Benjamin Franklin; Self-Revealed, Volume 2 (of 2)

The interest of Franklin in electrical science was but another sequel of the world-wide avidity with which learned men had recently turned to the study of that subject. One of them, Grey, had pursued a series of experiments for the purpose of determining the relative conductivity of various substances, another, Du Fay, had erroneously classified electricity as resinous and vitreous, and the perfected Leyden Jar particularly had given a new momentum to the progress of electrical investigation. Into this movement, after witnessing Dr. Spence's awkward experiments at Boston, Franklin threw himself with the utmost enthusiasm, and his discovery of the identity of lightning and electricity and his lightning-rod conception were but the chief fruits of this enthusiasm. Between the Autobiography and his letters, we are at no loss to follow closely the steps by which he reached all the results which have given him such a high position as an electrical investigator. "I purchased all Dr. Spence's apparatus …" he tells us in the Autobiography, "and I proceeded in my electrical experiments with great alacrity." How keen this alacrity became, after he had been rubbing for a time the glass tube, sent over to Philadelphia by Collinson, may be seen in what he wrote to Collinson himself on March 28, 1747:

For my own part, I never was before engaged in any study that so totally engrossed my attention and my time as this has lately done; for what with making experiments when I can be alone, and repeating them to my Friends and Acquaintance, who, from the novelty of the thing, come continually in crouds to see them, I have, during some months past, had little leisure for anything else.

The result of this experimentation was the various letters to Collinson and others that constitute Franklin's highest claim to distinction as a man of science. By following them in their chronological order, the reader can trace with little difficulty the genesis of each of his more valuable conclusions touching electricity. They are distinguished by remarkable simplicity and force of reasoning and by a clearness of statement as transparent as crystal. Moreover, they are even enlivened at times by gleams of fancy or humor. In a word they indisputably merit the judgment that Sir Humphry Davy, no mean judge of style as well as scientific truth, passes upon them:

The style and manner of his publication on electricity are almost as worthy of admiration as the doctrine it contains. He has endeavoured to remove all mystery and obscurity from the subject. He has written equally for the uninitiated and the philosopher; and he has rendered his details amusing as well as perspicuous, elegant as well as simple. Science appears in his language in a dress wonderfully decorous, the best adapted to display her native loveliness. He has in no instance exhibited that false dignity, by which philosophy is kept aloof from common applications; and he has sought rather to make her a useful inmate and servant in the common habitations of man, than to preserve her merely as an object of admiration in temples and palaces.

While recalling these words, it is not amiss to recall, too, what Lord Brougham had to say about the agencies with which Franklin conducted his experiments.

He could make an experiment [said Brougham] with less apparatus and conduct his experimental inquiry to a discovery with more ordinary materials than any other philosopher we ever saw. With an old key, a silk thread, some sealing wax and a sheet of paper he discovered the identity of lightning and electricity.

The truth of these observations is strikingly instanced in a story told of Franklin in Pettigrew's Life of Lettsom. When Henry Smeathman was insisting that the flight of birds is on inclined planes, and that they could not fly at all, but would simply float with the wind, if they were not heavier than the air, Franklin launched half a sheet of paper obliquely into the air, observing, as he watched its course, that that was an evident proof of the propriety of Smeathman's doctrines.

In a letter to Collinson, dated July 11, 1747, Franklin communicated to him the earliest results of his experimental use of the glass tube that Collinson had sent over to Philadelphia. The first phenomenon, which fixed his attention, was the wonderful effect of pointed bodies in drawing off the electrical fire. This was the lightning rod in its protoplasmal stage. The manner in which he described the experiment, by which this particular truth was demonstrated, is a good specimen of his remarkable faculty for simple and clear statement:

Place an iron shot of three or four inches diameter on the mouth of a clean dry glass bottle. By a fine silken thread from the ceiling, right over the mouth of the bottle, suspend a small cork ball, about the bigness of a marble; the thread of such a length, as that the cork ball may rest against the side of the shot. Electrify the shot, and the ball will be repelled to the distance of four or five inches, more or less, according to the quantity of Electricity. When in this state, if you present to the shot the point of a long slender sharp bodkin, at six or eight inches distance, the repellency is instantly destroy'd, and the cork flies to the shot. A blunt body must be brought within an inch, and draw a spark, to produce the same effect. To prove that the electrical fire is drawn off by the point, if you take the blade of the bodkin out of the wooden handle, and fix it in a stick of sealing wax, and then present it at the distance aforesaid, or if you bring it very near, no such effect follows; but sliding one finger along the wax till you touch the blade, and the ball flies to the shot immediately. If you present the point in the dark, you will see, sometimes at a foot distance, and more, a light gather upon it, like that of a firefly, or glowworm; the less sharp the point, the nearer you must bring it to observe the light; and, at whatever distance you see the light, you may draw off the electrical fire, and destroy the repellency. If a cork ball so suspended be repelled by the tube, and a point be presented quick to it, tho' at a considerable distance, 'tis surprizing to see how suddenly it flies back to the tube. Points of wood will do near as well as those of iron, provided the wood is not dry; but perfectly dry wood will no more conduct electricity than sealing-wax.

The repellency between the ball and the shot was likewise destroyed, Franklin stated, 1, by sifting fine sand on it; this did it gradually, 2, by breathing on it, 3, by making a smoke about it from burning wood, and 4, by candlelight, even though the candle was at a foot distance; these did it suddenly.

The same result was also produced, he found, by the light of a bright coal from a wood fire, or the light of red-hot iron; but not at so great a distance. Such was not the effect, however, he said, of smoke from dry resin dropped on hot iron. It was merely attracted by both shot and cork ball, forming proportionable atmospheres round them, making them look beautifully, somewhat like some of the figures in Burnet's or Whiston's Theory of the Earth.

Franklin also noted the fact that, unlike fire-light, sunlight, when thrown on both cork and shot, did not impair the repellency between them in the least.

In the same letter, guided by the belief that he had formed that electricity is not created by friction but, except when accumulated or depleted by special causes, is equally diffused through material substances generally, he also reached the conclusion that electrical discharges are due to circuits set up by substances that offer little resistance to the transit of the electrical current between bodies charged with more than the ordinary quantity of electrical energy and bodies not in that condition. In other words, electricity is always alert to restore its equilibrium when lost, and, if accumulated beyond its normal measure in one body, seeks with violent eagerness, as soon as a favorable medium of transmission is presented to it, to pass on its surplus of electrical energy to another body less amply supplied.

These conceptions, too, which lie at the very foundations of modern electrical science, are illustrated by Franklin with extraordinary simplicity and clearness as follows:

1. A person standing on wax, and rubbing the tube, and another person on wax drawing the fire, they will both of them, (provided they do not stand so as to touch one another) appear to be electrised, to a person standing on the floor; that is, he will perceive a spark on approaching each of them with his knuckle.

2. But, if the persons on wax touch one another during the exciting of the tube, neither of them will appear to be electrised.

3. If they touch one another after exciting the tube, and drawing the fire as aforesaid, there will be a stronger spark between them, than was between either of them and the person on the floor.

4. After such strong spark, neither of them discover any electricity.

These appearances we attempt to account for thus: We suppose, as aforesaid, that electrical fire is a common element, of which every one of the three persons above mentioned has his equal share, before any operation is begun with the tube. A, who stands on wax and rubs the tube, collects the electrical fire from himself into the glass; and his communication with the common stock being cut off by the wax, his body is not again immediately supply'd. B, (who stands on wax likewise) passing his knuckle along near the tube, receives the fire which was collected by the glass from A; and his communication with the common stock being likewise cut off, he retains the additional quantity received. To C, standing on the floor, both appear to be electrised: for he having only the middle quantity of electrical fire, receives a spark upon approaching B, who has an over quantity; but gives one to A, who has an under quantity. If A and B approach to touch each other, the spark is stronger, because the difference between them is greater: After such touch there is no spark between either of them and C, because the electrical fire in all is reduced to the original equality. If they touch while electrising, the equality is never destroy'd, the fire only circulating. Hence have arisen some new terms among us: We say, B, (and bodies like circumstanced) is electrised positively; A, negatively. Or rather, B is electrised plus; A, minus. And we daily in our experiments electrise bodies plus or minus, as we think proper. To electrise plus or minus, no more needs to be known than this, that the parts of the tube or sphere that are rubbed, do, in the instant of the friction, attract the electrical fire, and therefore take it from the thing rubbing: The same parts immediately, as the friction upon them ceases, are disposed to give the fire they have received, to anybody that has less. Thus you may circulate it, as Mr. Watson has shown; you may also accumulate or subtract it upon, or from anybody, as you connect that body with the rubber or with the receiver, the communication with the common stock being cut off.

The same letter recounts some of the tricks that Franklin and his fellow-experimenters were in the habit of making their new plaything perform. They fired spirits, lit candles just blown out, mimicked lightning, produced sparks with the touch of the finger, on the human hand or face, and gave electrical kisses. Other feats consisted in animating an artificial spider in such a way as to keep him oscillating in a very lifelike and entertaining manner between two wires, and lighting up the gilding on the covers of a book with a brilliant flash. This letter also shows that the provincial philosophers had already made improvements in the usual electrical methods. They had found that it was better to fill the phial with granulated lead than with water because of the superior facility with which the former could be warmed, and kept warm and dry in a damp place. They rubbed their tubes with buckskin, and, by observing certain precautions, such as never sullying the tubes by handling them, and keeping them in tight, close-fitting cases of pasteboard, lined with flannel, increased their efficiency. Their spheres for charging phials with electricity were mounted on iron axes with a small handle on one end, with which they could be set revolving like a common grindstone. It was in this same letter that Franklin with his usual generosity was careful to state that the power of pointed bodies to throw off as well as draw off the electrical fire was a discovery of his friend Hopkinson, and that the revolving sphere used by them was the invention of his friend Syng. About a month later, Franklin wrote to Collinson that, in the course of further experiments, he had observed several phenomena which made him distrust some of his former conclusions. "If there is no other use discover'd of Electricity," he said, "this however is something considerable, that it may help to make a vain man humble."

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