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Johannes Wilcke invented and then Alessandro Volta
perfected the electrophorus well over two hundred years ago. This device was quickly
adopted by scientists around the world because it filled the need for a
reliable and easy-to-use source of charge and voltage for experimental researches
in electrostatics [Dibner, 1957]. Many old natural philosophy texts contain
lithographs of the electrophorus.
A hand-held electrophorus can
produce significant amounts of charge conveniently and repeatedly. It is
operated by first frictionally charging a flat insulating plate called a "cake".
In Volta's day, the cake was made of shellac/resin mixtures or a carnauba wax
film deposited on glass. Nowadays, excellent substitutes are available.
TeflonTM, though a bit expensive, is a good
choice because it is an excellent insulator, charges readily, and is easy to
clean and maintain. The electrophorus is ideal for generating energetic
capacitive sparks required for vapor ignition
demonstrations.
The basic operational steps for the electrophorus are depicted
in the sequence of diagrams below. Note that the electrode, though making
intimate contact with the tribocharged plate, actually charges by induction. No
charge is removed from the charged cake and, in principle, the electrode can be
charged any number of time by repeating the steps depicted. Click
here to
view a neat animation of the electrophorus charging process. Ainslie describes
interesting experiments with an electrophorus that was charged in the Springtime
and then its charge monitored throughout the summer [Ainslie, 1982]. The
apparent disappearance of the charge during humid weather and its reappearance in
the Fall must be attributed to changes in the humidity.
The energy for each
capacitive spark drawn from the electrophorus is actually supplied by the action of
lifting the electrode off
the cake. This statement can be confirmed by investigating the strength of the
sparks as a function of the height to which the electrode is lifted. Layton
makes this point and further demonstrates with a small fluorescent tube the
dependence of the electrostatic potential on the position of the electrode
[Layton, 1991]. Lifting the electrode higher gives stronger sparks [Lapp,
1992]. The
electrophorus works most reliably if the charged insulating plate rests atop a
grounded plane, such as a metal sheet, foil, or conductive plastic. [See Bakken
Museum booklet, pp. 78-80.] The ground plane limits the potential as the
electrode is first lifted from the plate, thus preventing a premature brush
discharge. In dry weather, powerful 3/4" (2 cm) sparks can be drawn easily from
a 6" (15 cm) diameter, polished, nick-free aluminum electrode. Estimating the
potential of the electrode at V = ~50 kV and the capacitance at C =
~20 pF, we get for the charge and for the capacitive energy. This energy value
easily exceeds the minimum ignition energy (MIE) of most
flammable vapors. Click here to learn about a new
type of electrophorus invented by S. Kamachi. The web site of the world-famous
Exploratorium in San Francisco describes a simple
electrophorus made of aluminum pie plates and other inexpensive materials.
Young scientists should check out this page. In addition, the library references
below contain interesting information about the
electrophorus and other electrostatics demonstrations. One example is the
cylindrical electrophorus [Ainslie, 1980]. A simple
leaf electroscope attachment, shown in the figure below, makes it very easy to
reveal some of the important charging and charge redistribution phenomena of the
electrophorus. This accessory is especially handy because it works even on warm,
humid days when large, impressive sparks can not be coaxed out of the
electrophorus. Refer to the electroscope page
for details on how to make this convenient accessory. The electroscope is
operated in the same way as before, but now the electroscope reveals information
about the charge and its distribution on the electrode. In particular, it should
be noted that, as the electrophorus is lifted up, its charge does not change. The
leaves of the electroscope spread apart because the constant charge on the
electrode redistributes itself, with about half of the charge moving to the top
surface. Another thing to notice is that the leaves, which spread to a wide angle
when the electrode is first lifted, slowly come back together with time,
indicating the leakage of electric charge, presumably due to corona discharge
from the edges of the leaves. Another
simple accessory is a corona discharge point that can be attached to the
electrophorus. The attachment is a metal rod of diameter 1/16" or greater with
one end sharpened to a point. When the charged electrode is lifted, the electric
field at the sharpened tip exceeds the corona limit and a local discharge starts,
dissipating the charge on the electrophorus. If one listens closely as the
electrode is lifted, a soft, varied-pitch buzzing noise lasting just a few
seconds may be heard. This is the corona, and it stops after the voltage has
been reduced below the corona threshold. Passive corona discharge points are used
widely in
manufacturing to dissipate unwanted static
charge. The corona discharge
can be
largely suppressed by covering the sharpened point with a small piece of antistatic
plastic foam
of the type used for packaging ESD-sensitive electronic components. The figure below
shows how
this scheme -- called resistive grading -- works to reduce or stop corona
discharges. D.S. Ainslie, "Inversion of electrostatic charges in a
cylindrical electrophorus", Physics Teacher, vol. 18, No. 7, October,
1980, p. 530. D.S. Ainslie, "Can an electrophorus lose its charge and then
recharge itself?", Physics Teacher, vol. 20, No. 4, April, 1982, p. 254.
Bakken Library and
Museum, Sparks and Shocks, Kendall/Hunt Publishing Co., Dubuque, IA,
1996, pp. 53-55. B. Dibner, Early Electrical Machines, pub. #14, Burndy
Library, Norwalk, CT, 1957, p. 50-53. R.A. Ford, Homemade Lightning:
creative experiments in electricity (2nd ed.), TAB Books (McGraw-Hill), New
York 1996, chapter 10. O.D. Jefimenko, "Long-lasting electrization and
electrets," in Electrostatics and its Applications (A.D. Moore, ed.),
Wiley-Interscience, New York, 1973, pp. 117-118. D.R. Lapp, "Letters,"
Physics Teacher, November, 1992, p. 454. W. Layton, "A different light
on an old electrostatic demonstration," Physics Teacher, Vol. 29, No. 1,
January, 1991, p. 50-51. K.L. Ostlund and M.A. Dispezio, "Static electricity
dynamically explored," Science Scope, February, 1996, pp.
12-16.
Electroscope accessory
Corona discharge accessory
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Last modified: Sunday, 06-Mar-2011 16:15:15 EST