Ampère’s Law is one of the fundamental laws in electromagnetism. It describes the relationship between an electrical current and the magnetic field created by that current. Let me tell you about how Ampère’s Law was discovered, explain how it works and how we can make use of it in practice.
In 1820, in the month of July, Hans Christian Ørsted, a Danish physicist, carried out experiments with a Volta pile, an early type battery, that supplies current. Ørsted noticed that a compass needle moved when it came close to current carrying conductors on his workbench.
In those days the results of physical experiments were shared and discussed in scientific societies and what Ørsted had observed spread quickly over Europe.
The Académie in Paris asked André-Marie Ampère to verify Ørsted’s findings. In the summer of 1820 Ampère repeated the experiment and did some additional research as well. He placed two copper wires in parallel, let an electrical current flow through the wires and discovered that the wires move towards each other when both currents flow in the same direction and move away from each other when the currents flow in opposite direction.
Ampère derived a formula to calculate the forces between current carrying conductors. This force is proportional with the current value and inversely proportional with the square of the distance between the conductors.
Thanks to Ampère’s experimental work we are able to calculate the relation between the current through a conductor and the magnetic field in the space around the conductor This relationship is known as Ampère’s Law: the line integral of the magnetic field vector around a single closed path is equal to the current enclosed.
The magnetic field lines run in circles around the conductor and the magnetic field vector points in the direction that a corkscrew makes when it rotates in the direction of the current flow.
We can also explain why two parallel conductors attract each other when both currents flow in the same direction. In the space between the conductors, the magnetic field lines from the two conductors cancel each other out. Both conductors experience the force of the magnetic field at the outside and they move towards another.
Ampère also found out that when a straight piece of copper wire is bend in the shape of a turn, the magnetic field lines at the inside add up. By putting a number of turns in series we have a coil and the magnetic field lines are multiplied. The magnetic field can be amplified even more by placing ferromagnetic material inside the coil. A coil with a ferromagnetic core is what we call an electromagnet.Ampère described the results of his experiments in his book Electricté Dynamique