I was working on a video on the physics of the Faraday cage and it became clear that not only do you need to talk about electric fields to explain the cage but that Faraday had actually created the idea of electric fields for that very purpose in 1837. Now I had known that Faraday had created the idea of magnetic fields which he called “lines of magnetic force” in 1831, and I knew that Faraday had added lines of electric force before 1846 when he admitted to the wild idea that maybe the light was a vibration of these lines, but I didn’t know the details of his creation of electric fields.
The more I looked into it, the more meaningful it became. Faraday’s ideas of lines of force through space and vibrating lines of force in space, well, that is the backbone of all Physics today. Magnetic lines of force around magnets seem logical if you look at iron filings, but electric lines of force and electromagnetic waves? That took a truly unique brain. For that reason, I wanted to create a real deep dive into how and why Faraday did this, what we think he got right and wrong, and why these ideas were hated by almost everyone at the time. Just a little warning, most of my videos (and my new book: “The Lightning Tamers” which I have important news about at the end of the video), have about 80% personal stories and 20% science. This video, however, is the reverse (20% personal stories and 80% science), as I really want to get into the core of these ideas.
Table of Contents
I would like to begin on July 4, 1831. That was the momentous day when 39-year-old Michael Faraday quit his job attempting to manufacture high-quality lenses for the British government. He had worked on it for 10 years and felt that his only results were his own “nervous headaches and weakness.” We now think that quitting saved his life as Faraday was actually being poisoned by the heavy metals in the glass which caused severe health problems for Faraday for the rest of his life (glassmakers typically died in their 30s in the 1800s). Faraday didn’t know about all of that. Instead, Faraday was motivated to fulfill his lifelong desire to make contributions to science for science’s sake.
How Faraday Discovered Magneto-Electric Induction
8 weeks later, on August 29, 1831, Faraday started a new section in his laboratory notebook with the hopeful title of “Experiments on the Production of Electricity from Magnetism.” After all, Faraday’s friend André-Marie Ampère had demonstrated that current-carrying wire in the shape of a pancake or a coil acted like a bar magnet some 10 years earlier and it just made sense to Faraday that if electricity could make magnets, magnets could also make electricity. But how?
Faraday had experimented before but had no luck until he was inspired by a fellow Englishman named William Sturgeon who had found that an electrified coil wrapped around insulated iron made a far stronger electromagnet (or magnet made with electricity) than one wrapped around glass or air. Faraday thought that if iron makes a stronger magnet, maybe iron could help transfer electricity from one object to another.
For that reason, Faraday wrapped two separate coils of wire on a single iron ring with the hope that the magnetism from one coil would transfer to the iron to cause current to flow in the other coil. However, even though he put a very strong current into the first coil so that it was a strong electromagnet, the second coil had no current. Frustrated, Faraday disconnected the battery and an amazing thing happened: the compass needle next to the other coil twitched. Then, nothing. When he plugged in the battery again, the needle twitched again, in the other direction. Then, nothing again. Faraday repeated this experiment multiple times and found that he created a current in the second wire when the first one was charging up or discharging but never when it was flowing steadily—even when it had an incredibly large current. In other words, an electromagnet only creates a current in a separate coil when the magnetic strength is changing. If its strength is steady, nothing happens.
Now that he knew the trick for creating current, Faraday pulled a very strong magnet out of a coil of wire and pushed it in again. When the magnet was moving, the compass moved, too, meaning that a magnet created current without any battery needed. He also noted that not only would it only work while the magnet was moving inside the coil (or the coil was moving relative to the magnet), but if the magnet moved outside the coil, no electricity was created.
As electric induction was the term for moving charges without touching, Faraday called this magneto-electric induction.
The First Description of Magnetic Fields
Faraday then made a law of induction which he declared was, “very simple although rather difficult to express.” It was in order to describe magneto-electric induction that Faraday came up with the idea of magnetic fields. Now, for centuries people had noticed that if you sprinkle iron filings around a bar magnet, it creates patterns, and, after the discovery of electromagnets, it was found that current-carrying wire also made similar patterns. But, before 1831, no one thought to give these things names or had any use for them. However, Faraday decided that these were needed to describe induction. Faraday went even further and stated that these “magnetic lines of force” were always present around magnets and around current-carrying wires. The iron filings just made them visible. Moreover, Faraday decided that current is created (or induced) when the lines of force are broken or “cut” by a coil of wire.
Think about pushing a bar magnet into a coil of wire. As you push the magnet, imagine the magnetic field lines passing through the coil. Or if you turned on or off an electromagnet, imagine the magnetic lines appearing or disappearing and cutting through the neighboring coil. Faraday felt that it was these disturbances in the force (not to get too Star Wars about it) that created the current in the coil.
To recap, Faraday said that magnets and currents have magnetic lines of force/magnetic fields emanating from them or around them and you can see these lines if you sprinkle iron filings around them. If a coil of wire cuts through magnetic lines of force, a current is induced in the coil. Faraday even imagined that the N and S of a magnet will be attracted to each other because their magnetic lines of force combine and a N and a N or a S and a S will repel because their magnetic lines of force push against each other.
Scientists at the time had very little interest in explaining magnetic force with curved lines of invisible force, but as a method of understanding magneto-electric induction, it seemed an interesting trick. And the people were very interested in induction. Supposedly, Faraday’s discovery was so popular that the minister of finance and future prime minister named Willaim Gladstone dropped by the laboratory to see it firsthand. When Gladstone asked Faraday about its uses, Faraday replied, “I know not, but I wager that one day your government will tax it.”
[If this really happened then this comment is prescient as all non-solar generators use spinning electromagnets next to coils to generate electricity, and yes, the government does tax it].
Although Faraday could have easily profited from his new fame, Faraday was a quietly pious man whose religion, called the Glasites, later called Sandenians, believed that the accumulation of wealth was against the will of God. Faraday also took that to mean that he should reject the adornment of honors and insisted on being, “plain Michael Faraday to the last.”
Meanwhile, Faraday’s discovery of magneto-electric induction seemed to have opened the floodgates of the invention and Faraday published paper after paper after paper after paper on “Experimental Experiments in Electricity.”
How Faraday Discovered the Faraday Cage
In 1837, for his 11th paper on electricity in 6 years, Faraday decided to re-examine very old experiments with his new understanding of physics. In this, Faraday ended up creating the Faraday cage and the idea of electric fields, which he initially called “line[s] of inductive force.”
What initiated all these discoveries was when Faraday read a 50-year-old paper from Charles Augustin Coulomb, of Coulomb’s law fame who found that all charges on a conductive material will be, “only diffused over its surface, and does not penetrate into its interior parts.” Faraday then recreated the experiments and found, like those before him, that conducting containers would have all of their charges on the outside where the inside was protected from electric forces. Faraday even built a giant cage or cube that was large enough to “live in”. Faraday found that he couldn’t see any electric effects from the inside, “though all the time the outside of the cube was powerfully charged, and large sparks and brushes were darting off from every part of its outer surface.”
The First Description of Electric Fields & Dielectrics
But how did it work? Inspired by his “magnetic lines of force,” Faraday started imagining “inductive lines of force” emanating from charged objects and around changing magnetic lines of force. Like magnets, inductive lines of force (or electric lines of force or electric fields) combine to make opposite charges attract and push against each other to make like charges repel.
Unlike magnetic lines of force that only work on other magnets or select materials like iron, inductive lines of force (electric fields) work on all neutral objects because, as Faraday learned from Benjamin Franklin, neutral objects that we can see aren’t 100% neutral but instead have equal amounts of positive and negative charge. However, what happens in the neutral object in an electric field depends on what kind of object it is: conductor or insulator.
Conductors are the easiest to explain. See, conductors are conductors because they can easily conduct electricity. For that reason, the electric field lines can move the charges in the conductor until their charges balance out the external electric field. If there were any remaining electric lines of force left, the charges would be pushed to move until there weren’t. In fact, one definition of a perfect conductor is a material for whom any net charge will reside on the surface and the electric field inside is zero, even in the presence of an external electric field. This is why the Faraday cage protects you from outside fields, the charges in the metal move to cancel out external forces leaving you safe inside.
Insulators, on the other hand, have charges that are more stuck in place, which is why insulating objects usually cannot cancel out the electric fields inside them. However, just because they don’t cancel out the electric field doesn’t necessarily mean that they have no effect. In fact, Faraday initially thought (erroneously we now think) that all-electric forces, “never occurred except through the intermediate influence of the intervening matter.” What Faraday meant was that the molecules of air or the “ether” of the vacuum or whatever material the electric field was going through the line up electrically to transmit these lines of force. Faraday then experimented with different systems and found, for example, that parallel plate capacitors would change how much charge they would carry due to the properties of the insulating material between them. For this reason, he came up with a new term for insulators, he called them: “dielectrics, to express that substance through which the electric forces are acting” a term still in use today for insulators when they are used to alter the electric field
Faraday also realized that in an electric field dielectrics (i.e. insulators) can twist almost like the magnetic domains in a magnetic field, where how much dielectrics/insulators affect the electric field depends on the dielectric’s “specific inductive capacity” (another term Faraday created that is still in use). We currently think that dielectrics will bend the electric field depending on this capacity (also called the dielectric constant) but do not “propel” the electric field and are not needed for the electric field to travel in a vacuum. (Confusingly, many physicists use the terms insulators and dielectrics interchangeably as all insulators have some amount of specific inductive capacity, whereas many electrical engineers reserve the term “dielectric” for insulators with a large dielectric constant, and therefore use the expression, “all dielectrics are insulators but not all insulators are dielectrics.” It would probably be better to say, “all dielectrics are good insulators but not all insulators are good dielectrics.”)
Anyway, if this is all sounding a little strange and possibly unscientific, imagine how the scientists in the 1800s felt about it. They thought Faraday was bonkers. In 1855, the Royal Astronomer Sir George Airy, piped up with, “I declare, that I can hardly imagine anyone who practically and numerically knows… [physics to believe] anything so vague and varying as lines of force.” But the more Faraday debated and thought about it, the more that these lines of the force seemed to make sense to him. This brings us to how Faraday came up with the frankly crazy idea that maybe the light was an electromagnetic wave.
Short History of Polarization up to 1824
Now I should probably take a moment to talk about the polarization of light because it was very influential in Faraday’s paradigm-shifting idea of the electromagnetic nature of light. For many, many, years people knew about a strange crystal called an Icelandic spar that would split light not into various colors but into two identical images. However, if that crystal was used to look at light reflecting at a low angle, or glare, there was only one image through the crystal. This was then examined by a French engineer named Étienne-Louis Malus, who decided in 1811 that light has two of what he called “polarization,” where the glare only reflected one of those polarizations. Malus even came up with a theory about the relationship between the angle of polarized reflection and the index of refraction of the material but he was stymied by the poor quality of the glass available to him. (The equation was then found in 1815 by a man named Sir David Brewster, which is why this angle is called Brewster’s angle). Then, in 1828, a geologist named William Nicol used novel methods that he had devised to prepare thin sections of crystals and rocks to fashion two prisms of Icelandic spar together in such a way as to create a polarizing filter. Nicol’s “polarizing prism” would only have one image as it refracted one of the polarizations out the side of the prism. Therefore, if you look at glare through this prism, you could block out the light if you held it at the right angle and let all the glare through if you twisted it. It is kind of magical. By the way, polarizing sunglasses use this process to cut down glare.
Faraday experimentally discovers the relation between light & EM
That brings us to July of 1845 when a 21-year-old Irish student named William Thomson asked Faraday if he had ever studied if or how polarizing light changed when the light was going through a transparent dielectric.
Faraday was intrigued and therefore shined bright lamp light off of the glass at Brewster’s angle (to polarize it), then passed the light through a piece of glass with a high dielectric constant and then through a Nicol Prism that was twisted to cancel out the light. On September 13, 1845, Faraday found to his delight that a large magnetic field across the dielectric would cause the polarization to change so that he could see the light through the prism. Faraday crowed: “Thus is established, I think for the first time, a true, direct relation and dependence between light and the magnetic and electric forces.” Fun fact, it was in this paper that Faraday switched from “inductive lines of force” to “electric lines of force” which is a decidedly easier term to use.
Light as an Electro-Magnetic Wave
That is why, in late 1845 or early 1846, Faraday came up with an even more radical thought: since the polarization of light could be changed with an electromagnet, maybe *light* itself was not a wave in the invisible infinitely-strong ether, maybe the light was a wave of these lines of force. This wasn’t a completely new idea for Faraday. Way back in 1832, a few months after creating the idea of magnetic fields, Faraday came up with the idea that “magnetic action is progressive, and requires time… [and] I see the reason for supposing that electric induction…is also performed in a similar progressive way.” He even came up with the idea that “diffusion of magnetic forces from a magnetic pole” is like “the vibrations upon the surface of the water, or those of air in the phenomena of sound..and most probably to light.” In other words, back in 1832, Faraday conceived that magnetic forces and electric forces took time to travel and could move through the air like sound and light. Even that idea was so extreme that Faraday was loath to say it without experimental proof. Therefore he put it in a secret letter to be held in the Royal Societies’ storage box so he could prove he had the idea first if someone else experimentally proved it first.
However, before 1845, Faraday, like every other scientist in the world believed that light was a wave, though there was an invisible fluid everywhere called the ether that was the medium for light to transfer through. However, his experiments with polarization in 1845 combined with his thoughts from 1832 that magnetic and electric forces could make a vibrating force that takes time to travel through space made him think that maybe you don’t need a medium to transmit light. He even wrote in his diary, “so incline[d] to dismiss the ether.” By this time, Faraday had given up on the idea that all electrical induction had to do with the induction of all the ether in space, instead, “I… believe that when there are intervening particles of matter… they take part in carrying the [electric] force through the line, but that when there are none, the line proceeds through space.” However, Faraday was still internally debating these ideas when on April 3, 1846, the speaker at the Royal Society had a bout of stage fright and skipped out on his speech minutes before the talk. That is how Faraday ended up giving his one and only unprepared lecture of his career. Faraday began with a warning that he was only expressing, “the vague impressions of my mind…[without] sufficient consideration.” With that caveat out of the way, Faraday asked the audience to imagine a simple scenario, he asked them to imagine two objects A and B that are “distant from each other and under mutual action, and therefore connected by their lines of force.” Faraday then asked the audience to imagine if “one of the objects move” and how that would lead to a vibration of the lines of force which would then vibrate the second object.
Then, Faraday said what I think is one of the most radical and influential scientific postulates in history: “The view which I am so bold as to put forth considers, therefore, radiation as a high species of vibration in the lines of force… It endeavors to dismiss the ether, but not the vibrations.” However, he did think (erroneously we now feel) that the light was a back-and-forth wave or a “lateral vibration.”
Overview of Faraday’s Accomplishments
Let us take a moment to pause and consider what monumental and truly outside-the-box thinking went into this. In 1831, Faraday created the idea of magnetic lines of force or magnetic fields to explain how magneto-electric induction works and also found that they could be a way of envisioning how magnets attract and repel. Months later, he wrote a secret letter that magnetic and electric forces might take time to transmit and could make waves in space. 5 years later, in 1837, Faraday came up with the idea of inductive lines of force (or electric lines of force) to describe how a metal box can protect you from the electrical effects on the outside of the box. He also realized that insulators could alter an electric field and named a set of insulators dielectrics for their ability to affect the electric fields. Finally, in 1846, Faraday came up with the bizarre idea that maybe light itself is a vibration of these electromagnetic lines of force, and that you didn’t need the ether. And mind you, he did all this, and way, way more, with absolutely no math skills as Faraday never learned any mathematics aside from possibly some very simple Algebra.
Although people honored Faraday for his experimental discoveries, they did not feel the same about his theoretical constructs, especially this crazy one on the light. Therefore, Faraday’s “vague impressions” were almost universally rejected by scientists: the lines of the force seemed unscientific, physicists liked theories with math, and everyone knew that ether was real. It seems as if these ideas would have faded from view except for the work of a mathematical physicist named James Clerk Maxwell. Maxwell decided that he liked almost all of Faraday’s ideas aside from dropping the ether, so he just ignored that part. Maxwell then put Faraday’s lines of force into mathematical form in many papers culminating in a paper titled “A Dynamic Theory of the Electromagnetic Field,” published in 1864 which contained a primitive form of what is called “Maxwell’s Equations.” In this paper, Maxwell mathematically determined that the only way to explain polarization is with an up and down or transverse wave and that as a current or moving charge creates magnetic lines of force, a vibrating charge should make *both* vibrating electric lines of force and vibrating perpendicular magnetic lines of force, or what he called an “electromagnetic wave” or a wave in the electromagnetic “fields”, “because it has to do with the space in the neighborhood of the electric or magnetic bodies.” Soon all these lines of force were called fields, and we have called them electric fields and magnetic fields ever since.
Now that I have gone over in more detail the origin of the idea of magnetic fields, electric fields, and electromagnetic waves, it is finally time for me to give a deep dive explanation of how the Faraday Cage works, including why it works even with holes.