Draft:Hertz’s experiments on electromagnetic wave

A series of well-known experiments were conducted by Heinrich Hertz starting from 1887, which generated and detected the electromagnetic waves, and examined wave properties of EM waves such as interference and reflection. The experiments gave clear demonstration of the wave properties of the electromagnetic waves which was predicted by Maxwell in his theory of electricity and magnetism. Hertz's experiments were considered the final proof of Maxwell's theory, which unified electromagnetic waves and light.

Background
Faraday first proposed the concept of "lines of force". Following Faraday's concept, Maxwell published a series of papers under the title of On Physical Lines of Force. One of them derived the speed of light c from the expression of the velocity of the electromagnetic wave in relation to the vacuum constants. In 1865, Maxwell published A Dynamical Theory of the Electromagnetic Field, with Maxwell's equations in its final form. Maxwell wrote A Treatise on Electricity and Magnetism in 1873, which gave his full theory. Maxwell's theory predicted that the electromagnetic waves are the same as light waves. They both propagate in the same finite speed. The theory also gave the same wave equation for electromagnetic wave propagation and light. Maxwell died in 1879 before any direct evidence for his predictions was produced.

Physicists actively worked on the generation of electric magnetic waves from an electric source. Among them, Oliver Lodge and G. F. FitzGerald were well known. However, these experiments were not successful.

Another key question at that time was what is the carrying medium for such electromagnetic waves. From the Newtonian view, the vacuum is an empty space. Forces, such as gravitational force, should be able to act-at-a-distance instantly. Maxwell believed that the electromagnetic waves propagates through a medium pre-filling the space called ether. The electric force thus also was transmitted through this ether medium. Physicist at that time were interested to test if the speed of the electric force is finite or infinite; and whether there was an ether medium in space.

Maxwell's work on electromagnetism was translated by Helmholtz to German. Hertz was a student of Helmholtz, and he was well familiar with the Maxwell's EM theory. Starting from 1887, Hertz experimented various setups for finding relationships of electromagnetic forces and dielectric polarizations. He also aimed to determine if the electric force propagate with a finite or infinite speed. It is interesting that Hertz's discovery of the electromagnetic waves propagating in space was a serendipity.

Hertz's experimental setup
In his experiment clearly demonstrating the generation and detection of electromagnetic waves, Hertz used a resistor-inductor-capacitor circuit and connected it to a loop of wire with a gap to generate the EM waves. The capacitors Hertz used were two This circuit was called the primary circuit. When the capacitors were charged up, high voltages induced in the gap produced sparks that were visible to naked eyes. This is clear evidence of the generation of the EM waves. The receiving device was another loop of wire with a gap placed across the room (as far as 12 meters away) so that it was away from the source circuit. The gaps can be adjusted. Hertz called the receiving device "resonator". After the primary circuit was charged, the sparks were also seen in the gap of the detecting device. It showed that the EM waves generated can travel in the air across the room to reach the detecting device. The electromagnetic wave generated by the experiment was called the "Hertzian wave", after Hertz. Later, people realized the EM waves generated in Hertz experiments was actually the radio waves. Hertz also conducted experiments on the reflection, refraction, and interference patterns of electromagnetic waves, confirming their wave characteristics.

At that time, there were debates if the propagation of electric force is instant or with a finite speed. Thus, Hertz worked on determining the speed of EM waves is finite or not. Hertz determined the wavelengths from the interference patterns, and by estimating their frequencies, he calculated the propagation speed of EM waves. The experiment clearly showed the finite propagation speed of EM force. Furthermore, by comparing the speed of the EM wave and that of light, he supported Maxwell's theory on the unification of electromagnetic waves and light.

The Maxwell equations presented in the volume of A Treatise on Electricity and Magnetism was hard to understand. It was not widely read. It was believed that Hertz experiments on generation and detection of the electromagnetic waves greatly contributed to the acceptance of the Maxwell's theory on electromagnetism.

The publication of of Hertz's research work on electromagnetism were collected and translated to English into three volumes: Electric Waves, Miscellaneous Papers and The Principle of Mechanics.

Application
Hertz's experiment was not aimed to generate applications for practical use. However, his experiments in generating and detecting EM waves (radio waves) became the basis for future development of the radio technology.