140 Engineering Fundamentals
Chemical Action
Cells create electrical energy using a chem-
ical action. A battery is an electrical connec-
tion of two or more cells. An electrode is a solid
conductor through which electricity enters or
leaves a medium. When two electrodes made of
different materials are placed in an electrolyte,
electrons gather on the negative terminal and a
voltage builds up between the electrodes. The
electrolyte acts chemically on the electrodes
and conducts electrons between the electrodes.
Disposable batteries are called primary cells
and batteries that can be recharged are called
secondary cells.
Making a cell can be as simple as soaking a
paper towel in salt water and placing it between
a nickel and a penny. A very small voltage can
be read between the nickel and penny. Figure 8-7
shows a very simple cell that you can make.
Solar Cells
Solar cells use light to create electricity.
Inside a solar cell, a layer of positive semi-
conductor material is sandwiched together
with a layer of negative semiconductor mate-
rial. Semiconductors, such as silicon, have
conductive properties between those of conduc-
tors and insulators. When light shines on the
cell, some of the light energy is absorbed.
Figure 8-7.
This is a basic cell. Unlike metals are separated by a paper towel soaked in salt water.
Paper towel soaked in salt water
between a nickel and a penny
Copper
Zinc
Goodheart-Willcox Publisher
Electrical Engineering in History
Many people have contributed to the develop-
ment of batteries throughout history, but one man’s
contribution stands above the rest. An Italian physi-
cist named Alessandro Volta is credited with invent-
ing the battery. A colleague of Volta, Luigi Galvani,
discovered that a dead frog’s leg would twitch
when the nerves were touched with unlike metals.
He believed that the animal tissue contained a cell
potential, which he called animal electricity.
Alessandro Volta expanded on Galvani’s discov-
ery by demonstrating in 1791 that electrical current
can be produced by layering unlike metals (copper
and zinc) with cardboard or cloth soaked in salt water.
He later piled up numerous layers of unlike metals
(like cells wired in series) to create more current. This
is called the voltaic pile, and it was the first battery
capable of providing sustained electrical current.
Volta discovered that he could measure the
difference in charge or electromotive force from
the top of the stack to the bottom. The measure-
ment unit volt is named for him.
History