predict that a time may arrive when the energy of the wind or tide will
be employed to produce from the magnetic lines of force given out by the
earth's magnetism electrical currents far surpassing anything we have
yet seen or of which we have heard. Therefore let us not despise the
smallness of the force, but rather consider it an element of power from
which might arise conditions far higher in degree, and which we might
not recognize as the same as this developed in its incipient stage.

If the galvanometer be replaced by a telephone, no matter how the spiral
be moved, no sound will be heard, simply because the induced currents
produced consist of comparatively slow undulations, and not of sharp
variations suitable for a telephone. But by placing in circuit this
mechanical make and break arrangement the interruptions of the current
are at once audible, and by regulating the movement of the spiral I can
send signals, which, if they had been prearranged, might have enabled
us to communicate intelligence to each other by means of the earth's
magnetism. I show this experiment more with a view to illustrate the
fact that for experiments on induction both instruments are necessary,
as each makes manifest those currents adapted to itself.

The lines of force of light, heat, and sound can be artificially
produced and intensified, and the more intense--they are the more we
perceive their effects on our eyes, ears, or bodies. But it is not so
with the lines of magnetic force, for it matters not how much their
power is increased--they appear in no way to affect us. Their presence
can, however, be made manifest to our eyes or ears by mechanical
appliances. I have already shown you how this can be done by means of
either a galvanometer or a telephone in circuit with a spiral wire.

I have already stated that while engaged in these experiments I found