Biomedical Engineering Reference
In-Depth Information
In France, Charles F. DuFay, a member of the French Academy of Science, was intrigued
by Gray's experiments. DuFay showed by extensive tests that practically all materials, with
the exception of metals and those too soft or fluid to be rubbed, could be electrified. Later,
however, he discovered that if metals were insulated, they could hold the largest electric
charge of all. DuFay found that rubbed glass would repel a piece of gold leaf, whereas
rubbed amber, gum, or wax attracted it. He concluded that there were two different kinds
of electric “fluids,” which he labeled “vitreous” and “resinous.” He found that while unlike
charges attracted each other, like charges repelled. This indicated that there were two kinds
of electricity.
In the American colonies, Benjamin Franklin (1706-1790) became interested in electric-
ity and performed experiments that led to his hypothesis regarding the “one-fluid
theory.” Franklin stated that there was only onetypeofelectricityandthattheelectrical
effects produced by friction reflected the separation of electric fluid so one body
contained an excess and the other a deficit. He argued that “electrical fire” is a common
element in all bodies and is normally in a balanced or neutral state. Excess or deficiency
of charge, such as that produced by the friction between materials, created an imbalance.
Electrification by friction was, thus, a process of separation rather than a creation of
charge. By balancing a charge gain with an equal charge loss, Franklin had implied a
law—namely, that the quantity of the electric charge is conserved. Franklin guessed that
when glass was rubbed, the excess charge appeared on the glass, and he called that
“positive” electricity. He thus established the direction of conventional current from pos-
itive to negative. It is now known that the electrons producing a current move in the
opposite direction.
Out of this experimental activity came an underlying philosophy or law. Up to the end of
the eighteenth century, the knowledge of electrostatics was mainly qualitative. There were
means for detecting but not for measuring, and the relationships between the charges had
not been formulated. The next step was to quantify the phenomena of electrostatic charge
forces.
For this determination, the scientific scene shifted back to France and the engineer-
turned-physicist Charles A. Coulomb (1726-1806). Coulomb demonstrated that a force is
exerted when two charged particles are placed in the vicinity of each other. However, he
went a step beyond experimental observation by deriving a general relationship that
completely expressed the magnitude of this force. His inverse-square law for the force of
attraction or repulsion between charged bodies became one of the major building blocks
in understanding the effect of a fundamental property of matter-charge. However, despite
this wide array of discoveries, it is important to note that before the time of Galvani and
Volta, there was no source that could deliver a continuous flow of electric fluid, a term that
we now know implies both charge and current.
In addition to a career as statesman, diplomat, publisher, and signer of both the Decla-
ration of Independence and the Constitution, Franklin was an avid experimenter and
inventor. In 1743 at the age of 37, Franklin witnessed with excited interest a demonstra-
tion of static electricity in Boston and resolved to pursue the strange effects with investi-
gations of his own. Purchasing and devising various apparati, Franklin became an avid
electrical enthusiast. He launched into many years of experiments with electrostatic
effects.
