Biomedical Engineering Reference
In-Depth Information
a controversy that ultimately resulted in the creation of two separate and distinct
disciplines: electrophysiology and electrical engineering. The controversy arose from the
different interpretations of the data presented in this now famous article. Galvani was
convinced that the muscular contractions he observed in frog legs were due to some
form of electrical energy emanating from the animal. On the other hand, Allesandro
Volta, a professor of physics at the University of Padua, was convinced that the
“electricity” described in Galvani's experiments originated not from the animal but from
the presence of the dissimilar metals used in Galvani's experiments. Both of these inter-
pretations were important. The purpose of this section, therefore, is to discuss them in
some detail, highlighting the body of scientific knowledge available at the time these
experiments were performed, the rationale behind the interpretations that were formed,
and their ultimate effect.
12.2.2 Electricity in the Eighteenth Century
Before 1800, a considerable inventory of facts relating to electricity in general and
bioelectricity in particular had accumulated. The Egyptians and Greeks had known that cer-
tain fish could deliver substantial shocks to an organism in their aqueous environment.
Static electricity had been discovered by the Greeks, who produced it by rubbing resin
(amber or, in Greek,
) with cat's fur or by rubbing glass with silk. For example,
Thales of Miletus reported in 600 BC that a piece of amber when vigorously rubbed with
a cloth responded with an “attractive power.” Light particles such as chaff, bits of papyrus,
and thread jumped to the amber from a distance and were held to it. The production of
static electricity at that time became associated with an aura.
More than two thousand years elapsed before the English physician William Gilbert
picked up where Thales left off. Gilbert showed that not only amber but also glass, agate,
diamond, sapphire, and many other materials when rubbed exhibited the same attractive
power described by the Greeks. However, Gilbert did not report that particles could also
be repelled. It was not until a century later that electrostatic repulsion was noted by Charles
DuFay (1698-1739) in France.
The next step in the progress of electrification was an improvement of the friction
process. Rotating rubbing machines were developed to give continuous and large-scale
production of electrostatic charges. The first of these frictional electric machines was
developed by Otto von Guericke (1602-1685) in Germany. In the eighteenth century, elec-
trification became a popular science, and experimenters discovered many new attributes
of electrical behavior. In England, Stephen Gray (1666-1736) proved that electrification
could flow hundreds of feet through ordinary twine when suspended by silk threads.
Thus, he theorized that electrification was a “fluid.” Substituting metal wires for the sup-
port threads, he found that the charges would quickly dissipate. Thus, the understanding
that different materials can either conduct or insulate began to take shape. The “electrics,”
like silk, glass, and resin, held charge. The “nonelectrics,” like metals and water,
conducted charges. Gray also found that electrification could be transferred by proximity
of one charged body to another without direct contact. This was evidence of electrification
by induction, a principle that was used later in machines that produced electrostatic
charges.
elektron
