Hearing the Field 209
detailed picture of those currents in relation to the density and composition of the medium,
extending several centimeters above the plate.
This thickly substantial view of the medium supported Faraday ’ s consideration of the
interplay within it of longitudinal and transverse forces, “ considered as a pendulum vibrat-
ing to and fro under a given impulse, ” thereby giving rise to heaps of particles or crispations
of the liquid surface. Perhaps remembering his apprentice days in the work-yard, Faraday
instances the comb-shaped surface of “ the water in a pail placed in a barrow, and that on
the head of an upright cask in a brewer ’ s van passing over stones, ” which he duplicated
by tapping his plate ( figure 13.10a,b ). He was particularly pleased with “ a very simple
arrangement [that] exhibits these ripples beautifully ” ( figure 13.10c ); tapping transversely
created radial ripples, “ the results of that vibrating motion in directions perpendicular to
the force applied. ”^31 This significant transduction between perpendicular directions of
motion — singularly important in Ø rsted ’ s discovery — Faraday attributes to the properties
of the medium.ABCFigure 13.10
Faraday ’ s 1831 illustrations of (a) tapping a plate at × to excite (b) the comb-shaped ripples on the surface of
the water; (c) another device to excite strong radial ripples from a transverse tap at × on a rod attached to a cork
extending into a bowl of water.