il of
wire, and lifting the bar into the direction of the dipping needle, he
excited by this action a current in the coil. On reversing the bar, a
current in the opposite direction rushed through the wire. The same
effect was produced, when, on holding the helix in the line of dip, a
bar of iron was thrust into it. Here, however, the earth acted on the
coil through the intermediation of the bar of iron. He abandoned the bar
and simply set a copper-plate spinning in a horizontal plane; he knew
that the earth's lines of magnetic force then crossed the plate at an
angle of about 70 deg.. When the plate spun round, the lines of force were
intersected and induced currents generated, which produced their proper
effect when carried from the plate to the galvanometer. "When the plate
was in the magnetic meridian, or in any other plane coinciding with the
magnetic dip, then its rotation produced no effect upon the
galvanometer."

At the suggestion of a mind fruitful in suggestions of a profound and
philosophic character--I mean that of Sir John Herschel--Mr. Barlow, of
Woolwich, had experimented with a rotating iron shell. Mr. Christie had
also performed an elaborate series of experiments on a rotating iron
disk. Both of them had found that when in rotation the body exercised a
peculiar action upon the magnetic needle, deflecting it in a manner
which was not observed during quiescence; but neither of them was aware
at the time of the agent which produced this extraordinary deflection.
They ascribed it to some change in the magnetism of the iron shell and
disk.

But Faraday at once saw that his induced currents must come into play
here, and he immediately obtained them from an iron disk. With a hollow
brass ball, moreover, he produced the effects obtained by Mr. Barlow.
Iron was in no way necessary: the only condition of success was that the
rotating body should be of a character to admit of the formation of
currents in its substance: it must, in other words, be a conductor of
electricity. The higher the conducting power the more copious were the
currents. He now passes from his little brass globe to the globe of the
earth. He plays like a magician with the earth's magnetism. He sees the
invisible lines along which its magnetic action is exerted and sweeping
his wand across these lines evokes this new power. Placing a simple loop
of wire round a magnetic needle he bends its upper portion to the west:
the north pole of the needl