Kitabı oku: «Benjamin Franklin; Self-Revealed, Volume 2 (of 2)», sayfa 23

Another letter from Franklin to Collinson, written about two weeks later, communicated to him some valuable observations upon "M. Muschenbroeck's wonderful bottle" – the Leyden Jar. This bottle was a mere ordinary bottle, with a common cork in its neck, into which a common wire had been inserted. He wrote that, at the same time that the wire and the top of the bottle were electrised positively or plus, the bottom of the bottle was electrised negatively or minus, in exact proportion; the consequence was that, whatever quantity of electrical fire was thrown in at the top, an equal quantity went out at the bottom until, if the process was kept up long enough, the point was reached in the operation, when no more could be thrown into the upper part of the bottle, because no more could be drawn out of the lower part. If the attempt was made to throw more in, the fire was spewed back through the wire, or flew out in loud cracks through the sides of the bottle.

He also noted that an equilibrium could not be restored in the bottle by inward communication or contact of the parts, but only by a communication, formed without the bottle between its top and bottom.

He also noted that no electrical fire could be thrown into the top of the bottle, when none could get out at its bottom, either because the bottom was too thick, or because it stood on some non-conducting material, and likewise that, when the bottle was electrified, but little of the electrical fire could be drawn from the top by touching the wire, unless an equal quantity could at the same time get in at the bottom.

So wonderfully [he adds] are these two states of electricity, the plus and minus, combined and balanced in this miraculous bottle! situated and related to each in a manner that I can by no means comprehend! If it were possible that a bottle should in one part contain a quantity of air strongly comprest, and in another part a perfect vacuum, we know the equilibrium would be instantly restored within. But here we have a bottle containing at the same time a plenum of electrical fire, and a vacuum of the same fire; and yet the equilibrium cannot be restored between them but by a communication without! though the plenum presses violently to expand, and the hungry vacuum seems to attract as violently in order to be filled.

The letter concludes with an elaborate statement of the experiments by which the correctness of its conclusions could be established.

Franklin's next discovery communicated to Collinson in a letter dated the succeeding year was that, when the bottle was electrified, the electric fluid resided in the glass itself of the bottle. The manner in which he proved this fact is a good example of his inductive thoroughness.

Purposing [he said] to analyze the electrified bottle, in order to find wherein its strength lay, we placed it on glass, and drew out the cork and wire, which for that purpose had been loosely put in. Then taking the bottle in one hand, and bringing a finger of the other near its mouth, a strong spark came from the water, and the shock was as violent as if the wire had remained in it, which shewed that the force did not lie in the wire. Then, to find if it resided in the water, being crouded into and condensed in it, as confin'd by the glass, which had been our former opinion, we electrified the bottle again, and, placing it on glass, drew out the wire and cork as before; then, taking up the bottle, we decanted all its water into an empty bottle, which likewise stood on glass; and taking up that other bottle, we expected, if the force resided in the water, to find a shock from it; but there was none. We judged then, that it must either be lost in decanting, or remain in the first bottle. The latter we found to be true; for that bottle on trial gave the shock, though filled up as it stood with fresh unelectrified water from a teapot.

By a similar course of experimentation with sash glass and lead plates, he also demonstrated that the form of the glass in the bottle was immaterial, that the power resided in the glass as glass, and that the non-electrics in contact served only like the armature of a loadstone to unite the force of the several parts, and to bring them at once to any point desired; it being the property of a non-electric that the whole body instantly receives or gives what electric fire is given to, or taken from, anyone of its parts. These experiments suggested the idea of intensifying the application of electrical forces by grouping numerous electrical centres.

We made [he said] what we called an electrical battery, consisting of eleven panes of large sash-glass, arm'd with thin leaden plates, pasted on each side, placed vertically, and supported at two inches distance on silk cords, with thick hooks of leaden wire, one from each side, standing upright, distant from each other, and convenient communications of wire and chain, from the giving side of one pane, to the receiving side of the other; that so the whole might be charged together, and with the same labour as one single pane; and another contrivance to bring the giving sides, after charging, in contact with one long wire, and the receivers with another, which two long wires would give the force of all the plates of glass at once through the body of any animal forming the circle with them. The plates may also be discharged separately, or any number together that is required.

When the idea of the electrical battery was formed by him, Franklin was not aware that Smeaton and Bains had previously assembled panes of glass for the purpose of giving an electrical shock.

At the time that this letter was written, Franklin had added to his electrical exploits that of electrifying a mezzotint of the King in such a manner that, if anyone attempted to take the crown off his head, he would receive a "terrible blow."

If the picture were highly charged [he said], the consequence might perhaps be as fatal as that of high treason.

The operator [he continues], who holds the picture by the upper end, where the inside of the frame is not gilt, to prevent its falling, feels nothing of the shock, and may touch the face of the picture without danger, which he pretends is a test of his loyalty. If a ring of persons take the shock among them, the experiment is called The Conspirators.

Another far more significant exploit was the application of electrical energy in such a way as to set an electrical Jack revolving with such force and swiftness as to carry a spitted fowl around before a fire with a motion fit for roasting.

This wheel was driven by an electrical battery, but Franklin also devised what he called a self-moving wheel that was, by a different electrical method, revolved with so much force and rapidity that he thought that it might be used for the ringing of chimes and the movement of light-made orreries. And after observing that a thin glass bubble, about an inch in diameter, weighing only six grains, being half filled with water, partly gilt on the outside, and furnished with a wire hook, gave, when electrified, as great a shock as a man can well bear, Franklin exclaims, "How great must be the quantity (of electrical fire) in this small portion of glass! It seems as if it were of its very substance and essence. Perhaps if that due quantity of electrical fire so obstinately retained by glass, could be separated from it, it would no longer be glass; it might lose its transparency, or its brittleness, or its elasticity."

This letter also reaches the conclusion that bodies, having less than the common quantity of electricity, repel each other, as well as those that have none.

It concludes with a lively paragraph:

Chagrined a little that we have been hitherto able to produce nothing in this way of use to mankind; and the hot weather coming on, when electrical experiments are not so agreeable, it is proposed to put an end to them for this season, somewhat humorously, in a party of pleasure on the banks of Skuylkil. Spirits, at the same time, are to be fired by a spark sent from side to side through the river, without any other conductor than the water; an experiment which we some time since performed, to the amazement of many. A turkey is to be killed for our dinner by the electrical shock, and roasted by the electrical jack, before a fire kindled by the electrified bottle; when the healths of all the famous electricians in England, Holland, France and Germany are to be drank in electrified bumpers, under the discharge of guns from the electrical battery.

An electrified bumper, a note to the letter explained, was a small thin glass tumbler, nearly filled with wine, and charged, which, when brought to the lips of a person, gave him a shock, if he was close-shaved, and did not breathe on the liquor. Another note states that the biggest animal that the experimenters had yet killed was a hen.

A later letter to Collinson on the phenomena of thunder-gusts takes Franklin away from the Leyden Jar of the laboratory to the stupendous batteries of the outer universe – from the point of a bodkin to the lofty natural or artificial objects, upon which lightning descends from the illimitable sky. "As electrified clouds pass over a country," he remarks, "high hills and high trees, lofty towers, spires, masts of ships, chimneys, &c., as so many prominencies and points, draw the electrical fire, and the whole cloud discharges there." From this observation to the lightning rod was but a short step.

Another letter to Collinson in the succeeding year brings us to the lightning rod in principle if not in name. Speaking of what a sea captain had said of luminous objects, which had settled on the spintles at the topmast heads of his ship before an electrical shock, and burned like very large torches, he says:

According to my opinion, the electrical fire was then drawing off, as by points, from the cloud; the largeness of the flame betokening the great quantity of electricity in the cloud: and had there been a good wire communication from the spintle heads to the sea, that could have conducted more freely than tarred ropes, or masts of turpentine wood, I imagine there would either have been no stroke; or, if a stroke, the wire would have conducted it all into the sea without damage to the ship.

In the same letter, there is an adumbration of his grandest experiment, when he speaks of the flash from two of his jars as "our mimic lightning."

This letter also shows that with electricity Franklin had frequently imparted polarity to needles and reversed it at pleasure. Wilson, at London, he said, had failed to produce these results because he had tried it on too large masses and with too small force. The letter also evidences the fact that he had employed the electric spark for the practical purpose of firing gunpowder.

Another letter to Collinson dated July 29, 1750, is accompanied by an additional paper on the properties and effects of the Electrical Matter. It acknowledges the debt that Franklin owed to Collinson for the glass tube and the instructions which attended it, and to the Proprietary for the generous present of a complete electrical apparatus which "that bountiful benefactor to our library," as he calls him, had made to it. The telegraph, the Marconi tower, the telephone, the electric bulb, the electric automobile and the trolley car rise up before us when we read this observation in the paper that accompanied the letter: "The beneficial uses of this electric fluid in the creation, we are not yet well acquainted with, though doubtless such there are, and those very considerable." The paper is the most important that Franklin ever wrote on electricity; containing as it does the two suggestions which, when carried into execution, made his name famous throughout the world, that is to say, his suggestion, already quoted by us at length, that houses, churches and ships might be protected by upright rods of iron, and his suggestion, already quoted by us, too, as to how the identity of lightning and electricity could be established. The point of the bodkin and the electrified shot and ball, and the mimic brightness, agility and fury of the lurking fire in the wonderful bottle had led, step by step, to two of the most splendid conceptions in the early history of electrical science.⁵⁴

With the discovery that electricity and lightning were the same thing, the real achievements of Franklin in the province of electricity came to an end. But he still continued his electrical experiments with undiminished ardor. We find him on one occasion prostrating with a single shock six persons who were so obliging as to lend themselves to the pursuit of scientific truth. Twice he was the victim of his own inadvertence. Speaking of one of these occasions, in a letter to a friend in Boston, he said:

The flash was very great, and the crack as loud as a pistol; yet, my senses being instantly gone, I neither saw the one nor heard the other; nor did I feel the stroke on my hand, though I afterwards found it raised a round swelling where the fire entered, as big as half a pistol-bullet; by which you may judge of the quickness of the electrical fire, which by this instance seems to be greater than that of sound, light, or animal sensation… I then felt what I know not how well to describe; a universal blow throughout my whole body from head to foot, which seemed within as well as without; after which the first thing I took notice of was a violent quick shaking of my body, which gradually remitting, my sense as gradually returned, and then I thought the bottles must be discharged, but could not conceive how, till at last I perceived the chain in my hand, and recollected what I had been about to do. That part of my hand and fingers, which held the chain, was left white, as though the blood had been driven out, and remained so eight or ten minutes after, feeling like dead flesh; and I had a numbness in my arms and the back of my neck, which continued till the next morning, but wore off. Nothing remains now of this shock, but a soreness in my breast-bone, which feels as if it had been bruised. I did not fall, but suppose I should have been knocked down, if I had received the stroke in my head. The whole was over in less than a minute.

On the second occasion, while making ready to give a healing shock to a paralytic, he received a charge through his own head. He did not see the flash, hear the report or feel the stroke.

When my Senses returned [he told Jan Ingenhousz], I found myself on the Floor. I got up, not knowing how that had happened. I then again attempted to discharge the Jars; but one of the Company told me they were already discharg'd, which I could not at first believe, but on Trial found it true. They told me they had not felt it, but they saw I was knock'd down by it, which had greatly surprised them. On recollecting myself, and examining my Situation, I found the Case clear. A small swelling rose on the Top of my Head, which continued sore for some Days; but I do not remember any other Effect good or bad.

One of Franklin's contemporaries, Professor Richmann, of St. Petersburg, did not fare so well; for a stroke of the lightning that he had allured from the clouds brought his life to an end. Priestley, however, seems to have regarded such a death as a form of euthanasia. At any rate, in speaking of this martyr of science in his History of Electricity he terms him "the justly envied Richmann."

After Franklin learned how to impound lightning, his intercourse with electricity was more familiar than ever.

In September, 1752 [he wrote to Collinson], I erected an iron rod to draw the lightning down into my house, in order to make some experiments on it, with two bells to give notice when the rod should be electrify'd: a contrivance obvious to every electrician.

I found the bells rang sometimes when there was no lightning or thunder, but only a dark cloud over the rod; that sometimes, after a flash of lightning, they would suddenly stop; and, at other times, when they had not rang before, they would, after a flash, suddenly begin to ring; that the electricity was sometimes very faint, so that, when a small spark was obtain'd, another could not be got for some time after; at other times the sparks would follow extremely quick, and once I had a continual stream from bell to bell, the size of a crow quill: Even during the same gust there were considerable variations.

In the winter following I conceived an experiment, to try whether the clouds were electrify'd positively or negatively.

The result of these experiments, conducted with Franklin's usual painstaking completeness, was the conclusion on his part that thunder-clouds are, as a rule, in a negatively electrical state, and that, therefore, generally speaking, they do not discharge electricity upon the earth, but receive it from the earth. For the most part, he said, "tis the earth that strikes into the clouds, and not the clouds that strike into the earth."

The thoroughness with which he addressed himself to the study of electricity was very marked. His investigation was as searching and minute as that of an anatomist engaged in the dissection of nervous tissue. Under his hands, the bare Leyden Jar became a teeming storehouse of instruction and amusement. He collected electricity from common objects by friction, he brought it down from the sky, he sought its properties in amber, in the tourmaline stone, in the body of the torpedo; he thought that he discerned it in the radiance of the Aurora Borealis. He put it through all its vagaries, juggled with it, teased it, cowed it until it confessed its kinship with the tempestuous heavens. He tested its destructive effects upon hens and turkeys, its therapeutic value to paralytic patients, its efficacy as a corrective of tough meat. He even, it is said, charged the railing under his windows with it to repel loafers standing about his front door. And, in his relations to electricity, as to everything else, his purposes were always those of practical utility. In one of his papers, he admits that he cannot tell why points possess the power of drawing off the electrical fire;

nor is it of much importance to us [he adds] to know the manner in which nature executes her laws. 'Tis enough if we know the laws themselves. 'Tis of real use to know that china left in the air unsupported will fall and break; but how it comes to fall, and why it breaks, are matters of speculation. 'Tis a pleasure indeed to know them, but we can preserve our china without it.

He anticipated, or, in some instances, all but anticipated, several of the more important discoveries of modern electrical science. He knew that, when a number of Leyden jars are connected up under certain conditions, the extent, to which each jar can be charged from a given source, varies inversely as the number of jars. For a time, he was puzzled by the fact that the light of a candle, or of a fire-coal, or of red-hot iron, would destroy the repellency between his electrified ball and shot, but that the light of the sun would not. But it was not long before he hit upon this ingenious explanation:

This different Effect probably did not arise from any difference in the light, but rather from the particles separated from the candle, being first attracted and then repelled, carrying off the electric matter with them; and from the rarefying the air, between the glowing coal or red-hot iron, and the electrised shot, through which rarefied air the electric fluid could more readily pass.

Referring to what Franklin had to say about the action of sunlight in this connection, Arthur Schuster, in his Some Remarkable Passages in the Writings of Benjamin Franklin, observes: "Had Franklin used a clean piece of zinc instead of iron shot he might have anticipated Hertz's discovery of the action of strong light on the discharge of gases."

In the course of one of his experiments with an electrified can, Franklin reached the conclusion that a cork, which he had lowered into the can, was not attracted to its internal surface, as it would have been to its external, because the mutual repulsion of the two inner opposite sides of the can might prevent the accumulation of an electrical atmosphere upon them. From the same experiment, the genius of Henry Cavendish deduced his law that electrical repulsion varies inversely as the square of the distance between the charges.

Instead of declining, it can truly be said that the reputation of Franklin as an electrical investigator and writer has increased with the progress of electrical science. "We shall, I am sure," remarks Professor J. J. Thomson in his Electricity and Matter, "be struck by the similarity between some of the views which we are led to take by the results of the most recent researches, with those enunciated by Franklin in the very infancy of the subject." Nor should we omit a tribute of Dr. William Garnett, in his Heroes of Science, in regard to the statements in Franklin's first letters to Collinson. "They are," he says, "perfectly consistent with the views held by Cavendish and by Clerk Maxwell, and, though the phraseology is not that of modern text-books, the statements themselves can hardly be improved upon to-day."

If Franklin achieved a higher degree of success in the electrical than in any other scientific field, it was partly, at any rate, because he never again had the opportunity to give such continuous attention to scientific pursuits. To him this was at times a source of very great disappointment. In one of his letters to Beccaria, dated Sept. 21, 1768, he tells the latter that, preoccupied as he was, he had constantly cherished the hope of returning home, where he could find leisure to resume the philosophical studies that he had shamefully put off from time to time. In a letter, some eleven years later, from Paris, to the same correspondent, he said that he was then prevented by similar distractions from pursuing those studies in which he always found the highest satisfaction, and that he was grown so old as hardly to hope for a return of the leisure and tranquillity, so necessary for philosophical disquisitions. To Sir Joseph Banks he was inspired some years later, by recent astronomical discoveries, made under the patronage of the Royal Society, to write: "I begin to be almost sorry I was born so soon, since I cannot have the happiness of knowing what will be known 100 years hence," Indeed, to him, leisure, whether only the seclusion of a thirty-day voyage across the Atlantic, or the final cessation of public life, was but another term for recurrence to his scientific predilections. When he received his leave from Congress to return home from Paris, he wrote joyously to Ingenhousz: "I shall now be free of Politicks for the Rest of my Life. Welcome again my dear Philosophical Amusements." There was, to use his own expression, still too much flesh on his bones for his countrymen to allow him any time except for political experiments; but, for proof of the eager interest that he felt in science, and of the prominent position, that he occupied in the scientific world of America, until the last, we need go no further than the fact that, when he died, the meetings of the American Philosophical Society had, for some time, been held at his home in Philadelphia.

How far Franklin might have added to his reputation as a man of science, if he had not become engrossed by political duties and cares, is mere matter of surmise. But there can be no doubt that he was eminently fitted in many respects for scientific inquiry. The scientific temperament he possessed in the very highest degree. He loved the truth too much to allow the workings of human weakness in himself or others to deface its fair features. In reporting to Collinson the electrical achievements, which crowned him with such just renown, he almost invariably spoke of them as if they were the joint achievements of a group of collaborators, of whom he was but one. The generous alacrity, with which he credits to his friends Hopkinson, Kinnersley, or Syng exclusively special discoveries or inventions, made by them, shows conclusively enough how little this was true. There is no reason to believe that his letters to Collinson on electricity would ever have been published but for the unsolicited initiative of Dr. Fothergill and Collinson; or that they would ever have been translated into French but for the spontaneous persuasion that Buffon brought to bear upon D'Alibard. In a letter to Collinson, after expressing distrust of an hypothesis, advanced by him in former letters to the same correspondent, he declares that he is ashamed to have expressed himself in so positive a manner. Indeed, he said, he must request Collinson not to expose those letters, or, if he communicated them to any of his friends, at least to conceal the name of the author. His attitude towards his scientific triumphs was, when not that of entire self-effacement, always that of unaffected humility.

I am indebted for your preceding letter [he wrote in his forty-seventh year to John Perkins] but business sometimes obliges one to postpone philosophical amusements. Whatever I have wrote of that kind, are really, as they are entitled, but Conjectures and Suppositions; which ought always to give place, when careful observation militates against them. I own I have too strong a penchant to the building of hypotheses; they indulge my natural indolence: I wish I had more of your patience and accuracy in making observations, on which, alone, true philosophy can be founded.

Equally candid and noble are other observations in a subsequent letter to the same correspondent. Referring to certain objections, made by Perkins to his theory of water spouts, he observed:

Nothing certainly can be more improving to a Searcher into Nature, than Objections judiciously made to his Opinions, taken up perhaps too hastily: For such Objections oblige him to re-study the Point, consider every Circumstance carefully, compare Facts, make Experiments, weigh Arguments, and be slow in drawing Conclusions. And hence a sure Advantage results; for he either confirms a Truth, before too lightly supported; or discovers an Error, and receives Instruction from the Objector.

In this View I consider the Objections and Remarks you sent me, and thank you for them sincerely.

When he found that he was in error, it cost him no struggle to recant. For a while he believed the sea to be the grand source of lightning, and built up an imposing fabric of conclusions upon the belief; but he did not hesitate afterwards to admit that he had embraced this opinion too hastily. The same thing is true of the opinion that he held for a time, that the progress of a ship westward, across the Atlantic, is retarded by the diurnal motion of the earth. He supposed that the melting brought about by the action of lightning was a cold fusion until holes burnt in a floor by portions of a molten bell wire convinced him that this was not so.

I was too easily led into that error [he said] by accounts given even in philosophical books, and from remote ages downwards, of melting money in purses, swords in scabbards, etc. without burning the inflammable matters that were so near those melted metals. But men are, in general, such careless observers, that a philosopher can not be too much on his guard in crediting their relations of things extraordinary, and should never build an hypothesis on anything but clear facts and experiments, or it will be in danger of soon falling, as this does, like a house of cards.

In one of his letters to Collinson, he declared that, even though future discoveries should prove that certain conjectures of his were not wholly right, yet they ought in the meantime to be of some use by stirring up the curious to make more experiments and occasion more exact disquisitions. Following out the same thought in another letter to Collinson he concluded: "You are at liberty to communicate this paper to whom you please; it being of more importance that knowledge should increase, than that your friend should be thought an accurate philosopher." In a letter to John Lining, in which he described the experiment from which Cavendish deduced the law of which we have spoken, he observed:

I find a frank acknowledgement of one's ignorance is not only the easiest way to get rid of a dificulty, but the likeliest way to obtain information, and therefore I practise it: I think it an honest policy. Those who affect to be thought to know everything, and so undertake to explain everything often remain long ignorant of many things that others could and would instruct them in, if they appeared less conceited.

The fact is that Franklin had such a keen sense of the dignity and invincibility of truth that he could not be induced to enter into any personal controversy about it. His feelings with regard to such controversies are pointedly expressed in the Autobiography in connection with the attack made by the Abbé Nollet upon his electrical experiments.

I once purpos'd [he said] answering the abbé, and actually began the answer; but, on consideration that my writings contain'd a description of experiments which anyone might repeat and verify, and if not to be verifi'd, could not be defended; or of observations offer'd as conjectures, and not delivered dogmatically, therefore not laying me under any obligation to defend them; and reflecting that a dispute between two persons, writing in different languages, might be lengthened greatly by mistranslations, and thence misconceptions of one another's meaning, much of one of the abbé's letters being founded on an error in the translation, I concluded to let my papers shift for themselves, believing it was better to spend what time I could spare from public business in making new experiments, than in disputing about those already made.

But in this instance, too, after all, he acted upon the principle, stated in one of his letters to Cadwallader Colden, that he who removes a prejudice, or an error from our minds contributes to their beauty, as he would do to that of our faces who should clear them of a wart or a wen. He went through his experiments again, and satisfied himself that the Abbé had not shaken his positions. At one time, when he was hesitating as to whether he should reply to him, he heard that D'Alibard was preparing to do so. "Perhaps," he wrote to his friend, James Bowdoin, "it may then appear unnecessary for me to do anything farther in it. And will not one's vanity be more gratified in seeing one's adversary confuted by a disciple, than even by one's self?" When Wilson published a pamphlet, contending that lightning rods should be blunt rather than pointed, he simply observed, "I have not answered it, being averse to Disputes."