Benjamin Franklin (1706–1790) was born in Boston, the fifteenth of seventeen children of a candle and soap maker. He ran away to Philadelphia at seventeen, became a printer, and grew rich publishing a newspaper and Poor Richard's Almanack. He retired from business in 1748, partly to have time for science. Later he became one of the best-known statesmen of the American founding.
Key idea
How the idea came
In his Autobiography Franklin tells how he first saw electric tricks in Boston, and how a gift of a glass tube from London set him working. (Historians date the Boston demonstrations to 1743, a little earlier than he remembered.)
In 1746, being at Boston, I met there with a Dr. Spence, who was lately arrived from Scotland, and show'd me some electric experiments. They were imperfectly perform'd, as he was not very expert; but, being on a subject quite new to me, they equally surpris'd and pleased me. Soon after my return to Philadelphia, our library company receiv'd from Mr. P. Collinson, Fellow of the Royal Society of London, a present of a glass tube, with some account of the use of it in making such experiments. I eagerly seized the opportunity of repeating what I had seen at Boston.
Rubbing a glass tube with silk charged it. Franklin and his friends found that a person standing on wax could be charged, and that a pointed metal rod drew charge off quietly while a blunt one gave a spark. He wrote his findings in letters to Peter Collinson in London, who had them printed as a book in 1751.
Plus and minus
Others thought there were two kinds of electricity. Franklin suggested one: rubbing does not make it, it only moves it, so one object gains what the other loses. He called a surplus positive or plus, and a shortage negative or minus. We still use those words, and the idea that charge is conserved is one of the firm laws of physics.
Worked example: Charge moves, it isn't made
- The total must stay 0, since charge only moves.
- $6 + s = 0$, so $s = -6$.
Answer: −6 units.
Lightning is electricity
Franklin listed ways lightning and electric sparks were alike: light, crooked paths, a crack, melting metal, a sulfur smell. In 1750 he proposed a test: a tall pointed iron rod on a high building, insulated at the bottom, should draw charge from a storm cloud. In May 1752 French experimenters led by Thomas-François Dalibard did it at Marly, near Paris, and got sparks from the rod. Franklin, not yet knowing, tried a kite in Philadelphia. His letter describing it was printed that October.
As soon as any of the thunder clouds come over the kite, the pointed wire will draw the electric fire from them, and the kite, with all the twine, will be electrified, and the loose filaments of the twine will stand out every way, and be attracted by an approaching finger.
Never try this
Flying a kite in a thunderstorm can kill you. Franklin's kite was not struck by lightning; the string only picked up charge from the air, and even that was dangerous. In 1753 the scientist Georg Wilhelm Richmann was killed in St. Petersburg by lightning that came down an insulated rod during a similar experiment. Fly kites only in fair weather, in open fields far from power lines.
Story or history?
Franklin's letter does not give the date or say who helped. The details come from Joseph Priestley's History of Electricity (1767), written with Franklin's help: June 1752, with his son William. A few modern writers have questioned whether the kite flight happened at all, since Franklin wrote so little about it; most historians accept that it did. The famous pictures of lightning striking the kite are wrong either way.
The lightning rod
If a pointed rod can draw charge from a cloud, a grounded rod can lead a lightning strike safely down into the earth instead of through a building. Franklin published the idea in Poor Richard's Almanack for 1753. Rods spread across America and Europe, and grounded lightning protection is still used today, installed by trained professionals.
The stove and the bifocals
Around 1742 Franklin designed an iron fireplace that stood out in the room and warmed it better while burning less wood. He described it in a 1744 pamphlet. The governor offered him a patent, and he said no:
That, as we enjoy great advantages from the inventions 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.
In old age he needed one pair of glasses to see far and another to read. He had the lenses cut in half and put the two halves in one frame: reading half below, distance half above. He described his "double spectacles" in a letter of 1785. Others may have had the same idea, but Franklin's is the best-known early account.
| When | What happened |
|---|---|
| 1706 | Born in Boston |
| 1723 | Arrives in Philadelphia |
| c. 1742 | Designs the Pennsylvania fireplace (Franklin stove) |
| 1743–1746 | Sees electric demonstrations; starts experiments with a glass tube |
| 1750 | Proposes the sentry-box (rod) experiment and the lightning rod |
| 1751 | His letters on electricity printed in London |
| May 1752 | Dalibard draws sparks from a rod at Marly, France |
| June 1752 | The kite experiment in Philadelphia (date from Priestley) |
| 1753 | Copley Medal of the Royal Society |
| 1785 | Describes his double spectacles (bifocals) |
| 1790 | Dies in Philadelphia |
Worked example: Ages
- $1752 - 1706 = 46$. His birthday was in January, so by June he had turned 46.
Answer: 46.
What it led to
Franklin's words, positive, negative, battery, charge, conductor, are still the words of electricity. The lightning rod saved countless buildings and ships. And his refusal to patent shows one view of invention: that ideas built on other people's work should be shared.
Common mistake
Read more: Benjamin Franklin, Lightning rod, Franklin stove and Bifocals.