Benjamin Franklin
1706 - 1790
ELECTRIC-CURRENT-IN-A-SOLID
The distance at which electrons circle around their
nucleus is not arbitrary but limited to a number of
fixed orbits, also called bands or subshells. The first
band (the K-orbit in X-ray notation) can contain two
electrons at the utmost. For the second band (the
L-orbit) this maximum is eight, for the third (the
M-orbit) it is eighteen and the N- and O-orbits can
contain a maximum of thirty-two electrons each. The
outer bands (the P- and Q-orbits) can contain
eighteen, respectively eight electrons. Normally,
electrons can only be present in an outer band when
most of the inner bands are filled up until their
maximum. Carbon for instance contains two
electrons in the K-orbit and four in the L-orbit. With
mercury the K- until the N-orbit are filled up
completely while the O- and P-orbits contain
eighteen, respectively two electrons bringing the
total to eighty, equal to the number of protons within
the nucleus. With electrically conductive solids like
copper, the outer band (the conduction band) is not completely filled until its
maximum. This enables individual electrons in the outer (conduction) band to travel
more or less freely to the conduction bands of neighbouring atoms. Since the
average electron distribution stays the same there is, despite the electron
movements, no resulting electric current. When a potential difference is imposed
upon the outer ends of a conductor however, the free moving electrons in the
conduction bands will attempt to neutralise the potential difference by moving towards
the direction of the positive potential. By definition it is then stated that there is a
current flowing from the positive potential towards the negative potential although the
actual electron movement is just the other way. This confusion finds his origin in the
early experiments of Benjamin Franklin who, around 1750, defined the flow direction
of an electric current without completely understanding the mechanism behind it.
