On February 22, 2022, I got to climb into something called the “cave of doom” and then stand there as it was hit by huge lightning bolts from a giant Tesla coil.
How and why did Faraday build a giant cube or cage that was big enough for him to “live in” and why did he do this in 1837, 50 years before the discovery of Radio waves and 56 years before the invention of the Tesla Coil?
Table of Contents
First, a little background of the people, namely Benjamin Franklin and Charles Coulomb whose experiments inspired Faraday. The second is a description of what Faraday did in the 1830s to create the cage. Third, I will talk about how the actions of a young William Thomson (later ennobled Lord Kelvin) led over the years to the Faraday cage becoming popular and how their actions led to the cage being used with Heinrich Hertz’s radio waves and Nikola Tesla’s coil demonstrations. Forth I will go through a whirlwind overview of the influence of the cage: specifically, how knowledge of the Faraday cage led to the discovery of the electron and how it is used in the microwave oven and even EMPs and protection devices.
Part 1: Inspiration
In my mind, the story of the Faraday Cage does not begin in 1837, but 82 years earlier with Benjamin Franklin in 1755. This was 3 years after Franklin had done his famous key and kite experiment in the electrical storm and he was still trying his best to figure out how storm clouds created and stored electrical charges. For that reason, he conducted a lot of experiments electrifying different materials with different shapes and testing their charges. He knew that if a neutral object was placed near the outside of a charged jar, the neutral object would be attracted to the jar and, if it touched the outside of the jar, it would gain some of the electricity from the jar and then be repelled by the jar. In this experiment, however, Franklin tried electrifying a silver jar and then lowered a ball into the *inside* of the silver jar to see if the charges were more concentrated on the inside. To Franklin’s shock, the cork was not attracted to the jar. Moreover, if the cork ball touched the inside and then retrieved the cork would remain neutral.
In other words, not only had Franklin demonstrated that the charges on a conducting object were to be found on the outer surface, Franklin also accidentally found that the conducting silver jar somehow shielded the inside of the jar from any electrical effects. Franklin was completely flummoxed by this result writing, “The fact is singular. You require the reason; I do not know it. Perhaps you may discover it, and then you will be so good as to communicate it to me.”
The reason that I know about this experiment is that 14 years later, in 1769, Franklin included his description of this experiment in the 5th edition of his book on electricity. Then, in 1776 Franklin went to France and used his fame from electricity to assist his efforts to gain French support for the American Revolution. This was a resounding success; Franklin was a superstar. John Adams, a man who personally disliked Franklin, wrote that in France Franklin’s, “reputation was more universal than that of Leibnitz or Newton or Frederick or Voltaire, and his character more beloved and esteemed than any or all of them.”
In Paris, one of the many people interested in Franklin’s work and the visit was a 40-year-old mechanical engineer named Charles Augustin Coulomb. It was when Franklin arrived in France that Coulomb learned about a contest to make a very sensitive compass which inspired him to not only create a compass but also study the twisting force and create the torsional scale. Perhaps inspired by Franklin’s visit, or the memories of electricity that he learned in school, Coulomb started using his new torsional balance to study the electrical forces. By 1785 Coulomb experimentally determined that electrical force is proportional to one over the distance squared, which he declared to be a “Fundamental law of Electricity.” The following year, in 1786, Coulomb relooked at Franklin’s experiment with the silver jar and, decided that, even with his new super-accurate electricity meter, “when a conducting body is charged with the electric fluid [it] is only diffused over its surface, and does not penetrate into its interior parts.” It was Coulomb’s experiment that led Faraday to make his cage, which brings us to…
Part 2: Faraday and his Cage
Now I would like to fast forward to Michael Faraday in 1831. I picked that date because that is when Faraday discovered how to induce electricity with moving magnets or changing magnetic fields. His biographer and friend said that “after the discovery of magneto-electricity his fame was so noised abroad that the commercial world would hardly consider any remuneration too high for the aid of abilities like his.” However, Faraday decided that he could not make money or have honored and still dedicate himself fully to science. As he said it was a decision between “wealth or science,” and with the support of his beloved wife Sarah, they choose science. Faraday thus declined every honor and financial reward that came his way, and, as his wish, “remain[ed] plain Michael Faraday to the last.” Starting in 1831, Faraday wrote paper after paper about the “experimental researches in electricity” eventually writing 30 papers with that title, much of which affects our life and language to this very day. For example, in a single paper in 1834, his seventh on electricity in 3 years, Faraday created the names anode, cathode, electrolytes, electrodes, anions, cations, and ions. When a friend wrote him a complimentary letter about his work in 1835, he responded: “Your letter was quite refreshing for I had begun to imagine that I thought more about Electricity and Magnetism than it was worth: and so a notion was creeping over me that after all I was perhaps only a bore to my friends.” By the pivotal year of 1837, for his eleventh paper on electricity, Faraday decided to re-examine all of the old theories of electricity with all of this new knowledge. As he put it, “the science of electricity is in that state in which every part of it requires experimental investigation; not merely for the discovery of new effects, but what is just now of far more importance, the development of the means by which the old effects are produced.”
Faraday had read Coulomb’s 1768 paper and for Faraday, the fact that the charges in a conductor move to shield the electric forces proved that the conductor wasn’t producing extra charges but just moving the charges until all the forces are balanced. Faraday just saw it without math or even an explanation beyond a simple statement that, “the beautiful experiments of Coulomb … are sufficient, if properly viewed to prove that conductors cannot be bodily charged.”
Faraday wondered about insulators or non-conducting materials and declared that “with regard to …non-conductors, the conclusion does not at first seem so clear.” However, after various experiments, Faraday determined that it seemed true for insulators as well. For example, if part of a piece of glass was electrified with one charge by contact or proximity to a charged conductor, “it was always found that a portion of the inner surface of the contactor…or another part of the glass itself was in an equally opposite state.”
To examine this further, Faraday decided to examine how air (an insulator) reacted to charge. For this reason, Faraday redid Coulomb’s and Franklin’s experiment and upped the size by making a giant cage. Here is how Faraday described it: “I had a chamber built, being a cube of twelve feet [each side or 3.6 meters]. A slight cubical wooden frame was constructed, and copper wire passed along and across it in various directions, so as to make the sides a large net-work… and supplied in every direction with bands of tin foil, that the whole might be brought into good metallic communication.” After various experiments inside the cage, Faraday turned it around and had an assistant conduct experiment outside the cage while he, as he dramatically put it, “went into the cube and lived in it.”
Faraday, therefore, discovered that when he was inside his cage, he was protected from all electricity that happened outside. “I put a delicate gold-leaf electrometer within the cube, and then charged the whole by an outside communication, very strongly, for some time together; but neither during the charge [nor] after the discharge did the electrometer or air within show the least signs of electricity.” He added that he couldn’t see any electric effects, “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.”
Faraday was happily convinced by this and concluded, “that non-conductors, as well as conductors, have never yet had an absolute and independent charge of one electricity communicated to them, and that to all appearance such a state of matter is impossible.” Soon after this experiment, in the same paper, Faraday wrote about how he recreated “the torsion balance electrometer of Coulomb… with certain variations and additions,” the biggest of which is that he coated his electrical measurement devices in stripes of tin foil so that they would be protected from stray electrical fields. In other words, immediately after inventing a Faraday cage Faraday turned around and used one to protect his electrical devices.
Unfortunately, Faraday’s work started to slow by the late 1830s as he suffered from stress, anxiety, depression, and serious memory issues probably due to working with heavy metals without protective equipment (he spent 10 years trying to make high-quality glass lenses in the 1820s). In December 1839, his doctor recommended rest to keep him from a breakdown. This did not help and In the summer of 1840, 48-year-old Faraday wrote to a friend: “This is to declare … that I am not able to bear much talking … being at present rather weak in the head, and able to work no more.” That is not to say that Faraday stopped making important discoveries, just that his work happened in fits and spurts.
Part 3: Development of the Faraday Cage
The biggest promoter of the Faraday cage was most likely an Irish scientist named William Thomson. Thomson learned about Faraday as a teen from his tutor and became, as he put it, “inoculated with Faraday fire,” and then met his hero Faraday at a conference in 1845 when Thomson was just 21 years old. They became quick friends and Thomson then inspired Faraday to conduct an experiment that led Faraday to come up with the idea that light was a wave of electric and/or magnetic fields. Thomson did not like the idea of light being an electromagnetic wave, but it wasn’t enough to keep him from recommending Faraday to his younger friend James Clerk Maxwell, who ended up writing “Maxwell’s equations” based on Faraday’s ideas.
Meanwhile, Thomson talked about Faraday and his work and particularly emphasized Faraday’s cage. For example, in January 1860, Thomson complained that “even the best of ordinary electrometers hitherto constructed,” often give incorrect results, “as the inner surface of the glass… is liable to become electrified.” Thomson then lamented that “Faraday long ago showed how to obviate this radical defect by coating the interior of the glass case with a fine network of tinfoil; and it seems strange that even at the present-day electrometers…should be constructed with so bad and obvious a defect uncured by so simple and perfect a remedy.” Thomson then added the term cage when he noted that: “a cage made like a bird’s cage…may be substituted with advantage for the tinfoil network.”
During this time, Faraday’s health continued to deteriorate and he conducted his last experiment on March 12, 1862, at the age of 70, and wrote a friend, “Again and again I tear up my letters, for I write nonsense. I cannot spell or write a line continuously. Whether I shall recover—this confusion—do not know.” Faraday died on August 25, 1867, at the age of 75 and, as was his wish, had “a plain simple funeral, attended by none but my own relatives, followed by a gravestone of the most ordinary kind, in the simplest earthly place.”
A few weeks after Faraday’s death, the now knighted Sir William Thomson was on a committee to create standards of electrical resistance. In this report, Thomson stated again that it was a shame that people didn’t use Faraday’s system to protect their electronic equipment and added that the electroscope he was using was protected by using a “Faraday metal cage.” And, thus the term “Faraday cage” was born.
12 years later, in July of 1879, a 22-year-old German graduate student named Heinrich Hertz was told about a prize to experimentally prove “the theory of electrodynamics which was brought forth by Faraday and was mathematically executed by Mr. Maxwell.” It took Hertz eight years, but in 1887 Hertz discovered that if he added an antenna to an induction coil, it would make an invisible electromagnetic wave that he could “catch” across the room as a spark in a circular wire with a tiny gap in it. These waves were initially called Hertzian waves but by the 1920s they were called radio waves. As radio/Hertzian waves were created by vibrating electricity and moved at the speed of light, this seemed like a validation of the Maxwell-Faraday theories of the nature of light.
When William Thompson heard about it, he thought that radio waves would make it through a Faraday cage saying, “we all know how Faraday made himself a cage… [and] he saw no effects on his most delicate gold leaf electroscopes in the interior [but Faraday’s] attention was not directed to look for Hertz sparks, or probably he might have found them in the interior.” What Thomson didn’t know was that in July of 1889, Hertz tested out the Faraday cage for radio waves and found that it blocked his alternating signals too. For when Hertz placed his spark gap in a wheel of wires, he found that “not the slightest electrical disturbance would be detected in the wire in whatever direction waves were sent through the apparatus.”
At around the same time that Hertz was playing with Faraday cages, Nikola Tesla learned about Hertz’s experiments and later recalled that “the publication of Dr. Heinrich Hertz’s results caused a thrill as had scarcely ever been experienced before.” Tesla then, “concentrated my attention on the production of a powerful induction coil,” which eventually led to the development of the very high voltage Tesla coils. To Tesla’s delight, his new coils would make great sparks and could even electrify fluorescent bulbs if they were held near the coil. Tesla burst on the scene with these amazing displays in 1891 that even 12 years later were described as, “the most remarkable lectures ever delivered before a scientific audience.” Tesla’s experiments and demonstrations were actually at lower frequencies than Hertz’s and it was quickly found that Faraday’s cage worked to block Tesla’s waves just as it blocked Hertz’s and were soon incorporated into some of the Tesla coil demonstrations, as they continue to do to this very day.
Part 4: Influence of the Faraday Cage
But that is not the only use and influence of the Faraday cage. For example, 6 years after the Tesla coil, in 1897, a shy Englishman named J J Thomson (no relation to William Thomson) used his knowledge of the Faraday cage to discover electrons. See, Thomson was confused because Heinrich Hertz (the same guy who had discovered radio waves) had been unable to move a beam in a cathode ray tube with electrified plates even though another scientist had determined that the cathode ray had a negative charge and it was well known that the beam could be moved with a magnet just like negative electrical charges moving in a wire. Thomson then realized that trace amounts of gas in the tube were becoming conductive and screening the cathode ray “from the effect of electric force, just as the metal covering of an electroscope screen off all external electric effects.” In other words, JJ Thomson realized that the air was ionized and acting like a Faraday cage. Thomson then devised a more powerful tube and removed more air so that there was not enough air to be a faraday cage and the beam could be moved with electric forces as well as magnetic forces. The interaction between these two forces (magnetic and electric) is how Thomson determined that the cathode ray was composed of tiny negatively charged particles that are in everything which he called corpuscles (but we now call electrons).
In addition, as we had more technological electrical devices, there was a corresponding need for Faraday cages to protect delicate items as well as ourselves. For example, one day in early 1945, a very nice man named Percy Spencer was working with microwave radar at different frequencies to see if he could improve them. I mention this because on that particular day, Percy found that when he worked on his microwave radar the Mr. Peanut bar in his pocket melted! After testing it out with popcorn and an egg (which exploded!) Spencer realized that these waves, which are just high frequency (and therefore microwave) radio waves happen to have just the right frequency to make the water in food spin and heat food up. Spencer immediately filed for a patent for a microwave conveyer belt! Two years later, Spencer redesigned it into what he called a “microwave oven” where the oven was made of conductive material so that it would act like a Faraday cage and protect the operator.
Or another example, in July of 1962, the United States military tested out a 1.4 megaton hydrogen bomb (about 100 times more powerful than the bomb in Hiroshima) almost 250 miles above an island in the pacific. To the surprise of physicists, this caused circuit breakers to pop and burglar alarms to ring 800 miles away in Hawaii. By 1967 they determined that the bomb had interacted with the lower van Allen belt creating a huge electromagnetic pulse which they called by its initials EMP. However, at first, people weren’t too worried about EMPs as most of their equipment was built with vacuum tubes which are pretty impervious to damage from the very short spike of an EMP.
However, with the rise of semiconductor technology, there was more worry about damage from EMPs. By 1980, the secretary of defense publicly worried about “the widespread loss of connectivity which would be caused by a high altitude nuclear explosion and its resulting electromagnetic pulse.” By May of the following year, a science journalist named William Broad wrote a series of articles with the alarming subtitle of “a single nuclear blast high above the United States could shut down the power grid and knock out communications from coast to coast.” Many governments of the world took this seriously and made efforts to protect their vital electrical systems but the general public became convinced that all nukes would disable all electronics. Soon movies were showing planes falling out of the sky due to EMPs. My favorite is from a truly campy movie from 1994 called “Broken Arrow.” In this movie, an evil John Travolta detonates an underground nuclear device that destroys all of the operational electrical devices and causes a helicopter to crash. When he was told that the “shockwave took down the damn chopper” he responds “that was the EMP, electromagnetic pulse, nuclear blast sends it out for miles, everything electronic just shuts down including choppers and radios”
Note that this movie is just wrong. Small underground nukes do not make a big EMP, and even if they did, airplanes and helicopters tend to be pretty safe as they are made of metal and by themselves are Faraday cages (which is why they are safe when struck by lightning). In addition, it is possible to protect systems not in an airplane with a Faraday cage and/or surge protectors, which is called “hardening” the equipment. That is why most data centers and power centers are put behind metal gratings or metal walls. That is not to say that an EMP from a large nuke in the atmosphere is harmless as it takes money to protect equipment and it is too often skipped. For example, in 2019, the “Electric Power Research Institute” said that although the worst-case EMPs could cause widespread electrical damage, “on the order of several states or larger…[However,] none of the scenarios that were evaluated resulted in a nationwide grid collapse.” Of course, in a large EMP blast, your personal electronic equipment could be fried, and statewide electrical failure would be pretty catastrophic which is why people sometimes buy or make Faraday cages to protect their equipment for such a scenario.
In addition, with the rise of wireless communication, there was also an increased interest in using Faraday cages to protect data from electronic snooping. This is another reason for the portable Faraday cages to protect your cell phone. That is why the US government created secure facilities called SCIFs which are big Faraday cages to protect the people inside from outside interception of data. For all of these reasons, and many more, Faraday cages are abundant in electrical engineering.
So that brings us to the final question: what is going on with the electrons in the cage to cause it to protect the interior from both lightning and some electromagnetic waves but not others? I will use an 1843 experiment by our friend Michael Faraday with an ice bucket to explain the physics of the Faraday cage next time on the lightning tamers.
So that was the long history of the Faraday cage.