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1-6: The Discovery of Elementary Charge |
At the beginning
of the 19th century,
experiments on
electrolysis
were frequently
carried out.
For example,
it happens
that, if water is
electrolyzed,
oxygen and hydrogen
are generated.
We put electrolyte solution
in a container called
"electrolytic cell"
and put
positive and
negative electrodes in it.
When we send
a direct electric current
between the electrodes,
the negative ions
are attracted to the
positive electrode,
and the positive ions
to the negative electrode.
Thus the electolytic solution
is decomposed at these
electrodes.
This phenomena is
the electrolysis.
In the case of
the following figure,
a solution of copper sulfate
is to be electrolyzed.
[Faraday's Law of Electrolysis]
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M. Faraday
(UK, 1791 - 1867)
repeatedly carried out
experiments concerning
electrolysis
and discovered
the following law
called Faraday's law
of electrolysis
(1833).:
Here the chemical
equivalent means
the value obtained
by dividing
the atomic weight
by the valence
(or atomic value)
of the element.
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[The Valence]
The valence
of an atom is
its combining capacity.
Namely, it is the number
of "hands" of an atom,
by which the atom
combines with other
kinds of atoms.
For example,
hydrogen and oxygen
have one and two hands,
respectively,
so that one atom
of oxygen combines
with two atoms of hydrogen
to form a molecule of water.
Accordingly the valence
of hydrogen is 1,
and that
of oxygen is 2.
The atomic weights
of hydrogen and oxygen are
1 and 16,
respectively.
The chemical
equivalent of hydrogen
is therefore
1, and that
of oxygen is 8.
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[The Elementary Charge
from the Viewpoint
of Electrolysis]
Suppose that,
during an electrolysis,
the mass of
the separated element
is M whose atomic weight
is A
and its valence is v,
and the total electric charge
carried is Q.
Since the mass M
is proportional to the
chemical equivalent A/v,
we have
where 1/F is
the proportionality constant.
Now, let us consider
electrolysis
from the viewpoint
of atomism.
It is natural
to consider that
the charge carried
by an atom
should be proportional
to the number
of "hand" of the atom,
i.e. the valence.
Let the quantity
of charge carried
by one "hand"
be a unit of charge,
q.
Therefore the total
charge carried
by an atom
is vq.
Let us think
of the case
where electrolysis
decomposes 1 mole
of element
which includes
a number of atoms
equal to Avogadro's constant
NA.
Thereby the total charge
carried is
Since
M = A
in this case,
we have
from the above equation.
The constant F
is called
Faraday's constant
whose value is measured
to be
This means
that the electric charge
necessary to separate
1 gram equivalent
of any element
by electrolysis
is 96500 C
(Coulomb).
Here the gram equivalent
is the equivalent weight
of an element
or a substance
expressed in grams.
For hydrogen,
1 gram equivalent
is 1 gram of hydrogen.
In the case of
the electrolysis
of copper sulfate shown
in the
above figure
on this page,
since the valence
of copper is 2
and its atomic weight
is 63,
1 gram equivalent
of copper is
31.5 grams of elemental copper.
From the above-mentioned
result, we can see that
the minimum unit
of electric charge
in electrolysis
is considered to be
This is just the
elementary electric charge
or,
more briefly,
the elementary charge.
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[Millikan's Experiment]
In 1909,
about 80 years
after the discovery of
Faraday's law of electrolysis,
a precise measurement
of elementary charge
was carried out
by R. A. Millikan
(USA, 1868 - 1953)
in a physical way.
This is the famous
"Millikan oil
drop experiment".
The schematic diagram
of the experimental set up
of Millikan's experiment
is shown in the following
figure:
Fine oil drops
are scattered
between two metal plates
with a sprayer.
The reason oil
was used instead water
was to avoid the quick
evaporation of the drops.
The oil drops are
usually charged
during spraying,
but sometimes X-rays
are used
to charge them sufficiently.
Large oil drops
which are unsuitable for measurement
fall quickly out
of view of the microscope
and those of suitable
sizes remain.
Because there is
the air between the plates,
the gravitation
and the viscosity
resistance due to the air
work on an oil drop,
so that it slowly falls down
at a constant
speed.
This speed is measured
through the microscope.
If a voltage E
applied between
the plates,
an upward force acts
on a charged drop
and the speed of descent
changes.
The measurement
of the speed
is repeated many times,
varying
the voltage E.
Millikan measured
the charges on the oil drops
in this experiment
and he found that they
are integral multiples
of a minimum unit
of charge e
whose value is
agrees with that
of the elementary charge
obtained from the
experiment of electrolysis.
Thus it became
clear that
the basic minimum element
of electricity
is the elementary charge
e,
whose currently
accepted value is
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