I am asking two questions to improve my understanding of electromagnetic energy transported into free space from current in a conductor. At one time I thought all such energy was considered electromagnetic radiation (radio waves) but I think this is wrong. When Hertz did his original experiments in the 1880s that proved the existence of waves predicted by Maxwell in 1865, he used a spark-gap transmitter and a remote circular receiver.

First question -- How did Prof. Hertz know that the sparks appearing in his remote receiver were not caused (1) by induction (as described by Faraday) or (2) by what today is called "near-field radiation?"

Second question -- if we repeated Hertz's experiment with the circular receiver, but we kept moving the receiver farther away from the spark transmitter, what behavior would we expect to see in the receiver?
Let's say we start with the receiver physically adjacent to the transmitter and then keep moving it away in 1-foot increments. What response would we see in the receiver at each increment? Regarding terminology, is it scientifically accurate to describe the energy emitted by the transmitter as EM radiation, regardless of the distance from the transmitter? Thank you kindly for considering this request for clarification.

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    \$\begingroup\$ Sounds like homework \$\endgroup\$ – Passerby Oct 3 '17 at 2:27
  • \$\begingroup\$ I am afraid this is an example of "Problem-based learning", a constructionist method which allows students to learn about a subject by exposing them to multiple problems and asking them to construct their understanding of the subject through these problems. \$\endgroup\$ – Ale..chenski Oct 3 '17 at 2:37
  • \$\begingroup\$ Definition of electromagnetic energy: a form of energy that is reflected or emitted from objects in the form of electrical and magnetic waves that can travel through space. \$\endgroup\$ – Bruce Abbott Oct 3 '17 at 2:56
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    \$\begingroup\$ Come on, try and think about these questions yourself instead of expecting someone to dump the answer on a plate for you. \$\endgroup\$ – Finbarr Oct 3 '17 at 2:59

How did Prof. Hertz know that the sparks appearing in his remote receiver were not caused (1) by induction (as described by Faraday) or (2) by what today is called "near-field radiation?"

The spark at the receiver in response to a transmission from a spark gap transmitter was only the initial observation that inspired Hertz to do more conclusive experiments.

Hertz's experiments in 1886 to 1889 were explicitly an attempt to verify Maxwell's theories, introduced a decade earlier. Therefore he had an idea of the phenomena he was attempting to observe, and he knew the likely velocity of the waves, measured by Foucault in 1862.

Per Wikipedia

In the apparatus Hertz used, the electric and magnetic fields would radiate away from the wires as transverse waves. Hertz had positioned the oscillator about 12 meters from a zinc reflecting plate to produce standing waves. Each wave was about 4 meters long. Using the ring detector, he recorded how the wave's magnitude and component direction varied. Hertz measured Maxwell's waves and demonstrated that the velocity of these waves was equal to the velocity of light. The electric field intensity, polarity and reflection of the waves were also measured by Hertz. These experiments established that light and these waves were both a form of electromagnetic radiation obeying the Maxwell equations.

The main feature that distinguishes these waves from near-field or purely inductive effects, is the observation of standing waves from the transmitter placed in front of a reflector.


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