The key role of Oersted's and Ampère's 1820 electromagnetic experiments in the construction of the concept of electric current
We show that the concept of electric current was elaborated only after the discovery by Oersted in 1820 of a connection between electricity and magnetism, and thanks to the subsequent work of Ampère. In his study of the interaction between a compass and an electric circuit, Ampère set up a crucial experiment when he put a compass above his Voltaic pile, and another one above the connecting wire. Indeed, this experiment supported his creation of a new physical quantity, independent of the nature of physical phenomena, identical in the pile and in the wire, and only characterized by its direction and its intensity. To the experimental definition of this physical quantity—the electric current—by the oriented deviation of a magnetic needle, Ampère added in his manuscripts the substance of the two present theoretical definitions of the intensity of the current, namely, the ratio of charge to time q/t, and the ratio of electromotive force to the conducting wire's resistance E/R.