There are suspended in this room self-luminous bodies which enable us by
their rays or lines of force to see the non-luminous bodies with which
we are surrounded. There are also radiating in all directions from me
while speaking lines of force or sound waves which affect more or
less each one of you. But there are also in addition to, and quite
independent of, the lines of force just mentioned, magnetic lines
of force which are too subtle to be recognized by human beings,
consequently, figuratively, we are both blind and deaf to them. However,
they can be made manifest either by their notion on a suspended magnet
or on a conducting body moving across them; the former showing its
results by attraction and repulsion, the latter by the production of an
electric current. For instance, by connecting the small flat spiral of
copper wire in direct circuit with the galvanometer, you will perceive
that the slightest movement of the spiral generates a current of
sufficient strength to very sensibly affect the galvanometer; and as
you observe, the amplitude of the deflection depends upon the speed
and direction in which the spiral is moved. We know that by moving a
conductor of electricity in a magnetic field we are able to produce an
electric current of sufficient intensity to produce light resembling
in all its phases that of solar light; but to produce these strong
currents, very powerful artificial magnetic fields have to be generated,
and the conductor has to be moved therein at a great expenditure of heat
energy. May not the time arrive when we shall no longer require these
artificial and costly means, but have learned how to adopt those forces
of nature which we now so much neglect? One ampere of current passing
through an ordinary incandescent lamp will produce a light equal to ten
candles, and I have shown that by simply moving this small flat spiral a
current is induced in it from the earth's magnetic field equal to 0.0007
ampere. With these facts before us, surely it would not be boldness to