The experiment I wish to show you is no less than the conversion of electricity into magnetism; but it is a secret as yet. I will come to you at twelve on Monday, in the Poultry. If you will be so good as to order the battery to be charged to-morrow, it will be ready for us on Monday. Have you a dipping needle? This, and an air-pump, and the globe for taking sparks _in vacuo_ by points of charcoal, are all we shall want. Perhaps you will invite Dr. Babington, and our worthy friend Allen. I will show you the opening of quite a new field of experiment. Ever yours very sincerely, H. DAVY. The discovery of Professor Oersted was limited to the action of the electric current on needles previously magnetised. Davy ascertained that the _uniting conductor itself became magnetic_, during the passage of the electricity through it.[73] It was in consequence of having observed some anomaly, with respect to the way in which the uniting wire altered the direction of the magnet, that he was led to a conjecture which he immediately verified by a very simple experiment. He threw some iron filings on a paper, and brought them near the uniting wire, when immediately they were attracted by the wire, and adhered to it in considerable quantities, forming a mass round it ten or twelve times the thickness of the wire: on breaking the communication, they instantly fell off, proving that the magnetic effect entirely depended upon the passage of electricity through the wire. [73] It would appear that M. Arago likewise discovered this fact at about the same period; but it is evident that the French and English philosophers arrived at the result independently of each other; for the experiments which led to it were made by Sir H. Davy in October 1820; while the September number of the "Annales de Physique," containing the first account of the Researches of M. Arago, was not received in London until the 24th of November in that year; and it may be farther observed, that the numbers of this journal were very commonly published several months after the affixed date.