Induced EMF in the Earth's Magnetic Field
   "Currents of electricity may be induced by the influence of terrestrial magnetism, but in consequence of the feebleness of the action it is not easy to render it by the aid of wire coils alone. Deflections may, however, be obtained by connecting with a very delicate galvanometer a helix of coarse wire. A very evident effect may be produced by employing the instrument represented [below and to the right]. It consists of a small rod of soft iron wound with wire, and fitted to revolve on a horizontal axis, which is provided with a pole-changer. Upon the the segments of the pole-changer press two wires. ... Now on causing the bar to revolve by hand in one direction, each end of the iron will become alternately a north and south pole... and a current of electricity, whose directions changes twice in each revolution, will be induced in the surrounding wire. The two currents are turned into one direction by the pole-changer, and the needle of the galvanometer will be steadily deflected." (1841 Manual of Magnetism, pg 197)

   Earlier, Davis explained how the same instrument can be driven as a motor in the earth's magnetic field. The armature had to be "constructed with some delicacy", and it had to be placed with its extremities oriented north and south. "By placing a steel magnet in a proper position near the revolving bar, it will rotate with much greater speed than by the action of terrestrial magnetism along; its motion may be reverse, notwithstanding the opposing influence of the earth, by disposing the permanent magnet in a suitable manner."

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