The story takes place in 1826. A veteran named Sturgeon studied the coils wrapped around the iron rods and found that the iron rod after being energized is magnetic and can attract metallic iron objects. This is the electromagnet that the public is familiar with.

• How Does a Tesla Coil Work? A Historical Deep Dive[1]

01 Tesla Coil

  You may have seen the surprising discharge display of Tesla coils. How exactly does it work? Let’s hear Kathy tell us how each part of the Tesla coil was invented and what it works.

1. induced current

  The story took place in 1826. A veteran named Sturgeon studied the coil wrapped around the iron rod and found that the iron rod after powering on is magnetic and can attract metal iron objects. This is the electromagnet that is familiar to the public.

▲ Figure 1.1 Electromagnetic coils and electromagnets

  About six years later, in 1831, Faraday planned to try to use magnetic fields to generate current, but he was not so lucky. At that time, he did not have the available strong magnets , so he decided to use electromagnets to experiment. He wrapped two independent coils around both sides of a ring. It is found that when the first coil is turned on or off, a current pulse will appear on the second coil. Faraday uses the concept of a magnetic field to explain this phenomenon. When the magnetic field inside the coil changes, an electric current will be induced in the coil. But what made Faraday feel mournful was that he never could feel the voltage on the second coil. At that time, there were not many ways for people to measure voltage, only a few ways.

▲ Figure 1.2 Faraday electromagnetic induction coil

2. boost transformer

  A few years later, a pastor from Ireland , an amateur scientist named Nicholas Karen, who improved the Faraday device and could obtain stronger voltages. He wound the two coils on the same electric rod, separated by an insulating layer. He was surprised to find that when the primary coil was disconnected from the battery, a strong electric shock could be felt on the secondary coil. When the first coil is thin, that is, the number of turns of the coil is relatively small, the second coil is thicker, and the corresponding number of turns is relatively large, you can feel that the voltage output by the second coil is much stronger. Karen didn't know that he had invented a boost transformer.

▲ Figure 1.3 Karen's step-up transformer

  When the battery is connected to the original side, a magnetic field is generated and the core is turned into an electromagnet. When the battery is disconnected, the core loses its magnetism. Therefore, according to Faraday's new electromagnetic induction theory, whenever the battery is connected to the coil or is disconnected, an induced current will be generated on the secondary side. When the secondary coil is less than the primary coil, the induced voltage is smaller, but the current is generated. This also explains why Faraday can measure electrical current but does not feel the voltage stimulation on the coil. When the coils on the secondary side increase, a larger voltage can be generated. The stronger the voltage stimulus people feel, but the output voltage becomes relatively smaller.

3. High voltage pulse

 Karen also invented a gear that can continuously complete the battery entering the coil and disconnecting, which is called a "repeater", so that the induced voltage can be continuously generated. He said this is the best equipment that people have built so far to generate high pressure. Karen sent his experiment results to his friend Sturgeon, who also made the same equipment and improved Karen's plan. Later, many people sold the equipment as tormenting equipment, thus making a fortune.

▲ Figure 1.4 Transformer made by Sturgeon

 Incredible that induction coils are becoming more and more popular and are also used in medical fields. Many diseases can be treated by electric shock, including mucositis, honey disease, asthma, , cold, flu, headache, neuralgia, , rheumatism, ear pain, toothache, etc.

  So people began to compete to make devices that can output larger and more stable and continuous electric shocks or sparks.One of the most important inventions is to design a device for automatically turning on and off the coil, replacing the original gears and handles. When the primary coil is energized, the core is also magnetized, which attracts the armature on the spring to swing and disconnects the current. At this time, the coil is powered off and the core loses its magnetic properties. The armature is released and the line is reconnected under the action of the spring. This can create an on-off of 20 to 40 Hz, which was called an electromagnetic flux breaker at that time.

▲ Figure 1.5 Automatic electric spark equipment

IV, resonant loop

  Then, when the current is on and off, a large amount of sparks will be generated during electric shock. By 1853, a French physicist named Ann Armand Fitzha added Leiden bottle at both ends of the electromagnetic contacts, which was equivalent to adding capacitor . The Leiden bottle is actually wrapped in metal film inside and outside the glass bottle, and is also the earliest capacitor. The capacitance capacity of the Leiden bottle depends on the area of ​​the metal film and the thickness of the glass bottle wall. Since joining the Leiden bottle, Fitzha eliminated the spark of electromagnetic contacts, but also created a innovative device that can convert DC from the battery into AC . What is the principle?

▲ Figure 1.6 Leiden Bottle - High Voltage Capacitor

  Battery link Leiden Bottle stores opposite charges inside and outside. If the coil is shorted, the Leiden Bottle will discharge and generate current pulses. Then, if the short wire is connected to the coil, the current pulse passes through the coil, causing the magnetic field in the coil to change. The changing magnetic field will induce a new current in the coil, which is the self-induction phenomenon of the coil . Therefore, when the Leiden bottle is discharged in the coil, the discharge current will continue, and even the charge in the Leiden bottle will continue after it is discharged, which will produce the opposite charging charge in the Leiden bottle. So the Leiden bottle starts discharge in the opposite direction again. If there is no resistance loss in the discharge circuit, the discharge current will flow back and forth repeatedly. If there is resistance loss in the discharge circuit, the discharge current will decrease in each cycle until the electrical energy in the system is completely released and no current flows. The capacitor is connected to the coil, which is called a resonant loop, and is still used for oscillating currents.

▲ Figure 1.7 The resonant loop composed of Leiden bottle and coil

  The oscillation frequency depends on the capacitance in the resonant loop and the size of inductance . If you want to change the oscillation frequency, you can change the coil size and capacitance shape. Using this method, in the 1850s, people could convert the DC power of a battery into an AC pulse signal of several megahertz, covering the radio wave frequency range. Several years later, scientists introduced the differential equation to describe the working mechanism of the circuit. By 1886, German scientist Heinrich Hertz used an induction coil to observe radio waves for the first time. Then, Hz did not recognize the importance of his discovery, but only believed that it proved the correctness of the electromagnetic wave theory.

▲ Figure 1.8 LC resonant voltage waveform

V. Tesla coil

 Now let’s talk about Nikola Tesla . In the summer of 1889, Tesla came to the World Expo in Paris and heard about the magical experiments about electromagnetic wave . So he started to do the corresponding experiment and made the induction coil, but he made new innovations, removed the original electromagnetic flux breaker, and used an alternator, to generate an alternating current-driven primary coil. This is a very substantial improvement, no longer relying on mechanical devices to generate alternating currents, but instead using alternating generators directly to provide alternating power. For this purpose, Tesla also invented an alternator, a three-phase alternator. These devices have a lot of power, causing the coil temperature to rise, burning out the insulating layer of the coil, so he designed a hollow coil.

▲ Figure 1.9 Tesla and the three-phase alternator

  Later he found that the Leiden bottle in the primary level was sometimes good and sometimes bad, so he moved the Leiden bottle to the high-voltage secondary coil and could adjust the capacitance.By adjusting the secondary capacitance size, the secondary resonance frequency can also be changed to make it an integer multiple of the primary AC frequency. With this method, Tesla generates high-frequency and high-voltage currents. Using secondary capacitors, a tunable high-frequency electromagnetic wave is generated. At that time, Tesla was not interested in radio telegraph and wireless broadcasting, and he was obsessed with radio energy transmission lighting applications. To this end, he added an unexpected resonant circuit that uses a single wire to light the light bulb. He found that for neon lights, and fluorescent lights, there is no need for any wire connection, and it can emit light as long as it is close to the high-voltage coil. Later, people discovered that adding huge metal rings to the top of the high-voltage coil can produce a very beautiful discharge arc, which is also the most common display experiment for Tesla coils today.

▲ Figure 1.10 Patent image of Tesla's high voltage lighting neon bubble and fluorescent lamp

  Let us understand the working mechanism of modern Tesla coils and the charming discharge demonstration it can create. The boost transformer obtains higher AC voltage by increasing the secondary coil. The coil and the capacitor form a resonant loop. In the Tesla coil, the AC voltage is first increased by the boost transformer, and the corresponding output current is reduced. This is the same as Karen's transformer's effect back then. The resonant capacitor is discharged using the discharge gap, so that the resonant circuit generates a high frequency voltage. This high-frequency high-voltage alternating current is input to the primary side of another transformer, generating higher AC voltage, which can generate millions of volts of high-voltage alternating current in the secondary, and charge the metal ring body. Because the voltage on the top ring is very high, air ionization is caused, resulting in huge discharge.

▲ Figure 1.11 Circuit schematic diagram of contemporary Tesla coil

 Tesla was conquered by this discharge phenomenon. He even imagined building a huge discharge tower that fully charges the entire earth and illuminates the atmosphere. In reality, this is not feasible, and he even lobbies J.P Morgan to support him. During the same period, an Italian young man's idea was more compromised and showed that his name was Gullermo Marconi, who envisioned the implementation of sending wireless telegrams across the Atlantic , and later he finally succeeded using a Tesla coil.

▲ Figure 1.12 Guglielmo Marconi

Reference

[1]

How Does a Tesla Coil Work? A Historical Deep Dive: https://www.youtube.com/watch?v=IN9jb3fzZd0&t=96s