158 The Poetry of Physics and The Physics of Poetry
motion of the atom, so, too, the notion of electric fluids disappeared once
it was realized that electric phenomena were due to the underlying
electrical structure of atoms.
We now turn to a description of the experiments, which led to the
discovery of the underlying electrical structure of the atom. Students are
often baffled as to how the physicists know that an atom consists of a
positively charged nucleus about which negatively charged electrons
orbit. They become skeptical about the existence of the electron when
they discover that the electron has not and cannot be visually observed.
In order to counteract this skepticism, we shall carefully describe a series
of experiments stretching over a period of almost 100 years in which the
existence, charge and mass of the electron were established and in which
the underlying structure of the atom was discovered.
The first hint of any connection between the atom and electrical
phenomena came from the observation of the process of electrolysis. A
typical electrolysis set up involves an anode or positive electrode and a
cathode or the negative electrode, which are two metal rods connected to
the positive and negative terminals of a battery and are immersed in
water. A current is able to flow through the water because the water
molecules, H 2 O, separate chemically into two positively charged
hydrogen ions, H+ and one doubly negatively charged oxygen ion, O2–.
As the current flows in this circuit, hydrogen and oxygen gas will be
attracted to and collect at the negative cathode and the positive anode
respectively. The volume of hydrogen gas that collects is twice that of
the oxygen gas and is proportional to the total amount of electricity that
flows in the circuit. An ion is an atom, which carries a charge. Because
there is an excess of negative charge at the cathode, the H+ ions collect
there, become neutralized and form hydrogen gas. The oxygen ions
drawn by the positive charge collect at the anode, are neutralized and
form oxygen gas. The ratio of the volume of hydrogen gas to oxygen gas
is two to one because there are two hydrogen atoms to one oxygen atom
in the water molecule.
Other electrolysis experiments showed that most atoms are capable of
carrying a charge. One such experiment led to the process of silver
plating. Let us consider the following set up where we choose for the
anode, a pure silver rod, and for the cathode, a copper rod. Instead of
water as our conducting medium, we choose a solution of silver nitrate,
AgNO 3. When silver nitrate is dissolved in water, silver ions, Ag+, are
attracted to the copper cathode to which they attach themselves forming