The stove was constructed so that heat and smoke from the fire rose to warm an iron plate on top, then were carried by convection down a channel that led under the wall of the hearth and finally up through the chimney. In the process, the fire heated an inner metal chamber that drew clean cool air up from the basement, warmed it, and let it out through louvers into the room. That was the theory.
In 1744, he had a fellow Junto member who was an ironworker manufacture the new stove, and he got two of his brothers and several other friends to market them throughout the northeast. The promotional pamphlet Franklin wrote was filled with both science and salesmanship. He explained in detail how warm air expands to take up more space than cold, how it is lighter, and how heat radiates whereas smoke is carried only by air. He then included testimonials about his new design and touted that it minimized cold drafts and smoke, thus reducing the chance of fevers and coughs. It would also save on fuel, he advertised.
The new Pennsylvania Fireplaces, as he called them, were initially somewhat popular, at £5 apiece, and papers around the colonies were filled with testimonials. “They ought to be called, both in justice and gratitude, Mr. Franklin’s stoves,” declared one letter writer in the Boston Evening Post. “I believe all who have experienced the comfort and benefit of them will join with me that the author of this happy invention merits a statue.”
The governor of Pennsylvania was among the enthusiastic, and he offered Franklin what could have been a lucrative patent. “But I declined it,” Franklin noted in his autobiography. “As we enjoy great advantages from the invention of others, we should be glad of an opportunity to serve others by any invention of ours, and this we should do freely and generously.” It was a noble and sincere sentiment.
An exhaustive study by one scholar shows that Franklin’s design eventually proved less practical and popular than he hoped. Unless the chimney and lower channels were hot, there was not enough convection to keep the smoke from being forced back into the room. That made getting started a problem. Sales tapered off, manufacturing ceased within two decades, and most models were modified by their owners to eliminate the back channel and chamber. Throughout the rest of his life, Franklin would refine his theories about chimney and fireplace designs. But what is today commonly known as the Franklin Stove is a far simpler contraption than what he originally envisioned.3
Franklin also combined science and mechanical practicality by devising the first urinary catheter used in America, which was a modification of a European invention. His brother John in Boston was gravely ill and wrote Franklin of his desire for a flexible tube to help him urinate. Franklin came up with a design, and instead of simply describing it he went to a Philadelphia silversmith and oversaw its construction. The tube was thin enough to be flexible, and Franklin included a wire that could be stuck inside to stiffen it while it was inserted and then be gradually withdrawn as the tube reached the point where it needed to bend. His catheter also had a screw component that allowed it to be inserted by turning, and he made it collapsible so that it would be easier to withdraw. “Experience is necessary for the right using of all new tools or instruments, and that will perhaps suggest some improvements,” Franklin told his brother.
The study of nature also continued to interest Franklin. Among his most noteworthy discoveries was that the big East Coast storms known as northeasters, whose winds come from the northeast, actually move in the opposite direction from their winds, traveling up the coast from the south. On the evening of October 21, 1743, Franklin looked forward to observing a lunar eclipse he knew was to occur at 8:30. A violent storm, however, hit Philadelphia and blackened the sky. Over the next few weeks, he read accounts of how the storm caused damage from Virginia to Boston. “But what surprised me,” he later told his friend Jared Eliot, “was to find in the Boston newspapers an account of the observation of that eclipse.” So Franklin wrote his brother in Boston, who confirmed that the storm did not hit until an hour after the eclipse was finished. Further inquiries into the timing of this and other storms up and down the coast led him to “the very singular opinion,” he told Eliot, “that, though the course of the wind is from the northeast to the southwest, yet the course of the storm is from the southwest to the northeast.” He further surmised, correctly, that rising air heated in the south created low-pressure systems that drew winds from the north. More than 150 years later, the great scholar William Morris Davis proclaimed, “With this began the science of weather prediction.”4
Dozens of other scientific phenomena also engaged Franklin’s interest during this period. For example, he exchanged letters with his friend Cadwallader Colden on comets, the circulation of blood, perspiration, inertia, and the earth’s rotation. But it was a parlor-trick show in 1743 that launched him on what would be by far his most celebrated scientific endeavor.
Electricity
On a visit to Boston in the summer of 1743, Franklin happened to be entertained one evening by a traveling scientific showman from Scotland named Dr. Archibald Spencer. (In his autobiography, Franklin gets the name and year wrong, saying it was a Dr. Spence in1746.) Spencer specialized in amazing demonstrations that verged on amusement shows. He depicted Newton’s theories of light and displayed a machine that measured blood flow, both interests of Franklin’s. But more important, he performed electricity tricks, such as creating static electricity by rubbing a glass tube and drawing sparks from the feet of a boy hanging by silk cords from the ceiling. “Being on a subject quite new to me,” Franklin recalled, “they equally surprised and pleased me.”
In the previous century, Galileo and Newton had demystified gravity. But that other great force of the universe, electricity, was understood little better than it had been by the ancients. There were people, such as Dr. Spencer, who played with it to perform spectacles. The Abbé Nollet, court scientist to France’s King Louis XV, had linked 180 soldiers and then 700 monks and made them jump in unison for the court’s amusement by sending through them a jolt of static electricity. But Franklin was the perfect person to turn electricity from a parlor trick into a science. That task demanded not a mathematical or theoretical scholar, but instead a clever and ingenious person who had the curiosity to perform practical experiments, plus enough mechanical talent and time to tinker with a lot of contraptions.
A few months after Franklin returned to Philadelphia, Dr. Spencer came to town. Franklin acted as his agent, advertised his lectures, and sold tickets from his shop. His Library Company also received, early in 1747, a long glass tube for generating static electricity, along with papers describing some experiments, from its agent in London, Peter Collinson. In his letter thanking Collinson, Franklin was effusive in describing the fun he was having with the device: “I never was before engaged in any study that so totally engrossed my attention.” He commissioned a local glassblower and silversmith to make more such gadgets, and he enlisted his Junto friends to join in the experimenting.5
Franklin’s first serious experiments involved collecting an electric charge and then studying its properties. He had his friends draw charges from the spinning glass tube and then touch each other to see if sparks flew. The result was the discovery that electricity was “not created by the friction, but collected only.” In other words, a charge could be drawn into person A and out of person B, and the electric fluid would flow back if the two people touched each other.
To explain what he meant, he invented some new terms in a letter to Collinson. “We say B is electrised positively; A negatively: or rather B is electrised plus and A minus.” He apologized to the Englishman for the new coinage: “These terms we may use until your philosophers give us better.”
In fact, these terms devised by Franklin are the ones we still use today, along with other neologisms that he coined to describe his findings: battery, charged, neutral, condense, and conductor. Part of Franklin’s importance as a scientist was the clear writing he employed. “He has written equally for the uninitiated as well as the philosopher,” t
he early nineteenth-century English chemist Sir Humphry Davy noted, “and he has rendered his details as amusing as well as perspicuous.”
Until then, electricity had been thought to involve two types of fluids, called vitreous and resinous, that could be created independently. Franklin’s discovery that the generation of a positive charge was accompanied by the generation of an equal negative charge became known as the conservation of charge and the single-fluid theory of electricity. The concepts reflected Franklin’s bookkeeper mentality, which was first expressed in his London “Dissertation” positing that pleasure and pain are always in balance.
It was a breakthrough of historic proportions. “As a broad generalization that has withstood the test of 200 years of fruitful application,” Harvard professor I. Bernard Cohen has pronounced, “Franklin’s law of conservation of charge must be considered to be of the same fundamental importance to physical science as Newton’s law of conservation of momentum.”
Franklin also discovered an attribute of electrical charges—“the wonderful effects of points”—that would soon lead to his most famous practical application. He electrified a small iron ball and dangled a cork next to it, which was repelled based on the strength of the ball’s charge. When he brought the tip of a pointed piece of metal near the ball, it drew away the charge. But a blunt piece of metal did not draw a charge or spark as easily, and if it was insulated instead of grounded, did not draw a charge at all.
Franklin continued his experiments by capturing and storing electric charges in a primitive form of capacitor called, after the Dutch town where it was invented, a Leyden jar. These jars had a metal foil on the outside; on the inside, separated from the foil by the glass insulation, was lead or water or metal that could be charged up through a wire. Franklin showed that when the inside of the jar was charged, the outside foil had an equal and opposite charge.
Also, by pouring out the water and metal inside a charged Leyden jar and not being able to elicit a spark, he found that the charge did not actually reside in them; instead, he correctly concluded, it was the glass itself that held the charge. So he lined up a series of glass plates flanked by metal, charged them up, wired them together, and created (and gave a name to) a new device: “what we called an electrical battery.”6
Electricity also energized his antic sense of fun. He created a charged metal spider that leaped around like a real one, he electrified the iron fence around his house to produce sparks that amused visitors, and he rigged a picture of King George II to produce a “high-treason” shock when someone touched his gilded crown. “If a ring of persons take the shock among them,” Franklin joked, “the experiment is called The Conspirators.” Friends flocked to see his shows, and he reinforced his reputation for playfulness. (In one of the weirder scenes in Thomas Pynchon’s novel Mason & Dixon, Franklin lines up some young men in a tavern to jolt them from his battery, shouting “All hold hands, Line of Fops.”)
As the summer of 1749 approached and the rising humidity made experiments more difficult, Franklin decided to suspend them until the fall. Although his findings were of great historical significance, he had yet to put them to practical use. He lamented to Collinson that he was “chagrined a little that we have hitherto been able to discover nothing in the way of use to mankind.” Indeed, after many revised theories and a couple of painful shocks that knocked him senseless, the only “use discovered of electricity,” said the man who was always trying to tackle his own pride, was that “it may help make a vain man humble.”
The end of the experimenting season gave an occasion for a “party of pleasure” on the banks of the river. Franklin described it in a letter to Collinson: “A turkey is to be killed for our dinners by the electrical shock; and roasted by the electrical jack, before a fire kindled by the electrified bottle; while the healths of all the famous electricians in England, France and Germany are to be drank in electrified bumpers, under the discharge of guns from the electrical battery.”
The frivolity went well. Though turkeys proved harder to kill than chickens, Franklin and friends finally succeeded by linking together a big battery. “The birds killed in this manner eat uncommonly tender,” he wrote, thus becoming a culinary pioneer of fried turkey. As for doing something more practical, there would be time for that in the fall.7
Snatching Lightning From the Sky
In the journal he kept for his experiments, Franklin noted in November 1749 some intriguing similarities between electrical sparks and lightning. He listed twelve of them, including “1. Giving light. 2. Color of the light. 3. Crooked directions. 4. Swift motion. 5. Being conducted by metals. 6. Crack or noise in exploding…9. Destroying animals…12. Sulpherous smell.”
More important, he made a connection between this surmise about lightning and his earlier experiments on the power of pointed metal objects to draw off electrical charges. “Electrical fluid is attracted by points. We do not know whether this property is in lightning. But since they agree in all particulars wherein we can already compare them, is it not probable they agree likewise in this?” To which he added a momentous rallying cry:“Let the experiment be made.”
For centuries, the devastating scourge of lightning had generally been considered a supernatural phenomenon or expression of God’s will. At the approach of a storm, church bells were rung to ward off the bolts. “The tones of the consecrated metal repel the demon and avert storm and lightning,” declared St. Thomas Aquinas. But even the most religiously faithful were likely to have noticed this was not very effective. During one thirty-five-year period in Germany alone during the mid-1700s, 386 churches were struck and more than one hundred bell ringers killed. In Venice, some three thousand people were killed when tons of gunpowder stored in a church was hit. As Franklin later recalled to Harvard professor John Winthrop, “The lightning seems to strike steeples of choice and at the very time the bells are ringing; yet still they continue to bless the new bells and jangle the old ones whenever it thunders. One would think it was now time to try some other trick.”8
Many scientists, including Newton, had noted the apparent connection between lightning and electricity. But no one had declared “Let the experiment be made,” nor laid out a methodical test, nor thought of the practicality of tying this all in with the power of pointed metal rods.
Franklin first sketched out his theories about lightning in April 1749, just before his end-of-season turkey fry. The water vapors in a cloud can be electrically charged, he surmised, and the positive ones will separate from the negative ones. When such “electrified clouds pass over,” he added, “high trees, lofty towers, spires, masts of ships…draw the electrical fire and the whole cloud discharges.” It was not a bad guess, and it led to some practical advice: “Dangerous therefore it is to take shelter under a tree during a thunder gust.” It also led to the most famous of all his experiments.9
Before he tried to conduct his proposed experiments himself, Franklin described them in two famous letters to Collinson in 1750, which were presented to the Royal Society in London and then widely published. The essential idea was to use a tall metal rod to draw some of the electrical charge from a cloud, just as he had used a needle to draw off the charge of an iron ball in his lab. He detailed his proposed experiment:
On the top of some high tower or steeple, place a kind of sentry box big enough to contain a man and an electrical stand. From the middle of the stand, let an iron rod rise…upright 20 or 30 feet, pointed very sharp at the end. If the electrical stand be kept clean and dry, a man standing on it when such clouds are passing low might be electrified and afford sparks, the rod drawing fire to him from the cloud. If any danger to the man be apprehended (though I think there would be none) let him stand on the floor of his box, and now and then bring near to the rod the loop of a wire that has one end fastened to the leads; he holding it by a wax handle [i.e., insulating him from it]. So the sparks, if the rod is electrified, will strike from the rod to the wire and not affect him.
Fr
anklin’s one mistake was thinking that there would be no danger, as at least one European experimenter fatally discovered. His suggestion of using a wire held with an insulating wax handle was a smarter approach.
If his suppositions held true, Franklin wrote in another letter to Collinson, then lightning rods could tame one of the greatest natural dangers people faced. “Houses, ships and even towns and churches may be effectually secured from the stroke of lightning by their means,” he predicted. “The electrical fire would, I think, be drawn out of a cloud silently.” He wasn’t certain, however. “This may seem whimsical, but let it pass for the present until I send the experiments at large.”10
Franklin’s letters were excerpted in London by The Gentleman’s Magazine in 1750 and then published as an eighty-six-page booklet the following year. More significant, they were translated into French in early 1752 and became a sensation. King Louis XV asked that the lab tests be performed for him, which they were in February by three Frenchmen who had translated Franklin’s experiments, led by the naturalists Comte de Buffon and Thomas-François D’Alibard. The king was so excited that he encouraged the group to try Franklin’s proposed lightning rod experiment. As a letter to London’s Royal Society noted, “These applauses of his Majesty having excited in Messieurs de Buffon, D’Alibard and de Lor a desire of verifying the conjectures of Mr. Franklin upon the analogy of thunder and electricity, they prepared themselves for making the experiment.”
In the village of Marly on the northern outskirts of Paris, the Frenchmen constructed a sentry box with a 40-foot iron rod and dragooned a retired soldier to play Prometheus. On May 10, 1752, just after 2 in the afternoon, a storm cloud passed over and the soldier was able to draw sparks as Franklin had predicted. An excited local prior grabbed the insulated wire and repeated the experiment six times, shocking himself once but surviving to celebrate the success. Within weeks it was replicated dozens of times across France. “M. Franklin’s idea has ceased to be a conjecture,” D’Alibard reported to the French Royal Academy. “Here it has become a reality.”
Benjamin Franklin: An American Life Page 16