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.
Davy observes, it was easy to imagine that such magnetic effects could not be exhibited by the electrical wire, without its being capable of permanently communicating them to steel; and that, in order to ascertain whether such was the fact, he fastened several steel needles, in different directions, to the uniting wire, when those parallel to it were found to act like the wire itself, while each of those placed across it acquired two poles. Such as were placed under the wire, the positive end of the battery being east, had north poles on the south of the wire, and south poles to the north. The needles above were in the opposite direction; and this was constantly the case, whatever might be the inclination of the needle to the wire. On breaking the connexion, the steel needles, placed across the uniting wire, retained their magnetism,[74] while those placed parallel to it lost it at the moment of disunion. The most extraordinary circumstances, however, connected with these experiments were, first, that contact with the uniting wire was not found necessary for the production of the effect,—indeed, it was even produced, though thick glass intervened; and, secondly, that a needle which had been placed in a transverse direction to the wire, merely for an instant, was found as powerful a magnet as one that had been long in communication with it.
The distance to which magnetism is communicated by electricity, and the fact of its taking place equally through conductors and non-conductors, are circumstances which, in the opinion of Davy, are unfavourable to the idea of the identity of electricity and magnetism.
Davy subsequently ascertained by experiment, that the magnetic result was proportional to the quantity of electricity passing through a given space; and this fact led him to believe, that a wire electrified by the common machine would not occasion a sensible effect; and this he found to be the case, on placing very small needles across a fine wire connected with a prime conductor of a powerful machine and the earth. But as a momentary exposure in a powerful electrical circuit was sufficient to give permanent polarity to steel, it appeared equally obvious, that needles placed transversely to a wire at the time that the electricity of a common Leyden battery was discharged through it, ought to become magnetic; and this he found was actually the fact, and according to precisely the same laws as in the Voltaic circuit; the needle under the wire, the positive conductor being on the right hand, offering its north pole to the face of the operator, and the needle above, exhibiting the opposite polarity.