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, and the invisible ultraviolet waves which are so active in inducing chemical action.
Oscillation of Electric Charges within the Atom.—We have seen how these waves might be produced by the oscillation of two electrified rods, and it is supposed that the light coming from luminous bodies is produced in a similar way. There are many reasons for believing that there exist in the atoms of all substances, minute negatively electrified particles which may rotate in small orbits or oscillate to and fro within the atom. There also exists an equal positive charge within the atom. As the negative particles rotate or oscillate in the atom, it is evident that the field between them and the positively electrified part of the atom alternates, and so electro-magnetic waves are sent out.
CHAPTER II
GRAPHIC REPRESENTATION OF WAVES
A system of ripples on the surface of water appears in vertical section at any instant somewhat as in Fig. 10. The dotted line AB represents the undisturbed surface of the wafer, and the solid line the actual surface. If the disturbance which is causing the ripples is an oscillation of perfectly regular period the individual ripples will be all alike, except they will get shallower as they become more remote from the disturbance.
FIG. 10.