Hearing the Field 207
all the instruments preserve their distinctive qualities. ... Compared with an ordinary band,
heard at a distance through the air, the effect is as a landscape seen in miniature beauty
through a concave lens. ”^26 This miniature but faithful sonic transmission clearly was a step
on Wheatstone ’ s quest for telephony and telegraphy.
A decade later, he achieved long-distance telegraphy via electromagnetism, not solid
rods, which required the step Faraday himself was about to take in discovering electro-
magnetic induction. Their joint presentations had steeped Faraday in what would prove to
be two crucial elements: the relation between longitudinal and transverse vibrations
(already important to Ø rsted ’ s discovery) and the essentially transient nature of the effects
they had demonstrated. As Williams perceptively notes, “ Although Faraday or Wheatstone
did not remark the fact at the time, they were dealing with what might be called acoustical
induction. An arrangement of particles on one plate could be effected by another plate
thrown into a vibratory state. ”^27 Extending this insight (and giving more weight to Wheat-
stone ’ s contribution), I would like to emphasize the specific parts played by the two crucial
elements just listed.
Though Williams considers Wheatstone ’ s work merely “ suitable for the amusement of
the audience at the Royal Institution, ” serious substance ran through that work, particularly
the two persistent elements that moved Faraday to pursue sound and acoustical figures
for the six months immediately preceding his discovery of electromagnetic induction in1831.^28 His diary makes clear the scope of his acoustical work and helps us understand its
relation to what follows, as does a paper he published in the midst of this work, “ On a
Peculiar Class of Acoustical Figures; and on Certain Forms Assumed by Groups of Par-
ticles upon Vibrating Elastic Surfaces. ”^29
Faraday began his 1831 experiments by further investigating Chladni figures, using
many kinds of powder and liquid, as Wheatstone had begun to do. Faraday wanted to know
the fine details of exactly how and why these substances form their patterns on the plate,
not just the resulting shape. He tested and found wanting a 1827 paper by Savart giving
a simple account of the particles finding quiet resting places on the plate. To go more
deeply, Faraday set up all kinds of baffles and partitions, designed to show exactly where
the particles move, and when ( figure 13.8 ). Looking at the patterns formed on some of his
plates, Faraday surely compared them with the noticeably similar iron filing patterns he
had seen produced by magnets ( figure 13.9 ).
Above all, Faraday investigated the three-dimensional formation of these sound patterns,
which previously had been treated as purely two-dimensional. He studied the air currents
above the plate and how they affected the particles or liquids. Using a pump to lower the
air pressure above the plate allowed still further control over the conditions in the medium
forming the patterns. Faraday even did upside-down experiments, suspending liquids
underneath the vibrating plate. Where Savart based his model only on the motion of the
plate, Faraday concluded that “ the nature of the medium in which those currents were
formed ought to have great influence over the phenomena. ”^30 His observations provide a