Electricity from magnetism HEN AN ELECTRIC GUITARIST plucks a string in vast stadium, the metal string is almost silent. However, its vibrations are detected by an electromagnetic pick-up, boosted by the and this fills the stadium wi sound. The g pick-up of the of electrical devices, including generators and transformers, that rely manipulating magnetism to make electricity. The principle was demonstrated in 1831 by Michael Faraday in Britain and by Joseph Henry in the United States. In electromagnetic induction, a varying or moving magnetic field produces an electromotive force(e.m.f.) in a nearby conductor, and thus an electric current if the conductor is circuit(p. 20). The current flows only while the magnetic field varies