In 1874, Edison invented the quadruplex telegraph, which allowed the transmission of two signals in each direction down one wire.

FORBES: How You Can Profit From Thomas Edison's Inventions

The atoms of the wire would be magnetised in the direction of the field.

ECONOMIST: Spintronics

To compensate for this movement of ions, electrons have to move in the opposite direction and if the electrodes are connected by a conducting wire running through a useful circuit, that is the route they will take.

ECONOMIST: Electricity storage