flash.

It is found that a similar disturbance is caused, though the flow is in
the _opposite_ direction, when the coil of wire leaves the magnetic
field. And as the coil is revolving very rapidly we get a powerful
current of electricity that runs in alternate directions--an
"alternating" current. Electricians have apparatus for converting it
into a continuous current where this is necessary.

A current, therefore, means a steady flow of the electrons from atom to
atom. Sometimes, however, a number of electrons rush violently and
explosively from one body to another, as in the electric spark or the
occasional flash from an electric tram or train. The grandest and most
spectacular display of this phenomenon is the thunderstorm. As we saw
earlier, a portentous furnace like the sun is constantly pouring floods
of electrons from its atoms into space. The earth intercepts great
numbers of these electrons. In the upper regions of the air the stream
of solar electrons has the effect of separating positively-electrified
atoms from negatively-electrified ones, and the water-vapour, which is
constantly rising from the surface of the sea, gathers more freely round
the positively-electrified atoms, and brings them down, as rain, to the
earth. Thus the upper air loses a proportion of positive electricity, or
becomes "negatively electrified." In the thunderstorm we get both kinds
of clouds--some with large excesses of electrons, and some deficient in
electrons--and the tension grows until at last it is relieved by a
sudden and violent discharge of electrons from one cloud to another or
to the earth--an electric spark on a prodigious scale.

Sec. 11

Magnetism

We have seen that an electric current is really a flow of electrons. Now
an electric current exhibits a magnetic effect. The surrounding space is
endowed with energy which we call electro-magnetic energy. A piece of
magnetised iron attracting other pieces of iron to it is the popular
idea of a magnet. If we arrange a wire to pass vertically through a
piece of cardboard and then sprinkle iron filings on the cardboard we
shall find that, on passing an electric current through the wire, the
iron filings arrange themselves in circles round it. The magnetic force,
due to the electric current, seems to exist in circles round the wire,
an ether disturbance being set up. Even a single electron, when in
movement, creates a magnetic "field," as it is called, round its path.
Ther