182 Chapter 12
intervention in physics made invisible electricity take a visible form. In 1777, he completed
construction of a large electrophorus ( figure 12.1a ), whose resinous “ cake, ” two meters in
diameter, when rubbed with fur generated static electricity, whose polarities he symbolized
as + and –. By placing the insulated metal plate in contact with the charged “ cake ” and
then touching the plate ’ s upper surface to allow positive charge to escape ( “ charging by
induction ” ), the bottom of the plate had sufficient negative charge to generate sparks up
to 40 cm long (about 20,000 volts in modern units).
He noticed that the charged plate, when discharged, would cause the resin dust on his
table to assume striking shapes, now called Lichtenberg figures ( figure 12.1b,c ), “ innumer-
able stars, galaxies, and large suns ... finely formed branches, similar to those made by
frozen steam on window panes. ”^4 Consistent with his quest for the unity of nature, he
described the electrical patterns in the language of astronomy or botany, as if the electrified
dust visibly disclosed the deep affinity between all these realms. Further, he could clarify
the figures by scattering more dust on them, bringing out their fine details, which remained
clear even after several days.
Lichtenberg felt he had discovered a means of investigating the “ motion of electrical
matter ” similar to the long-known use of iron filings to make visible the action of magnets.
He noted the pronounced difference between the figures generated by positive ( figure
12.1b ) and negative (figure 12.1c) charge, taking this as evidence that electricity is com-
posed of two different fluids, not one (the subject of much controversy into the early
nineteenth century). He also realized that pressing black paper on his figures allowed them
to print themselves in far greater and more perfect detail than he could achieve in a
drawing. Some historians thus credit Lichtenberg with discovering the essential process
of xerography, now used in every copier or printer.^5
Lichtenberg ’ s figures found immediate resonance in the work of Ernst Chladni. The son
of a law professor, he quit the law after his father ’ s death to follow the “ study of nature,
which had always been my secondary and therefore dearest occupation. As an amateur of
music, whose elements I had begun to learn a bit late at age nineteen, I noticed that the
theory of sound was more neglected than many other branches of physics, which gave rise
to my desire to remedy this lack and be useful to this part of physics through some dis-
coveries. ” In 1785, Chladni began “ very imperfect ” experiments striking plates of glass or
metal at different points and trying to understand their sounds, compared to the familiar
vibrating string, which he studied from Bernoulli and Euler. Chladni then read of “ a
musical instrument made in Italy by abb é Mazzocchi, consisting of bells to which one
applied one or two violin bows, ” which he then decided to try on his plates. He obtained
sounds whose pitches were in the ratios of the squares of 2, 3, 4, 5, ... but felt he did not
understand the underlying motions. Reading Lichtenberg ’ s experiments made him
“ presume that the different vibratory motions of a sounding plate also ought to show dif-
ferent appearances if a little sand or a similar substance were spread on the surface. ”
His first trials on a round plate yielded ten- and twelve-pointed stars ( figure 12.2a ),