PROPAGATION OF ELECTROMAGNETIC WAVES e passage of an electric current through a conductor always generates a magnetic eld. You can check this by placing a small compass needle close to a conductor carrying a direct current (dc) with its axis of rotation parallel to the conductor. When you switch on the current, the needle deects, to lie on an imaginary circle drawn round the conductor. If you place a second compass needle, similarly aligned, somewhat farther away, it will also deect, apparently simultaneously. However, the deection isn’t quite simultaneous: the second needle moves a fraction of a second after the rst. is delay occurs because the magnetic eld doesn’t in fact appear instantaneously throughout space: it propagates outward from the source at approximately 300,000,000 m s-1 (186,000 miles  s-1) (Figure 3.1).