The Structure of the Atom 159
a silver plating on the cathode. The nitrate ions, NO 3 – , are attracted to
the silver anode where they combine chemically with the silver to
produce more silver nitrate. The level of silver nitrate remains the same
as a result of this process, silver, however, is transferred from the silver
anode to the copper cathode where it forms a uniform plate. One can
reverse this process and remove the silver plate from the copper rod by
changing the direction of the current. This is easily accomplished by
switching the terminals of the battery.
The electrolysis experiments first performed by Humphrey Davy and
Michael Faraday in the early part of the nineteenth century were
motivated by a desire to study the passage of an electric current through
a liquid, but they also revealed the existence of ions, i.e. atoms carrying a
charge. Faraday, during this period, also began to study the problem of
the passage of an electric current through a gas. He discovered that gases
are excellent insulators and that the conductivity of the gas increases as
its density decreases. Faraday’s research was limited by the primitive
state of vacuum techniques, at the time. It was not until 1854 when
Heinrich Geissler invented a vacuum pump, which enabled him to
evacuate a glass tube that the passage of an electric current through a
gas could be properly studied. The cathode and anode are attached
respectively to the negative and positive terminal of a high voltage
battery creating a strong electric field inside the discharge tube. The
passage of an electric current occurs as a result of the ionization of the
gas molecules (or atoms), the process whereby an electron is removed
from the gas molecule (or atom) leaving a positively charged ion. At the
time these experiments were first performed, it was thought that the
ionization was due to the strength of the electric field in the tube, which
was thought to tear apart the molecules of the gas. We have since
discovered that the ionization is due to the effects of cosmic radiation,
the high-energy charged particles, x-rays and gamma rays that are
continually streaming towards earth from outer space.
Once the molecule is ionized, the positive (negative) ion begins to
accelerate towards the anode (cathode), each gaining kinetic energy from
the electric field. If the gas is dense, they will soon collide with a neutral
molecule, lose their kinetic energy, and begin accelerating once again
toward their respective electrode. In this way, a small current is able to
pass through a dense gas. As the tube is evacuated so that the density of
the gas becomes significantly lower, the charge particles do not collide as
often with the neutral molecules of the gas. They, therefore, have time to