Take an ordinary flower-vase well dried and energetically rub it with a
silk handkerchief. The vase which thus becomes electrified will attract
any light body, such as a feather, as shown in the above illustration.]
Such cells are now made of zinc and carbon, immersed in sal-ammoniac,
but the principle is the same. The flow of electricity is a flow of
electrons; though we ought to repeat that they do not flow in a body, as
molecules of water do. You may have seen boys place a row of bricks,
each standing on one end, in such order that the first, if it is pushed,
will knock over the second, the second the third, and so on to the last.
There is a flow of _movement_ all along the line, but each brick moves
only a short distance. So an electron merely passes to the next atom,
which sends on an electron to a third atom, and so on. In this case,
however, the movement from atom to atom is so rapid that the ripple of
movement, if we may call it so, may pass along at an enormous speed. We
have seen how swiftly electrons travel.
But how is this turned into power enough even to ring a bell? The actual
mechanical apparatus by which the energy of the electron 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.