on current is
turned into sound, or heat, or light will be described in a technical
section later in this work. We are concerned here only with the
principle, which is clear. While zinc is very apt to part with
electrons, copper is just as obliging in facilitating their passage
onward. Electrons will travel in this way in most metals, but copper is
one of the best "conductors." So we lengthen the copper wire between the
zinc and the carbon until it goes as far as the front door and the bell,
which are included in the circuit. When you press the button at the
door, two wires are brought together, and the current of electrons
rushes round the circuit; and at the bell its energy is diverted into
the mechanical apparatus which rings the bell.

Copper is a good conductor--six times as good as iron--and is therefore
so common in electrical industries. Some other substances are just as
stubborn as copper is yielding, and we call them "insulators," because
they resist the current instead of letting it flow. Their atoms do not
easily part with electrons. Glass, vulcanite, and porcelain are very
good insulators for this reason.

What the Dynamo does

But even several cells together do not produce the currents needed in
modern industry, and the flow is produced in a different manner. As the
invisible electrons pass along a wire they produce what we call a
magnetic field around the wire, they produce a disturbance in the
surrounding ether. To be exact, it is through the ether surrounding the
wire that the energy originated by the electrons is transmitted. To set
electrons moving on a large scale we use a "dynamo." By means of the
dynamo it is possible to transform mechanical energy into electrical
energy. The modern dynamo, as Professor Soddy puts it, may be looked
upon as an electron pump. We cannot go into the subject deeply here, we
would only say that a large coil of copper wire is caused to turn round
rapidly between the poles of a powerful magnet. That is the essential
construction of the "dynamo," which is used for generating strong
currents. We shall see in a moment how magnetism differs from
electricity, and will say here only that round the poles of a large
magnet there is a field of intense disturbance which will start a flow
of electrons in any copper that is introduced into it. On account of the
speed given to the coil of wire its atoms enter suddenly this magnetic
field, and they give off crowds of electrons in a