+Electric Oscillations.+--If the position of the two rods is reversed, then of course the direction of the field at a point between them is reversed, and if this reversal is repeated rapidly, we shall have the direction of the field alternating rapidly. If these alternations become sufficiently rapid they are conveyed outwards in much the same way as the oscillations of position are conveyed in an ordinary ripple. Thus suppose the two rods are suddenly placed in the position in the diagram. The field is not established instantaneously, the lines of force taking a short time to establish themselves in their ultimate positions. During this time the lines of force will be travelling outwards to A in the direction of the dotted arrow. {20} Before they reach A let us suppose that the position of the rods is reversed. Then the direction of the lines is reversed and these reversed lines will travel outwards towards A, following in the track of the original lines. Thus a continuous procession of lines of force, first in one direction and then in the opposite direction, will be moving out perpendicular to themselves in the direction of the dotted arrow. This constitutes an electric wave. +Magnetic Oscillation, Lines of Force, and Field+.--Almost exactly the same kind of description applies to a magnetic wave. The space near to the North and South poles of a magnet is modified in somewhat the same way as that between the electrified rods, and the magnetic lines of force are the lines along which a small North magnetic pole would move. We may imagine a rapid alternation of the magnetic field by the rapid reversal of the positions of the North and South poles, and we may imagine the transmission of the alternations by means of the procession of magnetic lines of force. +Changes in Magnetic Field.+--But experiment shows that whenever the magnetic field at any place is changing an electric field is produced during the alteration, and _vice-versa_. Electric and magnetic waves must therefore always accompany one another, and the two sets of waves together constitute electro-magnetic waves. These are the waves which a huge amount of experimental evidence leads us to believe constitute heat, light, the electric waves used in wireless telegraphy, {21} and the invisible ultraviolet waves which are so active in inducing chemical action.