The electric field outside a conductor in which was flowing a current was observed experimentally by Oleg J. Jefimenko. "He had an ingenious idea of utilizing grass seeds as test particles near current carrying wires. They are electrically neutral in normal state so that they do not induce any charges in the conductor. On the other hand, they are easily polarized in the presence of an electric field, aligning themselves with it. The lines of electric field are then observed in analogy with iron fillings generating the lines of magnetic field." [Assis, A. K. T., Rodrigues, W. A., & Mania, A. J. (1999). The electric field outside a stationary resistive wire carrying a constant current. Foundations of Physics, 29(5), 729-753.]. Jefimenko presented his results in Plate 6 of his Electricity and Magnetism, Meredith Publishing. 1966.
