Andre-Marie Ampere work done in the field of Electromagnetism was his greatest contribution.
In September 1820 a Danish scientist, Hans Christian Orsted, produced some experimental results in the field of magnetism. Ampere influenced by his discovery, discovered the nature of the relationship between electric current-flow and magnetism, as well as the relationships of the behavior of electric currents in various types of conductors.
Besides that, Ampere showed that two parallel wires carrying electric currents magnetically attract to each other, if the currents are in the same direction and repel if the currents are in opposite directions.
This experiment led Ampere to formulate his law of electromagnetism, Ampere’s law, that describes mathematically the magnetic force between two electrical currents, which states that the mutual action of two lengths of current-carrying wire is proportional to their lengths and to the intensities of their currents.
Electric current passing through the wire creates magnetic lines of flux that encircle the wires. Magnetic flux, through the surface which is the component of B field, passing through it. The circulating magnetic field, is a closed loop, in the electric current passing through the loop where is the magnetic field.
Thus, this made him the first person to develop measuring techniques for electricity. The ampere is the unit for measuring electric current.