement is then set up in the window of a darkened room, so that one side is turned toward the sun, and the
Disks of Phosphoroscope.
other toward the observer; and, when the disks are turned, the object is alternately exposed to the light from one side, and to the eye from the other; that is, it is seen in a moment after exposure to light, and the duration of the moment can be determined by the rapidity of the rotation. The object, therefore, if not phosphorescent, will never be seen by the observer, as it is always in darkness, except when it is hidden by the intervening disk. But, if its phosphorescence lasts as long as an eighth part of the time of one rotation, it will become visible in the darkness. Suppose, now, that the disks are made to revolve a hundred times in a second, and that the body observed is visible, it is then proved that its phosphorescence lasts the one eight-hundredth of a second, that being the time which elapses between its exposure to the sun and its exposure to the eye. When examined in this way, a very large number of bodies show traces of phosphorescence, although in some cases it is found to last no longer than the ten-thousandth part of a second.
The question was thus opened whether phosphorescence is not a general property of matter, and, to determine this, with the conditions of its manifestation, a more thorough investigation of the subject was needed. Prof. Rood proposed to undertake it, using, if possible, an instantaneous source of illumination—the electric spark. But, in entering upon the inquiry, he soon found himself involved in preliminary difficulties with the spark itself. His phosphorescent investigations remain yet to be carried out, but the results obtained relative to the electric flash are of extreme interest. The full account of the research is given in a series of papers published in Silliman's Journal, and, if the reader finds the following statement insufficient in its details, he will know where to go for further explanations.
Since the time of Franklin, the lightning-flash has been regarded as a gigantic electric spark produced in the atmosphere; the inquiry, therefore, involved the nature of the meteorological discharge, as well as of the spark artificially produced. Various attempts to determine the duration of lightning have been made, with varying results. Faraday observed it, without any instruments for measuring the time, which seemed to last for a second, but he was doubtful if part of the effect was not due to the lingering phosphorescence of the cloud. Decharme observed the lightning-flashes from a distant storm, which also appeared to last for from a half to an entire second. Prof. Dove employed a revolving disk with colored sectors, and satisfied himself that single flashes of lightning often consisted of a number of instantaneous discharges. It is well known that, when a rapidly-moving train of cars is illuminated at night by lightning, it seems to stand still, that is, the duration of the flash is so brief that no motion of the train is perceptible while it lasts. The wheels are sharply defined as if perfectly motionless, but if they had a blurred aspect we should know that the illumination lasted sufficiently long to render the motion perceptible. Prof. Rood extemporized a simple contrivance for observing lightning, which acted upon this principle. It consisted of a white card-board disk, five inches in diameter, with a steel shawl-pin for an axis, on which it was made to revolve by striking the edge. He traced black figures near the circumference of the disk, and when it was in rapid motion these figures were sometimes seen as sharply as though they had been stationary, although they were often blurred as though the disk had moved through a few degrees during the act of discharge. He then cut narrow, radial apertures into the circumference of the disk, and observed the lightning through these openings. Here, again, the apertures were sometimes seen quite unchanged, but they were more frequently elongated into well-defined streaks some degrees in length. He afterward measured the average rate of rotation imparted to the disk in this way, and arrived at the conclusion that the lightning-flashes on the occasion referred to had a duration of about one five-hundredth of a second. Dissatisfied with the roughness of these observations, Prof. Rood arranged a small train of toothed wheels driven by a spring, which rotated a circular pasteboard disk with four open sectors. This instrument gave more regular and precise results; and, while it was shown that the flash sometimes lasts for a whole second, the suggestion of Dove was clearly verified that each flash "consisted of a considerable number of isolated and apparently instantaneous electrical discharges, the interval between the components being so small that, to the naked eye, they constituted a continuous act."
Several curious effects were observed in these experiments. Working with a disk having a single narrow opening, the multiple elements of the discharge were detected with great regularity, and Prof. Rood several times, instead of seeing the opening single, noticed that it had a form resembling the letter X or V, the lines in different positions of the disk having, as it were, got crossed in his eyes by their quick changes of position. On several occasions, when observing with the naked eye, the normal zigzag flashes lasted not less than a second, and the light seemed to pour steadily in a stream from the cloud to the earth. Observations made in the area occupied by a storm, out beyond its edge, and when it was quite distant, gave results that were identical, which the professor thinks furnishes an "argument in support of the hypothesis that zigzag lightning, heat and sheet lightning, etc., are really identical, being, in point of fact, due to the same cause but viewed under different conditions." As the result of these experiments, Prof. Rood concludes: "It is evident, from the foregoing, that the nature of the lightning-discharge is more complicated than has been generally supposed; it is usually, if not always, multiple in character, and the duration of the isolated constituents varies very much, ranging from intervals of time shorter than one one-thousandth of a second up to others at least as great as one-twentieth of a second; and, furthermore, what is singular, a variety of this kind may sometimes be found in the components of a single flash."
Such being the rough conclusions reached concerning the duration of the spark upon a grand scale, let us now consider the results of experiment upon it where all the conditions are in command. In 1835, Mr. Wheatstone attempted to measure the spark of a Leyden jar charged by a common frictional machine. The light from the spark was received upon a mirror mounted upon an axle capable of a high rate of revolution. The image of the spark, being thrown upon the mirror, was reflected to a distant point, and the time of the spark was inferred from the fixity or movement of the image. By using this arrangement, Mr. Wheatstone concluded that the discharge may take place within the millionth of a second; a result which was accepted by the scientific world for a quarter of a century. In 1858, a German named Feddersen, an accomplished physicist, dissatisfied with Wheatstone's results, entered upon a careful reexamination of the subject. He used the revolving-mirror arrangement with frictional electricity; but, as Wheatstone had driven his machinery by strings, Feddersen adopted a train of toothed wheels, and with this form of mechanism he found that the image of the spark was drawn out by the revolving mirror into a whitish streak which indicated that the time of the discharge was not less than the twenty-five-thousandth of a second, while it was inferred that the spark, instead of being a simple effect, is composite like the lightning, and is made up of several elements.
Such were the incomplete and discordant results of the investigation when it was undertaken by Prof. Rood. The arrangement ho devised consisted of two parts, one for the production of the spark, and the other for measuring it. Fig. 3 represents the first combination. A galvanic battery was used to generate the electricity; this was connected with a large Ruhmkorff coil, which was again connected with a Leyden jar, and this with the electrodes for producing the spark, S>
which were adjustable for varying its "striking distance." Connected with the wires between the battery and the coil was an automatic "interruptor" for breaking the circuit from three to six times in a second, by which the frequency of the discharges could be regulated. Leyden jars of different sizes could be used so as to give sparks of all degrees of strength and intensity.
In the second part of his arrangement, Prof. Rood, like his predecessors, employed a revolving mirror, turned by the gearing of Becquerel's phosphoroscope (Fig. 1), with the addition of an extra wheel and a weight to drive it. With this he could get 350 revolutions of the mirror per second, with a smooth and uniform motion. In order to measure exactly the rate of rotation, the cylinder on the lowest wheel was made to wind up a fillet of paper, upon which dots were made by an electro-magnetic apparatus, regulated by a seconds-pendulum, when a simple calculation gave the rate of the wheel to which the mirror was attached, and the regularity of the train was thus put to a sharp test. The light of the spark S (Fig. 4), passing through an achromatic lens, l, struck the mirror, m, and was reflected upward, forming an image at i, on the plate of ground glass G. The image of the spark on the ground glass was viewed from above, and its position and form were carefully measured by several methods. Of course, if the spark was absolutely instantaneous, its image thrown upon the ground glass would be exactly the same, whether the mirror was motionless or was revolving at the highest speed. But, if the spark had an appreciable duration, its image would be prolonged or drawn out into a streak, the length of which must depend upon the time of discharge. The rate of the mirror's rotation being known, also the distance, m i, and the length of the streak, it was easy to calculate the total duration of the spark.
Prof. Rood now had the subtile agent he was pursuing pretty effectually in his grasp, and the results that came out were very striking. The ordinary spark was found to be a highly-complex effect; to consist of diverse and successive elements, and, in fact, to have its periods and orderly history just like the geology of the globe. But, while the "vast durations" of Lyell and Dana are vague and inferential, these infinitesimal periods could be demonstrated with the greatest exactness. The previous discordant results were reconciled, Feddersen being justified in assigning a longer period for the total duration of the spark, and Wheatstone's time holding true of its elements.
With a Leyden jar of about a quart capacity (114.4 square inches of coating), and all the connections as short as possible, so as to offer the least amount of resistance to the electric flow, with brass balls as electrodes, with a striking distance of about the twenty-fifth of an inch, and the velocity of the mirror up to 223 per second, the image of the spark thrown upon the ground glass and viewed by the naked eye was drawn out into a streak one and a half or two inches long, the length, however, varying with the speed of the mirror. The aspects of the image are represented in Fig. 5. The first part was pure white, which shaded into a brownish-yellow tint, passing on into a pretty distinct green. When a polished plate of glass was substituted for the ground glass, and a small magnifier was used to observe the image, a series of bright points, on each side of the streak, became visible, in the positions indicated by the dots in Fig. 5. With high velocities, this succession of points was beautifully developed, and it consists of a series of separate discharges following the first. It was thus found that the Leyden jar furnished a number of single sparks, each time the coil was excited, the number varying between one and thirty, according to circumstances. The whole proceeding consumed an interval of time often as great as one-fiftieth of a second; that is, the jar loaded up and discharged itself twenty or thirty times in that period. Prof. Rood found the number of elements of the spark to vary with its length, the nature of the electrodes, and the size of the jar. Short sparks are more complex than long ones, small jars give more than large ones, and metallic points a greater number than balls. The point to be determined was, the duration of the several elements of the spark, and especially of its quickest element. In one case of a discharge lasting the fiftieth of a second, it began with an ordinary spark, followed by a pale-violet light, lasting about one-sixtieth of a second, and then came a compact
Images of Spark drawn out.
body of ten or twenty sparks, this last act continuing for about one two-hundredth of a second. The results of the inquiry are thus stated by Prof. Rood: "From the foregoing, then, it appears that, if a jar, having a metallic coating of about one hundred square inches, be connected, as above described, with an induction-coil, its discharge will be effected by a considerable number of acts, of which the first is by far the most intense. Further, the metallic particles, heated up by the first discharge to a white heat, almost instantly assume a lower temperature, marked by a corresponding change from white to brownish yellow; and, as their temperature continues to fall, the tint changes, in the case of brass electrodes, to green; in that of platinum, to a gray or violet-gray. These observation's further demonstrated the fact that four ten-millionths of a second is an interval of time quite sufficient for the production of distinct vision."
It was also shown that the first act of the electric explosion, represented by the white band, lasted through an interval of time so short as to be immeasurable. It was proved that it could not occupy more than the millionth or the half a millionth of a second, but how much less time it might occupy remained to be determined.
Prof. Rood now prepared for a more rigorous course of experiments. He used a small Leyden jar, with a surface of eleven inches, about equal to a moderate-sized wine-glass. To secure greater exactness of observation, he devised a peculiar micrometer, consisting of five lines ruled on a plate of glass smoked by lamp-black. This plate was placed between l and S (see Fig. 4), but quite near to the latter, and an image of the lines reflected from the mirror was formed on the clear glass at i. The lines were observed by a microscope magnifying ten diameters. In using this micrometer, the measurement was effected by noticing at what rate of the revolving mirror the lines in the image at i were obliterated, this obliteration being due to the circumstance that by the motion of the mirror the dark lines were superposed on the bright lines. The individual spark now produced was about a millionth of a second in duration, but the faint train was still observable. There was still the brilliant body of the spark appearing, first followed by a faint streak of less than one-hundredth the illuminating power of the first stage. The diagram, Fig. 6, represents the intensity
and time of the spark. The elevation, or peak, a b, shows the intensity of the first compact body of the spark, and the line a c the duration of the whole effect. The point was to get the time of a b, which Prof. Rood had proved must be regarded as a distinct act in the succession of effects. All precautions for observation being carefully made, the driving-weight was gradually increased, and the speed of the mirror carried up to 350 revolutions per second, when the lines of the image, which at first remained visibly as distinct as with a stationary mirror, became regularly less distinct, and at length vanished by the gradual superposition of the white and black lines. Prof. Rood says: "It was proved successively that the duration was less than eighty, sixty-eight, fifty-nine, fifty-five billionths of a second; and, finally, the lines, after growing fainter and fainter, entirely disappeared, giving as the result a duration of forty-eight billionths of a second." By reducing the striking distance, a still lower figure was reached, so that the professor states that "the duration of the first act of the electrical discharge is in certain cases only forty billionths of a second, an interval of time just sufficient to enable a ray of light to travel over forty feet." The duration was twenty-five times smaller than had ever before been measured. In this infinitesimal portion of time a strong and distinct impression upon the retina is made, so that "the letters on a printed page are plainly to be seen; also, if a polariscope be used, the cross and rings around the axis of crystals can be observed with all their peculiarities." Nor is this all; "as the obliteration of the micrometric lines could only take place from the circumstance that the retina retains and combines a whole series of impressions whose joint duration is forty billionths of a second, it follows that a much smaller interval of time will suffice for vision. If we limit the number of views of the lines presented to the eye in a single case to ten, it would result that four billionths of a second is sufficient for human vision."
We saw at the outset how much an act of vision involves, and we have now some idea of how long it takes. If the discharge of the thunder-cloud occupies, as was stated, the one five-hundredth of a second, the "interviews" of our philosopher with the "amber-spirit" were at least fifty thousand times "quicker than lightning."