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== Lightning rod ==
== Lightning rod ==
{{Main|Lightning rod}}
{{Main|Lightning rod}}
[[File:Sentry-box experiment.jpg|thumb|right|Franklin's sentry-box experiment of pointed iron rod to attract lightning]]
[[File:Sentry-box experiment.jpg|right |thumb|upright 1.50 |Franklin's sentry-box experiment of pointed iron rod to attract lightning]]
Franklin observed in using his electrostatic machine that pointed objects had a better effect of "drawing off" and "throwing off" electricity than a blunt object.{{sfn|Finger|2012|p=85}}{{sfn|LeMay|1987|p=600}} He then thought about 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 had a pointed wire on the top. Near the bottom of the string attached to the kite was a key just atop a Leyden jar. He held the kite string with a silk ribbon for insulation against the electrical charge from a lightning bolt. His 21-year-old son was with him during this experiment as an assistant and a witness. They hid under a shed roof for protection from the rain. A lightning bolt<!--a direct hit by lightning likely would have destroyed the kite--> hit the wire on the kite, traveled down the wet kite string through the key and sparked the Leyden jar, charging it just like his electrostatic machine did.{{sfn|McNichol|2011|page=20}} He proved from this experiment, and his electrostatic machine, that lightning is a giant electric spark.{{sfn|McNichol|2011|page=18}}<ref name=Britannica>{{cite encyclopedia |last=Chisholm |first=Hugh |author-link= |editor-last= |editor-first= |editor-link= |encyclopedia=The Encyclopædia Britannica |title= Electricity|trans-title= |url=https://books.google.com/books?id=Ek80AQAAMAAJ&pg=PA181 |access-date= |language= |edition=12 |date= |year=1910 |publisher=Encyclopædia Britannica Company|series= |volume=9 |location= |id= |isbn= |oclc= |doi= |pages=181 |quote= |ref=}}</ref>{{sfn|Cajori|1917|pages=121–134}}
Franklin observed in using his electrostatic machine that pointed objects had a better effect of "drawing off" and "throwing off" electricity than a blunt object.{{sfn|Finger|2012|p=85}}{{sfn|LeMay|1987|p=600}} He then thought about 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 had a pointed wire on the top. Near the bottom of the string attached to the kite was a key just atop a Leyden jar. He held the kite string with a silk ribbon for insulation against the electrical charge from a lightning bolt. His 21-year-old son was with him during this experiment as an assistant and a witness. They hid under a shed roof for protection from the rain. A lightning bolt<!--a direct hit by lightning likely would have destroyed the kite--> hit the wire on the kite, traveled down the wet kite string through the key and sparked the Leyden jar, charging it just like his electrostatic machine did.{{sfn|McNichol|2011|page=20}} He proved from this experiment, and his electrostatic machine, that lightning is a giant electric spark.{{sfn|McNichol|2011|page=18}}<ref name=Britannica>{{cite encyclopedia |last=Chisholm |first=Hugh |author-link= |editor-last= |editor-first= |editor-link= |encyclopedia=The Encyclopædia Britannica |title= Electricity|trans-title= |url=https://books.google.com/books?id=Ek80AQAAMAAJ&pg=PA181 |access-date= |language= |edition=12 |date= |year=1910 |publisher=Encyclopædia Britannica Company|series= |volume=9 |location= |id= |isbn= |oclc= |doi= |pages=181 |quote= |ref=}}</ref>{{sfn|Cajori|1917|pages=121–134}}


He developed and produced his lightning rod invention from what he learned from his electrostatic machine about static electricity principles.{{sfn|Maclean|1877|page=142}}{{sfn|Coulson|1950|page=32}} He determined that by placing a pointed iron rod at the top of a roof to protect a wooden structure, the electrical discharge from the cloud would hit it instead, and go harmlessly into the ground where the other end of the rod was anchored.{{sfn|Grimnes|2014|page=496}}{{sfn|McNichol|2011|page=21}} Before Franklin's discovery it was thought that lightning and thunder was some form of exploding gas.{{sfn|Baigrie|2007|page=40}}
Franklin developed and produced his lightning rod invention from what he learned from his electrostatic machine about static electricity principles.{{sfn|Maclean|1877|page=142}}{{sfn|Coulson|1950|page=32}} He published a proposal for an experiment to be done where a person was to stand on an insulated stool inside a sentry box protected from the rain and hold out a long pointed iron rod to attract a lightning bolt.{{sfn|Cohen|1990|page=28}} This was first performed with success in France on May 10, 1752.{{sfn|Cohen|1990|page=28}} It was repeated throughout Europe with success as Franklin had predicted, proving that lightning was electricity.{{sfn|Cohen|1990|page=28}} He determined from this experiment that by placing a pointed iron rod at the top of a roof to protect a wooden structure, the electrical discharge from the cloud would hit it instead, and go harmlessly into the ground where the other end of the rod was anchored.{{sfn|Grimnes|2014|page=496}}{{sfn|McNichol|2011|page=21}} Before Franklin's discovery it was thought that lightning and thunder was some form of exploding gas.{{sfn|Baigrie|2007|page=40}}


In 1750, Franklin gave a demonstration 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 and burst into flames. With another miniature he had a lightning rod attached to it that went into the ground. When it was hit with the same electric charge nothing happened, as it went safely into the ground bypassing the miniature house and steeple. Dollhouses were 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 explaining: "How to secure houses from being hurt by its destructive violence."{{sfn|Lemay|2014|page=91}}
In 1750, Franklin gave a demonstration 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 and burst into flames. With another miniature he had a lightning rod attached to it that went into the ground. When it was hit with the same electric charge nothing happened, as it went safely into the ground bypassing the miniature house and steeple. Dollhouses were 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 explaining: "How to secure houses from being hurt by its destructive violence."{{sfn|Lemay|2014|page=91}}

Revision as of 12:15, 9 November 2016

Museum model of Franklin's "electrostatic machine"

Franklin's electrostatic machine is a high voltage static electricity generating device that was used by Benjamin Franklin in the mid-eighteenth century for research on electricity. It was a hand cranked simple machine designed to save labor. It supplied sparks of momentary electricity that were stored in a Leyden jar that was designed to store charges of electricity. It consisted of a cranking wheel, a glass globe that was turned around on an axis, a rubbing pad that touched the spinning globe and a set of metal needles to collect electricity generated by the globe. Experiments from it eventually lead to the invention of the lightning rod and new theories about electricity.

Background

Main article: Benjamin Franklin § electricity

The concept to make man-made electricity by the use of a mechanical machine was not new in Franklin's time. It first came to European scientists in the middle of the seventeenth century. Otto von Guericke in 1663 was the first to make an apparatus that generated man-made static electricity and used a sphere of sulfur. Later in 1704 Francis Hauksbee made a more efficient machine for generating static electricity using a rotating glass sphere instead. By 1740 the basic design of Hauksbee was used throughout Europe as a means to generate a continuous supply of static electricity. The problem with these machines, however, was that these static charges lasted no longer than a second. In 1746 a means was developed to store these static electric charges with little loss of power over a long period of time. This was a type of electrical storage condenser capacitor that was referred to as a Leyden jar.[1][2]

Franklin knew that that if certain objects like amber, sulphur, or glass were rubbed they would produce a brief spark of electricity lasting less than a second.[3] He saw this "electrical fire" in the form of sparks. In order to conduct serious long term experimental research he needed a supply of electricity that lasted much longer than a second. Franklin at the age of 42 decided to retire from his printing business to spend more time on his hobbies. He turned over his entire operation to his partner David Hall in 1748.[4] Franklin then made a laboratory in his new home for experimenting and research.[4][5]

Peter Collinson, a businessman from London that corresponded with scientists, donated in 1745 a "glass tube" with operating instructions on how to generate static electricity.[6] Collinson was the library's London agent.[7][8] Franklin in 1746 started experimenting with electricity produced by a Leyden jar.[9] He wrote a letter to Collinson on March 28, 1747, thanking him for the "glass tube" and explained what he was doing with it. He wrote that the tube and the directions of operation that came with it motivated him and several of his colleagues to get excited about doing serious electrical experimentation.[10] Franklin experimenting with the Leyden jar battery discovered that it soon discharged totally in a short time and had no more power. He could recharge the jar to some small degree with sparks of electricity that he could produce by manually rubbing together certain objects on the "glass tube" that would make a temporary electrical charge. However this was very labor intensive for such a little amount of electricity that could be produced.[11] Franklin, known for his practical applications, used a hand operated friction machine as a leverage on labor to produce large quantities of static electricity that could be operated by one person.[11] The motion rubbed a large glass bulb that produced lots of static electricity and he therefore called this an "electrostatic machine."[7]

Description

Close-up of glass globe for producing static electricity with nearby medal needles on top to collect the charge made by the bottom pad rubbing the spinning globe.

Franklin's electricity generating machine was a simple hand cranked device designed to save labor for its purpose. It consisted of a rotating glass globe driven by a hand operated flywheel. The globe was made by Wistarburgh Glass Works of New Jersey.[12] His machine was unique in that he used a scientific glassware with certain specifications for the glass bulb given to the glass maker Caspar Wistar.[13] The glass globes were handled with great care and kept in flannel lined cases for protection from breaking and soiling the surface.[14] These were referred to as "electerizing globes and tubes."[15]

The glass globe was mounted on an axle so that it could rotate. The machine also had a piece of material in the form of a pad that rubbed the globe when it spun around. This action then changed the characteristics of the glass where it was electrically charged. The electricity, in the form of sparks then went to a set of metal needles that was very close to the spinning globe, passed through wires that lead to the Leyden jar that received the electricity.[16] A few revolutions charged a Leyden jar.[14] In 1746 Franklin began his electrical experiments[17] and by 1749 he had combined this electricity generating machine and the Leyden jar to store the electricity produced by the machine.[18] Franklin therefore had a complete electrical apparatus system with the addition of the Leyden jar for producing and storing all the electricity he needed for experimental research.[7] Wistarburgh Glass Works made the Leyden jars Franklin needed in the 1750s.[15]

Electrical principles

An electrical principle that Franklin proved in 1747 with his electrostatic machine was that of conservation of charge – that "positive" and "negative" charges come out in even amounts and are always balanced.[19] He was the first to use the terms "positive" and "negative" ("plus" and "minus") as applied to electricity.[20][21] From this idea he concluded that lightning was just giant sparks of opposing charges of electricity balancing themselves.[22] He referred to this static electricity as electric "fire" and sometimes as "electric fluid" and at other times as electric "matter."[23] Franklin had made a list in 1749 of several ways in which lightning had similar characteristics as that of "electric fluid" in matter.[22]

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.[23] Franklin called these a "battery", a military term used at the time for a group cannons.[24] There were new electric terms then developed from his name in the middle of the eighteenth century. For example, "Franklin currents" is electrostatic electricity.[25] "Franklinization" is electrotherapy where Franklin applied strong static charges from powerful Leyden jars, which were charged from his electrostatic machine, to treat patients with various illnesses.[26][27] In compliance with his will, Franklin's Leyden "battery" of jars was given to the Royal Society on April 1, 1836, by Thomas Hopkinson's grandson Joseph Hopkinson.[28]

Several Leyden jars connected together = a battery. (18 inches wide X 13 inches deep X 15 inches high)
Museum label for Leyden jar "Electrical battery"
Leyden jar battery at Franklin Museum and the associated museum label that is below it.

Lightning rod

Main article: Lightning rod
Franklin's sentry-box experiment of pointed iron rod to attract lightning

Franklin observed in using his electrostatic machine that pointed objects had a better effect of "drawing off" and "throwing off" electricity than a blunt object.[4][29] He then thought about 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 had a pointed wire on the top. Near the bottom of the string attached to the kite was a key just atop a Leyden jar. He held the kite string with a silk ribbon for insulation against the electrical charge from a lightning bolt. His 21-year-old son was with him during this experiment as an assistant and a witness. They hid under a shed roof for protection from the rain. A lightning bolt hit the wire on the kite, traveled down the wet kite string through the key and sparked the Leyden jar, charging it just like his electrostatic machine did.[30] He proved from this experiment, and his electrostatic machine, that lightning is a giant electric spark.[31][32][33]

Franklin developed and produced his lightning rod invention from what he learned from his electrostatic machine about static electricity principles.[17][34] He published a proposal for an experiment to be done where a person was to stand on an insulated stool inside a sentry box protected from the rain and hold out a long pointed iron rod to attract a lightning bolt.[3] This was first performed with success in France on May 10, 1752.[3] It was repeated throughout Europe with success as Franklin had predicted, proving that lightning was electricity.[3] He determined from this experiment that by placing a pointed iron rod at the top of a roof to protect a wooden structure, the electrical discharge from the cloud would hit it instead, and go harmlessly into the ground where the other end of the rod was anchored.[25][35] Before Franklin's discovery it was thought that lightning and thunder was some form of exploding gas.[36]

In 1750, Franklin gave a demonstration 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 and burst into flames. With another miniature he had a lightning rod attached to it that went into the ground. When it was hit with the same electric charge nothing happened, as it went safely into the ground bypassing the miniature house and steeple. Dollhouses were 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 explaining: "How to secure houses from being hurt by its destructive violence."[37]

Book on electricity

Main article: Experiments and Observations on Electricity

The significance of Franklin's electrostatic machine is that ultimately a science was developed out of electricity.[3] He sent many letters to his friend Collinson in London about his electrical experiments using the electrostatic machine and the Leyden jar. In them he included his theories on the principles of how electricity worked. These letters were eventually assembled and published in 1751 in a book entitled Experiments and Observations on Electricity which had four major editions and was translated into several European languages.[38][39]

See also

References

Citations

  1. ^ Grimnes 2014, p. 495.
  2. ^ "Benjamin Franklin tries to electrocute a turkey". Massachusetts Historical Society. 2016. Retrieved November 7, 2016.
  3. ^ a b c d e Cohen 1990, p. 28.
  4. ^ a b c Finger 2012, p. 85.
  5. ^ "Franklin's Lightning Rod". The Franklin Institute. 2016. Retrieved November 7, 2016.
  6. ^ Cohen 1990, p. 61.
  7. ^ a b c Crane 1954, p. 48.
  8. ^ Pasles 2008, p. 119.
  9. ^ Lemay 2014, p. 72.
  10. ^ "From Benjamin Franklin to Peter Collinson, 28 March 1747". Founders Online. National Historical Publications and Records Commission. Archived from the original on 28 October 2016. Retrieved July 15, 2015.
  11. ^ a b Tucker 2009, p. 40.
  12. ^ Lemay 2014, p. 75.
  13. ^ "The Wistars and their Glass 1739 – 1777". WheatonArts. 2015. Retrieved November 7, 2016.
  14. ^ a b Cohen 1956, p. 440.
  15. ^ a b Bridenbaugh 2012, p. 63.
  16. ^ Lemay 2014, pp. 65–75.
  17. ^ a b Maclean 1877, p. 142.
  18. ^ Garche 2013, p. 596.
  19. ^ The Papers of Benjamin Franklin. Vol. 3. Yale University Press. 1961. p. 142.
  20. ^ McNichol 2011, p. 16.
  21. ^ Malmivuo & Plonsey 1995, p. 13.
  22. ^ a b Morgan 2003, p. 12.
  23. ^ a b Cohen 1956, p. 460.
  24. ^ Lynn 2009, p. 136.
  25. ^ a b Grimnes 2014, p. 496.
  26. ^ Schiffer 2003, pp. 136, 137.
  27. ^ "Electro-therapeutics". The Encyclopedia Americana: A universal reference library comprising the arts and sciences. Scientific American Compiling Department. 1905.
  28. ^ Cohen 1956, p. 454B.
  29. ^ LeMay 1987, p. 600.
  30. ^ McNichol 2011, p. 20.
  31. ^ McNichol 2011, p. 18.
  32. ^ Chisholm, Hugh (1910). "Electricity". The Encyclopædia Britannica. Vol. 9 (12 ed.). Encyclopædia Britannica Company. p. 181.
  33. ^ Cajori 1917, pp. 121–134.
  34. ^ Coulson 1950, p. 32.
  35. ^ McNichol 2011, p. 21.
  36. ^ Baigrie 2007, p. 40.
  37. ^ Lemay 2014, p. 91.
  38. ^ McNichol 2011, p. 19.
  39. ^ "Experiments and observations on electricity". Smithsonian Libraries. Retrieved 28 October 2016.

Sources

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