Faraday was the recipient of many scientific honors, including the Royal and Rumford medals of the Royal Society; he was also offered the presidency of the society but declined the honor. Faraday's earliest researches were in the field of chemistry, but the research that established Faraday as the foremost experimental scientist of his day was in the fields of electricity and magnetism. In 1821 he plotted the magnetic field around a conductor carrying an electric current; the existence of the magnetic field had first been observed by the Danish physicist Hans Christian Oersted in 1819. In 1831 Faraday followed this accomplishment with the discovery of electromagnetic induction and in the same year demonstrated the induction of one electric current by another. During this same period of research he investigated the phenomena of electrolysis and discovered two fundamental laws: that the amount of chemical action produced by an electrical current in an electrolyte is proportional to the amount of electricity passing through the electrolyte; and that the amount of a substance deposited from an electrolyte by the action of a current is proportional to the chemical equivalent weight of the substance. Faraday also established the principle that different dielectric substances have different specific inductive capacities. In experimenting with magnetism, Faraday made two discoveries of great importance; one was the existence of diamagnetism, and the other was the fact that a magnetic field has the power to rotate the plane of polarized light passing through certain types of glass. In addition to a number of papers for learned journals, Faraday wrote Chemical Manipulation (1827), Experimental Researches in Electricity (1844-1855), and Experimental Researches in Chemistry and Physics (1859). The unit of capacitance, the Farad, was named in his honor.