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Could somebody check whether my understanding of this experiment is correct, because I feel like I assume Maxwell's results.

When a spark forms across gap, a spark is also created across the receiving loop. Since the current is ac, an electric field is produced that oscillates with the current, which causes the electric field across the gap to travel like a wave. enter image description here

The spark across the receiving loop is explained by a changing magnetic field perpendicular to the electric field(*), then by Faraday's law we have an emf in the receiving loop and the spark forms. It can be verified the magnetic field is perpendicular by rotating the loop, no sparks form when it is perpendicular to the transmitter gap.

Is (*) too generous? I feel like this could be explained without the magnetic field, that when the electric field is at a maximum, the spark forms in the receiver.


2 Answers 2


"I feel like this could be explained without the magnetic field [...]" The receiving loop can be rotated in its own plane through any angle, so the spark-gap can be at various orientations: it is found that sparks always occur with roughly the same intensity. This confirms the induction of an emf all round the loop by the magnetic component of the e-m wave (at right angles to the plane of the loop), rather than by the electric component of the wave. To detect the latter, a metal rod with spark gap is needed; the strongest sparks are when the rod is parallel to the transmitter's spark gap.


I ask the questioner to assume the followings (for which a basic theory may be found in a book entitled “Planets and electromagnetic waves”). The followings are based on mathematical interpretations for solutions of Maxwell equations. (1) If current exceeds some limit (depending on source of production), then magnetic field waves are produced. These waves are called in general radio waves. (2) If voltage exceeds some limit (depending on source of production), then electric field waves are produced. These waves are called in general light waves. (3) If current exceeds the corresponding limit and the voltage also exceeds the corresponding limit then both magnetic field waves and electric field waves exist separately; not in combined form (that is, not as everyone gives a figure for em waves with two component waves). A peculiar situation for (3) is the Hertz’s experiment. I refer to your first figure. The place for spark gap has enough current as well as voltage to produce light rays (electric field waves) as well as magnetic field waves (radio waves). Magnetic field waves produced in spark gap reaches receiving loop and produce emf in receiving loop. The light rays produce do not do any activity. That is in the place “electromagnetic waves”, there are light rays and radio waves, which independently propagate. Radio waves act in producing emf and light rays do not act.


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