Franklin's electrostatic machine

Franklin's first electrical machine 1764

Franklin's electrostatic machine is a high voltage static electricity generating device that was used by Benjamin Franklin for research in the mid-eighteenth century. Experiments from it eventually lead to the invention of the lightning rod and new theories about electricity.

Benjamin Franklin

Benjamin Franklin had knowledge of the fact that if certain objects were rubbed together they would produce a brief spark of electricity lasting less than a second. In order to conduct experimental research however Franklin would need a supply of electricity that lasted significantly longer.[1] Peter Collinson, a wealthy businessman from London, donated a Leyden jar rechargeable storage battery to Franklin's Library Company of Philadelphia in 1746. Franklin wrote a letter to him on March 28, 1747, thanking him for the Leyden jar. He pointed out that this battery and the directions of operation that came with it motivated him and several of his colleges to get excited about doing serious electrical experimentation.[2] Franklin advanced the principle of generating a spark of electricity and went about to develop an electrostatic machine to continually generate electrical sparks for a supply of electricity stored by a Leyden jar.[3]

Description of machine

The electrostatic machine was designed and built with help by Thomas Hopkinson, Ebenezer Kinnersley and Philip Syng, associates of Franklin in these electrical experiments.[4][5] It supplied sparks to a Leyden jar so Franklin would have a source of electricity on demand. It was Benjamin Loxley, a Philadelphia carpenter, who constructed the electrostatic machine from wood.[6] The apparatus consisted of a rotating nine-inch (23 cm) glass globe made by Wistarburgh Glass Works of New Jersey.[6] The glass globes were handled with much care and kept in flannel lined cases for protection from breaking and soiling the surface.[7] The globe was mounted on an axle so that it could rotate.[7] The machine also had a piece of chamois leather buckskin in the form of a pad and a Leyden jar to receive the electricity collected.[6]

Franklin mentioned how the Europeans charged the glass globe by rubbing it by hand and how tiring that was. His machine was far superior for getting a static charge.[7] The globe was turned by a leather belt that was attached to a larger cranked wooden wheel. The globe built up a static electrical charge since it rubbed against the chamois pad.[8] A set of connected metal knitting needles then just barely touched the charged globe which caused sparks. The electricity passed through a metal link wire to charge the Leyden jar.[9][10]

Franklin then did electrical experiments with this supply of electricity available on demand in his laboratory.[3][11] A few revolutions charged a Leyden jar.[7] He started his experiments on electricity in the winter of 1746–1747.[12][9] Franklin had several of these electrostatic machines made.[13] Cadwallader Colden and Lewis Evans ordered some of these machines and paid between ten and twelve pounds for them.[6]

Electrical principles

An electrical principle that Franklin proved with his electrostatic machine was that of conservation of charge – that "positive" and "negative" charges come out in even amounts and are always balanced.[14] He was the first to use the terms "positive" and "negative" ("plus" and "minus") as applied to electricity.[15] Franklin devised the theory from this that electricity is a single "fluid" that is in all matter.[16][17] He referred to this static electricity as electric "fire" and sometimes as electric "fluid" and other times as electric "matter."[18] Franklin currents is electrostatic electricity and of his namesake.[19] "Franklinization" is electrotherapy where Franklin applied strong static charges, from powerful Leyden jars that were charged from his electrostatic machine, to treat patients of various illnesses.[20][21]

Franklin's experiments in charging a Leyden jar with his electrostatic machine developed into connecting a bank of Leyden jars in series with one hanging on the tail of the other that could all be charged at once with his electrostatic machine globe.[18] Franklin called these a "battery", a military term used at the time for a group cannons.[22]

Franklin's Leyden "battery" of jars was given in compliance to Franklin's will from Joseph Hopkinson (Thomas Hopkinson's grandson) to the Royal Society on April 1, 1836.[23]

Lightning rod

Main article: Lightning rod
Franklin's sentry-box experiment of iron rod

Franklin in his letters to Peter Collinson suggested an experiment be performed that would prove principles he learned from his electrostatic machine concerning sharp points attracting electricity. A sentry box was to be built big enough to hold a man and an insulated clean dry stand (see bottom of illustration "Franklin's electrostatic machine"). There was to be a bent pointed iron rod going out the door that rose to the sky 20 to 30 feet (6.1 to 9.1 m). A man was to stay inside the box holding the rod with an insulated wax handle.[24] The idea was that if clouds contained electricity then it would be attracted to the rod with a point. The experiment suggested by Franklin was first done at Marly-la-Ville in the spring of 1752. The iron rod attracted sparks of fire from the electrified clouds. This experiment was repeated several times in different places in Europe.[25]

Franklin observed the principle that pointed objects had a better effect of drawing off and throwing off electricity than a blunt object. He then got thinking how this could be applied to practical use and wrote Collinson about it. Franklin went a step further and made a kite that attracted the electrical charge from the clouds. The kite on the top had a pointed wire. Near the bottom of the string flying the kite was a key just atop a Leyden jar. He held the kite string with a silk ribbon for insulation against the electricity of a lightning bolt. His 21-year-old son was with him during this experiment for assistance and as a witness. They hid under a roof of a shed for protection from the rain. A lightning bolt hit the wire on the kite, traveled down the wet kite string and through the key and sparked the Leyden jar, charging it just like his electrostatic machine did.[26] He proved from this experiment and his electrostatic machine that lightning is a giant electric spark.[24][27][28]

He developed and produced his lightning rod invention from what he learned about static electricity principles from his electrostatic machine.[13][17][29] He figured out then to put a pointed iron rod at roof tops to protect a wooden structure as the electrical discharge from the cloud would hit the pointed iron rod instead and go directly down into the ground harmlessly where the other end of the rod was anchored.[17][19][30] Before Franklin's discovery it was thought that lightning and thunder was some form of exploding gases.[31]

Franklin even gave a demonstration in 1750 of his proof that lightning was just a giant static electricity discharge by setting up miniature houses and steeples with removable lightning rods. These had little bits of gun powder in them. When one of these miniatures was hit with a strong electric charge from his machine apparatus they exploded going into flames. With another miniature he had a lightning rod attached to it that went into the ground. It would also be hit with this same electric charge and nothing happened, as the electric charge went safely into the ground bypassing the miniature house or steeple. There were dollhouses used in lecture demonstrations in 1751 to show this same idea. An advertisement for Franklin's lightning rod appeared in the Pennsylvania Gazette in April of 1751 telling "How to secure houses from being hurt by its destructive violence".[32]

Book on electricity

Franklin sent back many letters to his friend Collinson in London on his electrical experiments of using his electrostatic machine and the Leyden jar.[25] He included his theories on the principles on how electricity worked. These letters were eventually assembled and published as a book, Experiments and Observations on Electricity.[25]

See also

References

Citations

  1. ↑ "Benjamin Franklin". Complete Dictionary of Scientific Biography. 2008. Retrieved July 16, 2015.
  2. ↑ "From Benjamin Franklin to Peter Collinson, 28 March 1747". Founders Online. National Historical Publications and Records Commission. Retrieved July 15, 2015.
  3. 1 2 Fleming 2014, p. 49.
  4. ↑ Spencer 2015, p. 1014.
  5. ↑ Cohen 1956, p. 434.
  6. 1 2 3 4 Lemay 2014, p. 75.
  7. 1 2 3 4 Cohen 1956, p. 440.
  8. ↑ Cohen 1956, p. 441.
  9. 1 2 Feldman 2014, p. 36.
  10. ↑ Heilbron 2003, p. 459.
  11. ↑ Van Vleet 2007, p. 98.
  12. ↑ Grimnes 2014, p. 495.
  13. 1 2 Maclean 1877, p. 142.
  14. ↑ Labaree 1961, p. 142.
  15. ↑ McNichol 2011, p. 16.
  16. ↑ Warkentin-Glenn 2006, p. 86.
  17. 1 2 3 Radmanesh 2005, p. 231.
  18. 1 2 Cohen 1956, p. 460.
  19. 1 2 Grimnes 2014, p. 496.
  20. ↑ Schiffer 2003, pp. 136, 137.
  21. ↑ "Electro-therapeutics". The Encyclopedia Americana: A universal reference library comprising the arts and sciences. Scientific American Compiling Department. 1905.
  22. ↑ Lynn 2009, p. 136.
  23. ↑ Cohen 1956, p. 454B.
  24. 1 2 McNichol 2011, p. 18.
  25. 1 2 3 McNichol 2011, p. 19.
  26. ↑ McNichol 2011, p. 20.
  27. ↑ Chisholm, Hugh (1910). "Electricity". The Encyclopædia Britannica 9 (12 ed.). Encyclopædia Britannica Company. p. 181.
  28. ↑ Cajori 1917, pp. 121–134.
  29. ↑ Coulson 1950, p. 32.
  30. ↑ McNichol 2011, p. 21.
  31. ↑ Baigrie 2007, p. 40.
  32. ↑ Lemay 2014, p. 91.

Sources

External links

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