This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
When a magnet is placed within a coil of wire insulated by being covered with silk thread, the latter assumes a polar condition the reverse of that of the magnet; and, if the magnetic circuit and that of the coil be closed at the same time, a current takes place in the latter in a direction opposite to that of the former. A powerful horse-shoe magnet is thus capable of inducing an electric movement in a coil of wire, which gives rise to sensible phenomena, and with certain arrangements may be made to act with great energy. When the body is connected with the two opposite poles by any conducting material, a slight shock is felt upon the closing of the magnetic circuit, after which no sensation is perceived, and no obvious effect produced, until the circuit is broken, when another sensation stronger than the first is experienced, by the instant alteration of the current before its entrance into repose. If, by any contrivance, this interruption of the circuit be made rapidly, the quick succession of the shocks becomes painful, and the effect may be increased so as to be quite insupportable. Upon this principle it is that the electro-magnetic machine is composed.