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|1-6: The Discovery of Elementary Charge|
At the beginning
of the 19th century,
that, if water is
oxygen and hydrogen
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]
(UK, 1791 - 1867)
repeatedly carried out
the following law
called Faraday's law
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.
[The Elementary Charge
from the Viewpoint
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.
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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