_I._--Am I to understand that, if this room were perfectly dark, you could tell whether it contained a magnet, without being informed of the fact? _Medium._--I should know of its presence on entering the room. _I._--How? _Medium._--I should be rendered instantly ill. _I._--How do you feel to-day? _Medium._--Particularly well; I have not been so well for months. _I._--Then, may I ask you whether there is, at the present moment, a magnet in my possession? The young lady looked at me, blushed, and stammered, ‘No; I am not _en rapport_ with you.’ _I sat at her right hand, and a left-hand pocket, within six inches of her person, contained a magnet._” Tyndall adds, “Our host here deprecated discussion as it ‘exhausted the medium.’” CHAPTER VII THE PRODUCTION OF MAGNETISM BY ELECTRICITY [Illustration: FIG. 15.--Diagram to illustrate Magnetic effect of an Electric Current.] In the previous chapter attention was drawn to the fact that there are many close parallels between electric and magnetic phenomena, and in this chapter it will be shown that magnetism can be produced by electricity. In the year 1819 Professor Oersted, of the University of Copenhagen, discovered that a freely swinging magnetized needle, such as a compass needle, was deflected by a current of electricity flowing through a wire. In Fig. 15, A, a magnetic needle is shown at rest in its usual north and south direction, and over it is held a copper wire, also pointing north and south. A current of electricity is now sent through the wire, and the needle is at once deflected, Fig. 15, B. The direction of the current is indicated by an arrow, and the direction in which the needle has moved is shown by the two small arrows. If the direction of the current is reversed, the needle will be deflected in the opposite direction. From this experiment we see that the current has brought magnetic influences into play, or in other words has produced magnetism. If iron filings are brought near the wire while the current is flowing, they are at once attracted and cling to the wire, but as soon as the current is stopped they drop off. This shows us that the wire itself becomes a magnet during the passage of the current, and that it loses its magnetism when the current ceases to flow. [Illustration: FIG. 16.--Magnetic Field round wire conveying a Current.]