Note the metal comb at the top of the sketch.  This is an applications of one of Franklin's first observations

This was demonstrated in class using a Van de Graaff generator.  We first positioned a grounded metal sphere a few centimeters away from the generator.  Periodically, once sufficient charge built up on the dome of the generator, an audible & visible spark (about 3 inches long) would jump to the grounded sphere.

When a pointed, grounded, rod was brought near the Van de Graaff, the sparking to the grounded ball stopped.  The pointed rod draws off electricity from the dome of the generator before sufficient charge is able to build up and spark across to the grounded ball.  Once the pointed rod was moved away, sparking to the sphere resumed.

Early machines often used some material (my guess would be silk) rubbing against a spinning glass sphere or a glass cylinder.  The glass would acquire one charge (positive charge I suspect), the rubbing surface the other polarity.  The charge could be drawn from the glass cylinder by a metal comb.


Here's a rough translation of the French phrase at the bottom of the figure: "I know, where is best found this almost magical virtue, wisely named electricity; young beauties it's in your eyes.")

“By being removed so far from the European centres of experimentation and discussion of electrical events.  Franklin was able to view his own observation with a freshness not encumbered by the earlier notions of others.  He therefore regarded an electrically undisturbed body as being under neutral charge or as in a state of electrical equilibrium." (See the Dibner 1977 reference at the end of today's notes)

A Van de Graaff generator (invented in 1929) is a more modern way of producing electric charge like was used in some of the early electrical experiments.  A Van de Graaff generator itself is an example of a friction machine and it is worth taking a moment to understand how they work.  A photo is show below at left.  The dome has been removed in the close up below at right.  You can see the belt stretching around an upper polyethylene pulley and a wire mesh just above (but not touching) the belt.  The wire mesh (a very fine wire comb) "draws" charge off the belt and carries it to the metal dome where the charge collects.  It is important that the dome itself be smooth, points and sharp edges would "throw" off electrical charge into the air.  It is also important that the plastic support that the dome rests on be clean to prevent leakage currents from flowing from the dome to the bottom of the generator.

Here's a short description of triboelectric charging (from Wikipedia)

"The triboelectric effect (also known as triboelectric charging) is a type of contact electrification in which certain materials becom electrically charged after they come into contact with another different material through friction.   Rubbing glass with fur, or a comb through the hair, can build up triboelectricity. Most everyday static electricity is triboelectric. The polarity and strength of the charges produced differ according to the materials, surface roughness, temperature, strain, and other properties.

Thus, it is not very predictable, and only broad generalizations can be made.  Amber, for example, can acquire an electric charge by contact and separation (or friction) with a material like wool.   Other examples of materials that can acquire a significant charge when rubbed together include glass rubbed with silk, and hard rubber rubbed with fur"

And here is list many materials and the charge they acquire (from the same article in Wikipedia).  This was on a handout distributed in class.

The Triboelectric Series

Positively charged (most charging at the top of the list)	Negatively charged (most charging at the top of the list
polyurethane foam Hair, oily skin nylon, dry skin glass acrylic, lucite leather rabbit's fur quartz mica lead cat's fur silk aluminum paper (small positive charge) cotton No charge wool steel	ebonite silicone rubber teflon silicon vinyl (PVC) polypropylene polyethylene (like Scotch tape) plastic wrap orlon sytrene (Styrofoam - polystyrene foam) polyester synthetic rubber acetate, rahyon gold, platinum brass, silver sulfur nickel, copper hard rubber resins rubber balloon polystyrene sealing wax amber wood (small negative charge)

This is what was used to determine the charging of the plexiglas and polyethylene rollers in the Van de Graaff generator

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The Wimhurst generator was developed between 1800 and 1883 by a British inventor and is another method for separating and collecting charge.  Friction and triboelectric charging are not involved in this case.

Rather than attempting to draw two disks one in front of the other, we'll draw an end view of two concentric cylinders.  The geometry is different but the principle is the same (the base diagram below is from:  www.coe.ufrj.br/~acmq/whyhow.html)

We imagine that purely by chance a positive charge is found on one of the metal segments on the outer cylinder.  It could easily have been a negative charge.  This is how the Wimshurst machine gets started, a charge appears somewhere on one metal segments.

The two cylinders, which travel in opposite directions, have turned 45 degrees.  The original outer positive charge causes negative charge to be induced in the upper half of one of the cross members.  This results in negative charge being placed on one of the inner segments.  Positive charge is found on the opposite end of the cross member and is placed on a segment on the opposite side of the inner cylinder.

The cylinders rotate another 45 degrees.  The original positive charge is "drawn" off the outer cylinder at left and stored.    There are two charges left on the inner cylinder.	The two charges on the inner cylinder have moved into positions that are in line with the other cross member.  New induced charges appear on opposite segments on the outer cylinder.

Negative and positive charge on the inner cylinder are collected at right and left respectively.	Charges on opposite sides of the outer cylinder cause charges to be induced and placed on the outer cylinder.  Charge will be collected again at left and right after the next 45 degree turn.

Here's a good demonstration and explanation.

The figure above does not show the Leyden Jar capacitors (invented independently in 1644 and 1645-46) that were used to store electrical charges produced by the friction machines and the Wimshurst machine.  Franklin described a demonstration involving a dissectible Leyden jar.  When assembled the two metal cups form a capacitor with a glass dielectric in between.  The Leyden jar can be charged up and then disassembled.  No spark is observed when the two metal cups touch.  A spark is observed when the Leyden jar is reassembled and the inner and outer cups are connected.  Franklin believed (incorrectly) that it demonstrated that charge is stored on the dielectric that is between the two pieces of metal.

You'll find a short description (and some explanation) of the experiment on Wikipedia.  A portion is reproduced below

A popular but misleading demonstration with a Leyden jar involves taking one apart after it has been charged and showing that the charge is stored on the dielectric not the plates. The first documented instance of this demonstration is in a 1749 letter by Benjamin Franklin.  Franklin designed a "dissectible" leyden jar, shown below, which was widely used in demonstrations.

The jar in the demonstration is constructed out of a glass cup nested between two fairly snugly fitting metal cups. When the jar is charged with a high voltage and carefully dismantled, it is discovered that all the parts may be freely handled without discharging the jar. If the pieces are re-assembled, a large spark may still be obtained.

When not properly explained, this demonstration promotes the myth that capacitors store their charge inside their dielectric. This erroneous theory, due to Franklin, was taught throughout the 1800s, and is still sometimes encountered. However this phenomenon is a special effect caused by the high voltage on the Leyden jar.  In the dissectible Leyden jar, charge is transferred to the surface of the glass cup by corona discharge when the jar is disassembled; this is the source of the residual charge after the jar is reassembled. Handling the cup while disassembled does not provide enough contact to remove all the surface charge.

My interpretation of all this is that the charge is stored on the two metal surfaces of the capacitor when it is assembled.  Charge moves to the inner and outer surface of the dielectric when the capacitor is disassembled.  Once reassembled charge moves from the glass dielectric and back onto the metal plates until the capacitor is discharged.

Here's a pretty good video of the demonstration
(you'll also see a Wimshurst machine in action)

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Several demonstrations were shown in time remaining in the class.

Volta's hailstorm


The Volta Hailstorm apparatus consisted of two square metal plates mounted on the bottom and top of a clear plastic or glass cylinder.   Small round balls made of aluminum foil were inside.  The apparatus was placed on top of a Van de Graaff generator.   The top metal plate was connected to ground.

When the generator is turned on, the foil balls which are in electrical contact with the generator dome acquire some charge (assumed to be positive). 


The balls are repelled by the bottom electrode and travel up to the top metal disk (left figure above).  They transfer their charge to the top disk and then fall back to the bottom disk (right figure).  The positive charge in the top plate then flows to ground.  The foil balls again acquire charge from the bottom disk and the whole process repeats itself.


The apparatus will sometimes operate even when the top plate is not directly connected to ground.  The charge on the top disk bleeds off off the corners of the top plate and through the pieces of wire connected to the plate (the pointed pieces of wire are "throwing off" the electrical charge; the charge eventually travels to ground and completes the circuit).  The foil balls again acquire charge from the bottom disk and the whole process repeats itself. 

Here's a video of Volta's Hailstorm in action.

Franklin chimes

A Franklin chimes is a device that will detect (and perhaps measure) cloud electrification outdoors.  The device consists of two bells - one is connected to ground, the other is connected to a pointed vertical rod placed outdoors.  A small metal ball hangs from an insulated string in between the two bells.


A Franklin chimes can be seen at the left edge of this picture.  Note also the lightning rod visible outside the window  (see I. Gonta and E. Williams (1994) for a clearer version and more information about this image)


The photograph below shows the apparatus that was used.




The left bell is connected to ground.  The right bell was insulated from ground on a teflon support.  A pointed rod extended upward from the right bell.  The point at the end of the rod was placed 3 or 4 inches from the top of the Van de Graaff generator.  A small metal ball (actually a piece of rod) was suspended on an insulating string midway between the two bells.

When the Van de Graaff was turned on the metal ball began to swing between the two bells.  The operation is explained in the following series of pictures.

The pointed rod draws charge from the Van de Graaff generator and charges the right bell.  The metal ball between the two bells is initially uncharged.   Charging the right bell causes charges to be induced on the metal ball as shown above.


The metal ball is attracted to the right bell.  Contact with the bell neutralizes the negative charge on the ball.  The ball is then repelled by the positive charge on the bell.


Once the ball touches the grounded bell, the positive charge flows to ground. 


The ball is again uncharged.  The whole process repeats itself.  The overall effect of the swinging ball is to transport charge from the right bell to ground.

Here's an online demonstration that uses a slightly different geometry.  An inner bell is connected to the Van de Graaff generator and bells on the right and left that are connected to ground.

The last demonstration is an "electric whirl" ( I referred to it as an electric motor in class).

Here are several photographs (source of all 3 photos, that source also describes an "electric wind" demonstration that I'll have to try in the next edition of this course)


The middle picture is most like the whirl used in class.  In class the whirl was positioned on a needed that was attached directly to the top of the Van de Graaff generator.


We assume the Van de Graaff generator dome and the whirl are negatively charged in this sketch.  The points at the ends of the arms will "throw off" negative charge.  The negatively charged arms will be repulsed by the negative cloud of space charge near the ends of the arms and the whirl will spin as shown.

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References:

(1) B. Dibner, Benjamin Franklin, Lightning, Ch. 2 in Lightning Vol. 1, ed. by R.H. Golde, Academic Press, London, 1977.
(2) I. Gonta and E.. Williams, "A Calibrated Franklin Chimes", J. Geophys. Res., 99, 10671-10677, 1994.