earth, the induction set up by the electrified wire being a perfect imitation of that which exists on the globe. The opposite effect to that already described--viz., the rotation of one pole of a magnet round the electrified wire, was also arranged by Faraday in the following manner. (Fig. 205.) [Illustration: Fig. 205. N S. A little magnet floating in mercury contained in the glass A A; the north pole is allowed to float above the surface of the quicksilver, and the south pole is attached to the wire passing through the bottom of the glass vessel. The electricity passes in at B, and taking the course indicated by the arrows travels through the glass of quicksilver to the other pole of the battery at C. Directly contact is made with the battery, the little magnet rotates round the electrified wire, W. The dotted line shows the level of the mercury in glass.] In the examination of the magnetic phenomena obtained from wires transmitting a current of electricity, it should be borne in mind that any conducting medium which forms part of a closed circuit--_i.e._, any conductor, such as charcoal, saline fluids, acidulated water, which form a link in the endless chain required for the path of the electricity,--will cause a magnetic needle placed near it to deviate from its natural position. These positions of the electrified wire and the magnetic needle are of course almost unlimited, and in order to assist the memory with respect to the fixed laws that govern these relative movements, Monsieur Ampère has suggested a most useful mechanical aid, and he says:--"Let the observer regard himself as the conductor, and suppose a positive electric current to pass from his head towards his feet, in a direction parallel to a magnet; then its north pole in front of him will move to his right side, and its south pole to his left. "The plane in which the magnet moves is always parallel to the plane in which the observer supposes himself to be placed. If the plane of his [Page 215] chest is horizontal, the plane of the magnet's motion will be horizontal, but if he lie on either side of the horizontally-suspended magnet, his face being towards it, the plane of his chest will be vertical, and the magnet will tend to move in a vertical plane." This very lucid comparison will be seen to apply perfectly to the direction of the rotations in Figs. 203 and 205.