Electromagnetism
In 1820,
a professor from the University of
Copenhagen, Denmark, named Hans Christian Oersted
accidentally discovered the compass needle moved when
the electrical current was turned on.
Photo Credit: https://www-istp.gsfc.nasa.gov/earthmag/oersted.htm
In a series of rigorous experiments, Ampere discovered that the current flowing in the same direction in two circular loops would attract each other whereas current flowing in the opposite direction would repel each other (Ampere's circuital law). These experiments were influenced by Oersted's discovery.
Photo Credit:
https://www-istp.gsfc.nasa.gov/earthmag/oersted.htm
Ampere also found that two currents that are straight and parallel generated a force that was inversely proportional to the distance between them. The force generated is proportional to the intensity of the current flow (Ampere's force law).
Photo credit:
http://www.calctool.org/CALC/phys/electromagnetism/ampere_law