Ben Franklin and His Inventions: A Study in Deductive Reasoning- By Erin Maloney

At a time when the majority of the population thought that the sun rose more than once, electricity consisted of two distinct fluids, and the four humors still dictated a person’s health, the Scientific Revolution took hold in America.[1]  Out of this unenlightened period of scientific development, one of the first true renaissance men of his time and a household name, Benjamin Franklin became a master of deductive reasoning and logical thinking.  He approached all unknown aspects of the world around him as a study in the scientific method by hypothesizing on a problem, experimenting, and then drawing his conclusions based on those experiments.[2]  Once he understood the material and made his conclusions, the next step was to invent a product or publish a pamphlet that would render his findings relevant to the general population.[3]  Through this method, Franklin invented, discovered, and popularized many inventions and scientific facts that we take for granted in modern science such as daylight savings, the odometer, and bifocals.[4]  Franklin’s contributions to the worlds of mathematics and science were not his equations, though this formula for calculating interest is still used today.  Rather, his most notable contributions were direct results of superior logic, as seen in the invention of the Franklin stove and the lightning rod.

Franklin’s stove was a logical invention borne out of an attempt to conserve fuel while utilizing the heat output in the most efficient way.  At this time, conventional fireplaces used more wood and emitted less useable heat than was economical, not to mention the prevalence of smoke back up and chimney fires due to unclean burning.[5]  However, the alternative was even less appealing, freezing during the winter months.  Faced with this dilemma, Benjamin Franklin endeavored to invent a fireplace that would not only save money by economizing fuel consumption and removing excess smoke, but would keep the aesthetically pleasing sight and light of an open fire.[6]  To do this, he had to understand the relationship between air and heat.  To understand this relationship, he experimented with air filled balloons, putting them near the fire and watching the air in them expand, then placing them in a cool corner of the room and watching the air shrink back to the original size.[7]  From this, Franklin deduced that as warm air heats up, it looks for an escape.  As warm air moves away from the heat, cold air rushes in to replace it, thus beginning a cyclic motion through the chimney.  Franklin observed this in the nature of Nor’easters and applied that knowledge to the design of his stove.[8]  The stove consisted of an iron box with an open mouth for the fire and a hole in the bottom connected to a pipe leading from under the floor into the chimney with vents on the side, suspended above the ground by four legs.[9]  Cool air entered the warming box through a hole in the bottom of the fireplace, allowing the warm air to escape through the vented sides.[10]  The smoke was forced down under the floor through a hole cut in the bottom, then pushed up a pipe and out the chimney by the displacement of cold air.  The result was an open air, metal fireplace.[11]

However, being a master of reason is not the same as being infallible, nor is that a reasonable responsibility.  Though it was an ingenious invention and had potential, there was one small problem in the design of Franklin’s stove.

    Because his understanding of the relationship between air and heat was rudimentary and unrefined, he did not realize that, to draw the smoke away from the fire, the floor must be warm, thereby creating the necessary downdraft to pull the smoke through the hole.[12]  Franklin knew that warm air expanded, but did not know that warm air also rose.  This was not taken into account in Franklin’s design, so his original stove did not work properly.  However, David Rittenhouse, one of Franklin’s contemporaries, mended the flaw by creating an L-shaped chimney and the Franklin stove became an integral piece of Americana.[13]

Though Franklin’s “Pennsylvania Fireplace” revolutionized the heating of colonial homes and was one of his earliest inventions derived from deductive reasoning, his most famous experiments came with his discovery that lightning was electricity and the subsequent invention of the Lightning Rod.  Franklin began his experiments with electricity after watching Archibald Spencer use glass tubes, iron rods, silk thread, cork, chains, Ledyn jars (early batteries) and a young boy suspended from the ceiling to conduct an electrical exhibition that sent sparks flying out of a person’s fingers and hands.[14]  When Franklin decided to repeat Spencer’s experiment, he followed the directions perfectly with one minor change.  Instead of suspending the boy from the ceiling, Franklin seated the boy on a glass stool “for insulation.”[15] 

It would take many more observations and experiments before Franklin would discover that the glass stool would lead to the invention of the lightning rod.  By 1749, Franklin was already hypothesizing that lightning was indeed electricity by comparing lightning to the effects of the electrical experiments he had already conducted.[16]  Because Franklin knew that electricity was conducted through metal, pointed metal seemed to attract more electrical charge, and Franklin believed that lightning was electricity, Franklin devised an experiment to prove this theory.[17]  Franklin invented the lightning rod in September of 1752, though its purpose was not to protect the house, as would be the tradition.  This rod, which he attached to his house in Philadelphia, extended nine feet above the chimney, winding from the chimney through the staircase and finally into his study where it split into two rods with a bell at the end.  Between the points of these two rods, a metal ball hung on a silk thread.[18]  Soon after this experiment, Franklin conducted his famous kite experiment, proving conclusively that lightning was electricity.[19]  In July 1753, nine months after his famous experiment with the kite, Dr. Franklin observed the relationship between lightning and metal “conductors.”[20]  A house in Philadelphia was hit by lightning during the wee hours of the morning and, because of the building materials used, a person could trace the path the lightning took, including where the house was undamaged.  The undamaged portions of the house all had one thing in common, they all contained metal.[21]  Franklin also made the observation that houses whose roof was made of metal or lead and who had a downspout were rarely damaged by lightning strikes.[22]  This conclusively proved that lightning is electricity and that, by fixing a metal rod to a house, the metal would preserve the house from fire due to lightning strike. 

Franklin was a master of deductive reasoning.  There were several instances during his scientific career that Franklin could have made an assumption based on his own beliefs, but he chose to know the truth as conclusively as possible within the information he had available.  His experiments and their findings have stood the test of time, many becoming ideas that we now take for granted, and variations of his inventions are still in use today.  Franklin’s amazing contributions to every day American life came about as direct results of deductive reasoning.

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