David Nightingale: Franklin's Electricity

Jun 28, 2015

At the time of the Declaration of Independence it wasn't known what electricity was. A fluid, perhaps? Or a fire?

The 1st English edition of Franklin's book "Experiments and Observations on Electricity, made at Philadelphia in America" had been printed in London in 1751. By 1776 it had gone through 5 editions --  3 in French and at least 1 in German and 1 in Italian.

Franklin is of course well-known as one of the 56 signers of the Declaration of Independence, but he spent much time in the 1740s and -50s experimenting at home in Philadelphia. Since his first visit to England, at 19, where he worked for two London printing companies, he had come home and built up his printing shop and his newspaper, as well as Poor Richard's Almanac. As he approached 40, he gave the work of his printing business to a partner and began to spend time on what he loved  -- experimenting. He wrote to a friend  "...I hope soon to be quite a Master of my own Time... with Leisure to read, study, make Experiments, and converse at large with such ingenious and worthy Men as are pleas'd to honour me with their Friendship or Acquaintance ...." [Ref.1, pp.6,7]

As we said, in the 1750s there was little electrical knowledge. The Greek philosopher Thales, around 600 BC, had said that rubbing amber could --  in dry weather --  enable it to pick up pieces of straw, or pine needles.  Also, it was known that two small pieces of rubbed amber repelled each other.

The same thing happened with glass. Further, glass and amber seemed to possess some 'opposite' fluid, for they tended to attract each other.

Larger bodies could be charged up --  like the charge that builds up in the winter on a woman's silk dress. Many experimenters began to play with bigger and bigger charges, sometimes getting shocks. A large sphere of glass with an axle could be rotated and thus rubbed quite fast, building up a very large charge. Franklin also knew that European experimenters had learned how to store that charge in a jar -- the so-called Leyden jar, devised in the Dutch city of Leyden. 

He came to his discoveries about lightning rods after he had found in his home experiments that a pointed conductor would discharge an electrified body without actually touching it. [Ref.1,p.130].

Franklin didn't write papers, he wrote letters. To his friend Peter Collinson, an English botanist, he wrote:

Place a large shot [metal ball] of 3 or 4 inches diameter on the mouth of a clean dry glass bottle. By a fine silken thread from the cieling [sic], right over the mouth of the bottle, suspend a small cork ball... so that the cork ball may rest against the side of the metal [ball]. Electrify the metal, and the cork will be repelled, four or five inches, more or less... [This is of course just the usual Coulomb repulsion experiment beginning students may perform in school today.] He goes on  ... If you now point a long, slender, sharp bodkin [a pin or a needle would suffice today] ... the electrical fire will be drawn off by the point ... 'tis surprising to see how the cork ball [then] falls back ...

This electrical property of a sharp metal point -- to discharge nearby things -- is one of the most important discoveries Franklin ever made, and is profound. He didn't know for sure that lightning was just electricity, but he went on to perform, with his son, the famous and dangerous kite experiment to clinch the matter. This same experiment had been attempted by the physicist Georg Richmann, in St Petersburg in Russia, in1753, and had killed him.

Meanwhile, in London, a Royal Society Commission recommended the use of Franklin's sharp points in lightning conductors, but one of the Commission members dissented, saying that it would be better to use knobs, which were blunter than Franklin's points. The matter became political --  blunt vs sharp -- with King George III later ordering all lightning conductors to be changed to blunt !!!  Franklin knew from his home experiments that sharp worked better, but merely said (in a 1777 letter to a friend [Ibid, p.138] )"...  Disputes are apt to sour one's temper, and disturb one's quiet. I have no private interest in the reception of my inventions by the world, having never made, nor proposed to make, the least profit by any of them."

Finally, after another century of work by experimenters worldwide, it was discovered, just before 1900, that electricity wasn't a fire or a fluid. The basic carrier of electricity turned out to be the tiny electron -- from the Greek word 'ylektron' for amber.

References:

1." Experiments and Observations on Electricity (1774)", by B.Franklin, ed by I.Bernard Cohen; Harvard Univ Press, Cambridge, Massachusetts, 1941.

Dr. David Nightingale is Professor Emeritus of Physics at the State University of New York at New Paltz, and is the co-author of the text,  A Short Course in General Relativity.

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