Electromagnetic Pulse Generation

The following illustration is from the textbook "Fundamentals of Physical Optics" by Jenkins and White first published in 1937. In part (a) of the illustration below, lines of force are drawn about a small electric charge at rest at the point A. Lines of force are used to represent the electric and magnetic fields such that a tangent to the line of force at any point represents the direction of force on a small test charge placed at that point.

For an electric charge of part at reset the force lines are straight, diverge in every direction, and are uniformly distributed in space as shown in part (a). The same force lines apply if the charge is moving uniformly and non-relativistically in the direction A to B. For these two cases: charge at rest, and charge in uniform motion, there is no radiation of an electromagnetic wave.

In order to produce electromagnetic radiation, charge must be accelerated. This is illustrated in part (b). The charge initially at rest at point A is accelerated in the direction AB to the point B, once the charge has reached point C the charge moves with uniform velocity. The lines of force at C must be different then the origional force lines. However, they can only differ out to the arc RR' after which the two lines of force must be same. This because an electromagnetic disturbance can not be propagated faster than the speed of light, c. At the point C the velocity of the charge is uniform, thus the force lines must be straight up to the arc QQ'. Additionally, the force lines must be straight beyond the arc RR', because the charge was initially at rest. We see that there is a kink in the lines of force between the two arcs QQ' and RR'. The exact form of the kink will depend on the nature of the acceleration and is largest in directions perpendicular to the direction of acceleration.

Part (c) is one particular line of force from part (b). At any point along this line, the tangent determines the direction of the electric field at that point. A tangent is drawn at point P and can be resolved into two components, E₀ and E_t. E₀ is the electric field before acceleration and E_t is the emitted field. If we find E_t for various parts along the kink and place them side by side we obtain the pulse shown in part (d). From this simple analysis we see that electromagnetic radiation is always emitted when charge is accelerated.

While Maxwell's equations predicted that Electromagnetic waves are generated when charge was accelerated, it was up to Heinrich Hertz to verify experimentally that this was true. Below is an illustration of his experiments. Two brass plates were connected to a spark gap, SG, and sparks were caused to jump across the gap by charging the plates to high voltage with an induction coil. The spark discharge of the plates is oscillatory.

Each time the potential difference between the gap reaches the dielectric breakdown of air, charge flows between the knobs. At this point the signs of the charges on the two plates reverses and causes a return surge, another reversal of sign followed by another surge in the opposite direction and so forth until all the energy has dissipated by the resistance of the gap. Charge is being accelerated and therefore should be radiating electromagnetic waves.

The detector consisted of a resonating circuit constructed by a circular wire broken by a very narrow spark gap of adjustable length. The changing magnetic field in the wave induces an alternating e.m.f. in the circular wire. This causes sparks to jump the gap. Using this device Hertz was able to show that electromagnetic waves traveled at the speed of light, that they were plane polarized, they could be reflected and focused, and they could be refracted by a prism. He showed that they behaved as visible light waves, thus showing experimentally the validity of Maxwell's equations across the whole electromagnetic spectrum.