Electrophorus

The electrophorus is one of the simplest and most fascinating of the early electrostatic devices. It is a manually operated static electric generator, capable of developing very high-voltage charges. While Johan Carl Wilcke is credited with the device's invention during the latter half of the 18th century, Alessandro Volta improved the original and gave it a name.

The construction of the electrophorus is every bit as simple as it appears, and in fact, my own version is somewhat over complicated. The basic elements are a dielectric plate and a conducting disc with an insulating handle. It relies upon electrostatic induction and triboelectric charging to generate very high potentials. Operation and theory of the electrophorus is explained in many books and on many websites, so I will here only offer my unique observations.

The first electrophoruses employed cakes of resin or wax as dielectric, later followed by hard rubber; these were the best insulators available at the time. While I would have liked to procure hard rubber, or "ebonite," I chose polycarbonate for its availability, relative low cost, and scratch resistence. I dyed the polycarbonate black to hide the adhesive holding it to the wooden base. The base is ambrosia maple. The disc is aluminum and the handle acrylic. Traditionally, the disc would be brass or copper and the handle would be varnished glass. I use the fur side of a rabbit hide to exploit the triboelectric effect in charging the device - one of the few traditional materials I used.

My electrophorus performs very well. When humidity is not too high, I have been able to obtain sparks of well over an inch; enough to light up a compact florescent bulb brightly for a brief moment. The oft quoted breakdown rating of air is 30,000 volts per centimeter. I do not know under what conditions this value was obtained, but in my experience a 1cm gap will strike an arc at about half this value. In any case, the potentials generated by the device must be in the several tens of thousands.

While my electrophorus works very well, I have observed some things which, were I to build another, would suggest improvements in the construction. First, the device works much better on a well grounded surface. When the electrophorus is operated on a well insulated table, the charge obtained is weak or imperceptible, but on a grounded surface or, especilly, on the user's lap, the spark strength increases considerably. I believe this is due to a capacitive effect of the generator. The top of the dielectric being charged, surely the electrons in whatever lies beneath the dielectric will arrange themselves so as to present an opposite charge, this being easiest in a conductive medium. Photos and descriptions of antique electrophoruses show that most utilized a metal base (or container in the case of resinous dielectrics), perhaps for this very reason.

My second observation has to do with the tolerances to which the electrophorus was made. The wooden base I used in my electrophorus is bowed downward in the center. The deflection may only be a few thousands of an inch, but it is enough that the electrophorus disc makes incomplete contact with the dielectric. As with capacitors, the charge strength or capacitance, is directly related to the proximity of the two plates; thus, a thinner dielectric brings the plates of a capacitor closer together and increases the amount of charge it can hold. Electrostatic induction works much the same, strength being directly related to proximity. Thus, if I am correct in my belief that the electrophorus acts much like a capacitor, the closer the metal disc to the opposite plate (the ground), the higher the charge

Several months ago I made measurements with an electronic meter of the capacitance in a plate capacitor. My observations proved that even a slight wrinkling or bending of the plates caused a large reduction of capacitance. Pressing the plates firmly against the glass dielectric provided the highest readings. I have also noticed that if I press down on the handle of the electrophorus disc so as to improve contact with the dielectric (and proximity to ground), the charge seems stronger. This would also suggest that, as in a capacitor, a thinner dielectric plate would improve performace as long as the dielectric material itself does not breakdown.

Keeping in mind the capacitive nature of the electrophorus during construction is important. The base of the device should be able to easily arrange electrons in response to the charge built on the dielectric; and the dielectric should be as flat as possible, and no thicker than necessary. Care in electrophorus construction will certainly yield improved performance.

Created by Sean Corron, April 2, 2011.