then crossed the plate at an angle of about 70degrees. 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