A Brief History of Semiconductors – Electricity and Magnetism – Discovery of Electromagnetic Waves

“Maxwell’s electromagnetic theory governs all macroscopic electromagnetic phenomena including static electricity, steady magnetic field, electromagnetic induction, circuits, electromagnetic waves, etc., and unifies optics within this theoretical framework. The greatest achievement of physics before its advent.

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Maxwell’s electromagnetic theory governs all macroscopic electromagnetic phenomena including static electricity, steady magnetic field, electromagnetic induction, circuits, electromagnetic waves, etc., and unifies optics within this theoretical framework. The greatest achievement of physics before its advent.

But this theory, even long after Maxwell’s death, has not been experimentally proven or widely accepted. At that time, the most influential was the electromagnetic theory of Weber and Neumann. Because both of them are located on the European continent, this theory is also known as continental electrodynamics. Of course, their theories have not been experimentally proved.

Maxwell’s theory, based on the Faraday field, differs from all other schools’ explanations of electrical and magnetic phenomena. Maxwell’s description of the electromagnetic field was too complicated, and too complicated in its initial form. Even in the UK, Maxwell’s theory has not received much attention, and the vast majority of physicists express confusion about the theory.

There are also a small number of people who are willing to believe everything about Maxwell. Among them, in addition to the British Heaviside who simplified Maxwell’s equations, there were also Irish George FitzGerald and British Oliver Lodge. One thing in common between these three and Maxwell is that they are all from the British Isles.

Fitzgerald and Lodge have been loyal supporters of the theory since Maxwell published The General Theory of Electromagnetism in 1873. In 1878, the two decided to cooperate after meeting in Dublin to verify Maxwell’s electromagnetic theory from both theoretical and experimental aspects.[40]. They frequently exchanged each other’s discoveries through letters and forged a connection with Heaviside.

Fitzgerald believed that discharging a capacitor in a closed circuit would induce rapid current oscillations that could generate electromagnetic waves with wavelengths in the centimeter or meter range. He even calculated exactly what wavelengths could be produced using this method. Fitzgerald conducted many experiments, maybe the experiments he conducted did generate electromagnetic waves, but he did not find a suitable electromagnetic wave detection method.

Rocky, who is more hands-on, almost became the first person to discover electromagnetic waves. In 1987, Lodge was invited to work on the optimization of lightning rods. At that time, the lightning rod was directly grounded with a copper rod with extremely low resistance. However, many facts have found that in many cases, this method has not achieved a good lightning protection effect. In many cases, lightning does not go through the copper rod this designated path, but other higher resistance lines.

Lodge postulates that lightning is not composed of continuous direct currents, but alternating currents. After a lot of experiments using the Leiden jar to simulate lightning, he believed that the copper rod with very low resistance has a large inductive reactance under the action of the periodic changing current, so the overall impedance is not the lowest. This discovery greatly optimizes the design of lightning rods.

Today, lightning rods are generally composed of two parts, one is the outer shell and the other is the central ground rod. Among them, there is a distance of several millimeters between the shell and the center ground rod, forming a coupling capacitor. When the lightning strikes the lightning rod, the DC part is grounded and discharged through the casing, and the AC part is grounded and discharged through the coupling capacitor, and the lightning current is immediately discharged to the ground.

In 1888, Lodge conducted a series of experiments, one of which almost rewrote the history of electromagnetism. Rocky used two Leiden jars and placed a pair of spark gaps on top of them when experimenting with lightning rod optimization. When the Leyden jar discharges, an arc of light will appear in the spark gap, which Lodge uses to simulate lightning. In one experiment, Lodge used two 29-meter-long wires to connect the Leiden flask, and between the two wires, several spark gaps B1, B2, and B3 were placed, as shown in Figure 1‑20.

Figure 1‑21 Rocky’s electromagnetic wave experiment

When the Leiden bottle is discharged, an arc will appear in the spark gap A, and then Rocky is surprised to find that B1, B2 and B3 also have arcs correspondingly, and the arc at the end of B3 is the longest. Rocky understands that this is because the oscillation generated by the oscillation at the gap A propagates along the wire and is reflected at the end B3, where the incident wave and the reflected wave are in phase, and the voltage generated is twice that of point A. .

In addition, Lodge demonstrated the presence of standing waves along the wires. In a dimly lit room, he observed a visible glow on the wires. In the summer of 1888, he also conducted a series of experiments until he believed that he had emitted and received the electromagnetic waves that Maxwell had predicted 24 years earlier.

Rocky then went on vacation to the Alps, and he was ready to report the surprising result when he returned. But when he was riding a train, he accidentally discovered an article called “Electromagnetic Waves in the Air and Their Reflections” in the July 1888 “Almanac of Physics”, signed by a German researcher who was not known at the time. , Heinrich Hertz.

Hertz had a very famous teacher, Hermann von Helmholtz. Helmholtz was one of the founders of the law of conservation of energy, and he was the one who could understand Maxwell’s theory of electromagnetism at that time. At that time, the continental electrodynamics represented by Weber and Neumann was more mainstream.

However, Helmholtz discovered that one of the core concepts of continental electrodynamics, “Weber force”, does not obey the law of conservation of energy created by him; some subsequent experiments proved that Neumann’s potential theory also has no small problems. At this time, he had no better choice, and only Maxwell, who was not optimistic, was left in front of him.

In his own way, Helmholtz re-described and popularized Maxwell’s theory, leaving a crucial step to verify it experimentally. In the winter of 1879, the German Academy of Sciences in Berlin, on the initiative of Helmholtz, issued a scientific competition award to seek experimental proof of Maxwell’s theory of electromagnetic fields.

Helmholtz encouraged his student Hertz to experiment in this area. Between 1886 and 1888, Hertz tested Maxwell’s theory through a series of experiments. This experiment is known as the Hertzian experiment. The electromagnetic wave transmitting and receiving device used in the Hertz experiment is shown in Figure 1-22.

Figure 1-22 The electromagnetic wave transmitting and receiving device used in the 22 Hz experiment

Before carrying out this experiment, Hertz first started by observing the propagation of electromagnetic action in the wire, verified the existence of displacement current through a large number of experiments, and then observed that the “wire wave” propagated at a limited speed. In November 1887 and January 1888, Hertz published two articles, “On the Electromagnetic Effects Produced by Electrical Disturbances in Insulators” and “On the Finite Speed ​​of the Propagation of Electromagnetic Actions”, expounding two findings.[42].

At this time, Hertz had almost discovered electromagnetic waves, and he was ready to further verify the propagation of electromagnetic effects in the air. This is the most important stage of Hertz’s discovery of electromagnetic waves, and it is also the most difficult moment in his “discovery of electromagnetic waves”.

In the process of verifying the displacement current and wire wave, Hertz has become very good at how to adjust the capacitance and inductance, and how to control the arc generated at the spark gap S.[42]. This device is the left half of Figure 1‑22, which is equivalent to an electromagnetic wave generator.

In the experiment, Hertz connected the two ends of the induction coil with two copper rods. When the current of the induction coil is suddenly interrupted, the high voltage generated by the induction will cause the spark gap S to spark, and then the electric charge will pass through the spark gap in the zinc plate. Oscillation between C. According to Maxwell’s theory, this spark will generate electromagnetic waves.

In the Hertz experiment, the most difficult thing is how to design the receiving device, that is, the detector, to detect the electromagnetic waves propagating in the air. Detector in English is Detector, a device whose main function is to obtain useful information from radio signals, and is the most important part of a radio receiving system.

Constrained by the times, Hertz could only design a very crude detector. He bent a small piece of wire into a circle, leaving a small spark gap at each end of the wire. If electromagnetic waves can travel from the air to this small coil, an induced voltage will be generated and an arc will be created at the spark gap M.

The experiment was extremely difficult. Hertz’s transmitter does produce electromagnetic waves, but the detectors used in this experiment were too crude. Hertz spent a lot of time adjusting the transmitter and the detector. The specific work was a series of trivial tasks such as drilling holes, winding coils, adjusting the size of the capacitor and the distance of the spark gap, and adjusting the position of the detector. Until one day, Hertz discovered sparks in spark gap M in a dim laboratory.

In March 1888, Hertz shared this result with Helmholtz, and on the 31st of the same month, copied the article “Electroelectric Waves in Air and Their Reflections” to the Annals of Physics. This is the first time in human history that the transmission of electromagnetic waves in the air has been observed.

Hertz used experimental data to calculate the speed of electromagnetic waves in the air. Although there is a large error between this propagation speed and the speed of light, it is at least in an order of magnitude.

The Englishman Rocky passed by this great honor, but his attention was not on the fact that Hertz took the lead. Two other scientists from the British Isles, Fitzgerald and Heaviside, also became interested in the Hertz experiment. In the following time, the four communicated frequently, increasing Hertz’s understanding of electromagnetic waves.

In December 1888, Hertz published “On Electromagnetic Radiation”[42], discusses the research methods of the polarization, reflection and refraction phenomena of electromagnetic waves, and provides the experimental results. At the end of the article, Hertz is very confident that electromagnetic waves have the same properties as light. These properties can be deduced both from an optical perspective and from an electromagnetic perspective.

So far, Maxwell’s theoretical deduction of the identity of light and electromagnetic waves has obtained sufficient experimental basis. After the Hertz experiment, only time could stop the advent of wireless communication. With the combined efforts of scientists, entrepreneurs, engineers and countless ordinary people, this time has been shortened infinitely.

Different people, with different goals, have entered this blue ocean opened up by Faraday, Maxwell, Hertz and others. The huge energy contained in this blue ocean has distorted this history, so that many participants can only have the honor behind them.

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