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.