How Benjamin Franklin Discovered Electricity

How did Benjamin Franklin discover about electricity?  And what did Franklin discover about electricity once he heard about it? Well, I’ll tell you and along the way I will talk about how we got positive and negative charges, the longest-standing rule of Physics, a terrifying experiment with lightning and a simple mistake that still plagues us today. 

Table of Contents

Peter Collinson and Benjamin Franklin

The True Nature of Electricity

Benjamin Franklin’s Sentry Box Experiment

Peter Collinson and Benjamin Franklin

In 1746, an Englishman named Peter Collinson read an article in a magazine about some ‘wonderful’ German experiments with electricity like giving electric kisses and electrifying a person, and using a spark from that person to light alcohol on fire. 

Collinson was a Quaker merchant and amateur scientist who was friends with Benjamin Franklin in Philadelphia (Yes, that Franklin, the one who helped write the declaration of independence and who is frowning on the $100 bill).  Collinson sent Franklin a glass tube as well as a copy of the magazine with a shipment of other materials for Franklin’s library. 

Franklin was immediately hooked,  “For my part, I never was before engaged in any study that so totally engrossed my attention and my time as this has lately done and my Friends come continually in crowds to see them, I have, during some months past, had little leisure for anything else.”

At the time Franklin was 40 years old.  He had a successful printing company, post office, and stationary shop and just two children to support, 15-year-old William, and 3-year-old Sally.  (In comparison, Franklin was from a family of seventeen children!  Seven from his father’s first wife who died in childbirth and ten from Ben’s mother who survived bearing ten children and raising seventeen!)Around this time Franklin found a manager for his businesses, and felt that he was free to “having no other tasks than such as I shall like to give myself” with “leisure to read, make experiments, and converse at large with such ingenious and worthy men as are pleased to honor me with their acquaintance.”

The True Nature of Electricity

Franklin began writing to Collingson about his electricity discoveries while he thanked him for his “kind present of an electric tube with directions for using it”.  The first thing that Franklin related, after he wrote about how much fun electricity was, was that he noticed that sparks tended to both originate and land on a sharp point.  This was to be vital towards the invention of the lightning rod which we will get to in a little bit.

Franklin built an electrical machine but unlike other machines at the time, which had one person to spin the wheel and a separate person to rub the wheel with their bare hand, Franklin’s had a brush rub against the glass.  Interestingly, you could charge up another person by putting them on wax and having them touch the spinning glass OR the brush. 

Because of this, Franklin quickly came to the supposition that, contrary to the thinking of all those before him, rubbing a material does not create charge but instead moves charge.  In other words, all things contain charge, and you cannot create or destroy it, only move it.  This is called conservation of charge and is the longest-standing rule of Physics!

Franklin thought that if you rubbed glass with a wool brush the wool gained “electrical fire” and the glass lost “electrical fire”. He gave these processes a name that we are stuck with: if an object had too much “electrical fire” he called it electrified positively, too little electrified negatively, and uncharged objects were called neutral. 

His proof was that if you charged up two people by having one touch the glass and one touch the brush they would give each other a terrible shock but after that they both would be uncharged.  In other words, the “glass” person got the equal and opposite charge of the “brush” person.

It was decidedly a shopkeeper’s Physics, with all of the plusses and minuses like an accountants ledger, and it was almost correct.  One of the difficulties is that when you see (or experience) a spark it is impossible to tell which direction it is going in.  It is just as painful to lose electrons as it is to gain electrons. 

Franklin arbitrarily decided that the positive was the charge that moved.  It took a further 150 years to realize that the “electrical fire” that was flowing was really electrons and they are negative.  So, when he thought electricity flowed one way it really flowed the other way!  Oops. 

We are still stuck with this mistake today, so that we say “conventional current” flows from positive to negative when we know that what is really going on is that tiny negative electrons are flowing the other way! 

Once again, oops.  Still, despite that mistake, the idea that all objects contain charges and we only can move them was a brilliant observation that changed how people understood electricity: at least for the few people who understood and believed Franklin!

Meanwhile, Franklin was not only creating new rules for electricity, but he was also having a lot of fun. Franklin came up with many ingenious demonstrations like flying “spiders” and discharging a Leyden jar through gold leaves in a book to make it spark to amuse his friends and visitors.

He did have a little trouble when he tried to electrically kill his turkey for Christmas as he almost electrocuted himself but mostly he seemed to find electricity endlessly entertaining.

Benjamin Franklin’s Sentry Box Experiment

While playing with his electricity machines and electrifying turkeys Franklin started to think about lightning.  He became convinced that it was the same as the sparks he got at home but on a larger scale.   On November 7th, 1749, Franklin wrote his friend a list of twelve similarities between sparks and lightning.  

On July 29th of 1750, Franklin came up with a slightly terrifying experiment to test whether lightning was caused because the clouds were electrified.  He proposed that you could put a large iron rod in a tiny house, like a guardhouse or what he called a sentry box. 

He thought that if a cloud passed by if the clouds were electrified, then the rod would also be electrified by induction (moving charges at a distance) and could produce sparks.

Notice, that despite most laymen’s impressions at the time and today, Franklin was assuming that the metal rod would create sparks because it was near an electrified cloud, not because it was hit by lightning.  It was this experiment, the soldier in a box one, that thrust Franklin into international fame.

See in 1749, Collinson decided to publish Franklin’s letters as a book.  When they did, it initially did not do well in England.  Ever the politician, Franklin realized that his ideas of moving positive and negative charges would have an even harder time in France, as it was contrary to the prevailing theory that was created by a Frenchman named Charles Francois Cisterney Du Fay. 

Now Du Fay had died from smallpox in 1739, but his protégée: a man named Nollet seemed unlikely to enjoy Franklin’s work.  Therefore, Franklin “happened” to send it to Nollet’s rival: a pompous man named Buffon. 

Now Buffon had no interest in electricity, but he was delighted to humiliate a rival.  And that story, and how it led to the lightning rod (as well as Franklin’s famous kite experiment) is next time on the Secret History of Electricity.

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