162 The Poetry of Physics and The Physics of Poetry
liquid is heated, the atoms move faster and, hence, one expects a shift to
higher frequencies. Both the plum pudding and solar system models of
the atom could account for the continuous spectrum of radiation. The
spectra from the gases were not as easily explained. Each gas had its own
characteristic set of discreet lines implying that this radiation was due
to internal motion within the atom. This hypothesis was confirmed by
the fact that the heating of the gas did not change these characteristic
frequencies. They were unaffected by the external motion of the atom
and, hence, were due to the internal motion of the atom. The plum
pudding model explained these discreet lines as due to the oscillations of
the electrons in the pudding of positive charge. Proponents of the solar
system model tried to explain these frequencies in terms of the frequency
of the electron’s orbit about the nucleus. According to Maxwell’s theory,
however, a charged particle moving in a circular orbit is constantly
accelerating and, hence, would be continuously radiating light. As the
electron continued to radiate light, it would lose energy, and, hence, fall
into an orbit closer to the nucleus where it would lose more energy and
so on until it spiraled into the nucleus. This defect of the solar system
model of the atom, known as the “spiral death”, caused it to fall into
disfavour. The solar system atom would arise again and triumph but, for
the time being, the plum pudding model provided the best description
then of the atom.
Although Thomson’s gas discharge experiments had established the
existence of the electron, the charge of the electron had never been
measured nor had the hypothesis that the electron was the ultimate
unit of charge been verified. It wasn’t until 1909 that Robert Millikan
completed the work begun twelve years earlier by Thomson with his
famous oil drop experiment. Millikan sprayed oil drops between two
metal plates through the narrow nozzle of an atomizer. Many of the
oil drops became charged in this process losing or gaining electrons
through the act of subdividing or by rubbing against the inner wall of the
nozzle or each other. After spraying the oil drops between the two metal
plates, he would apply a known electric field between the plates, which
would accelerate the oil drops upwards. By measuring the rate of
the upward acceleration, Millikan was able to determine the ratio of
charge to mass on each oil drop. He would then turn off the electric field
and observe the oil drops falling under the influence of gravity. By
measuring their terminal velocity, he was able to determine the mass
of the oil drop. Once he knew the mass of the oil drop he could then