The discovery of these two kinds of electricity has, however, enabled us
to understand very fairly what goes on in electrical phenomena. The
outlying electrons, as we saw, may pass from atom to atom, and this, on
a large scale, is the meaning of the electric current. In other words,
we believe an electric current to be a flow of electrons. Let us take,
to begin with, a simple electrical "cell," in which a feeble current is
generated: such a cell as there is in every house to serve its electric
bells.
In the original form this simple sort of "battery" consisted of a plate
of zinc and a plate of copper immersed in a chemical. Long before
anything was known about electrons it was known that, if you put zinc
and copper together, you produce a mild current of electricity. We know
now what this means. Zinc is a metal the atoms of which are particularly
disposed to part with some of their outlying electrons. Why, we do not
know; but the fact is the basis of these small batteries. Electrons from
the atoms of zinc pass to the atoms of copper, and their passage is a
"current." Each atom gives up an electron to its neighbour. It was
further found long ago that if the zinc and copper were immersed in
certain chemicals, which slowly dissolve the zinc, and the two metals
were connected by a copper wire, the current was stronger. In modern
language, there is a brisker flow of electrons. The reason is that
the atoms of zinc which are stolen by the chemical leave their
detachable electrons behind them, and the zinc has therefore more
electrons to pass on to the copper.
[Illustration: DISINTEGRATION OF ATOMS
An atom of Uranium, by ejecting an Alpha particle, becomes Uranium X.
This substance, by ejecting Beta and Gamma rays, becomes Radium. Radium
passes through a number of further changes, as shown in the diagram, and
finally becomes lead. Some radio-active substances disintegrate much
faster than others. Thus Uranium changes very slowly, taking
5,000,000,000 years to reach the same stage of disintegration that
Radium A reaches in 3 minutes. As the disintegration proceeds, the
substances become of lighter and lighter atomic weights. Thus Uranium
has an atomic weight of 238, whereas lead has an atomic weight of only
206. The breaking down of atoms is fully explained in the text.]
[Illustration: _Reproduced by permission from "The Interpretation of
Radium" (John Murray)._