How Radio Waves Were Discovered

本站添加: 2026-05-05

[00:00:00]In the 1780s, an anatomy professor in Bologna named Luigi Galvani was dissecting frogs to study how electricity affected nerves and muscles.

[00:00:09]He prepared lower limbs with the crural nerves exposed from [music] the spinal cord to the legs and a metal wire passing through the vertebral canal, creating what he called reduced preparations on his bench.

[00:00:21]One day in his laboratory, Luigi's assistant, who was most likely his wife Lucia, touched one of the frog's exposed nerves with a lancet and the dead legs contracted vigorously. At the exact moment of the contraction, a spark drew from a nearby charged electrostatic machine.

[00:00:37]Repeating the setup, they found that every time a spark discharged from the machine, the frog's legs kicked, even though there was no continuous wire running from the machine to the animal.

[00:00:47]Galvani also found that very small discharges from a nearly discharged Leyden [music] jar, which were too weak to register on his best electroscopes, could still cause full muscular contractions.

[00:00:58]This suggested to Galvani that the external electricity did not supply all the energy of the movement. Instead, it acted as [music] an exciting cause that released a force already stored inside the animal tissue. To explain this phenomenon, Luigi built upon the 18th-century concept of irritability. A core idea [music] in irritability was that the way an organism reacted to an external influence was based on how that organism was structured internally, and this internal reaction was entirely independent of the nature of the influence. Based on this popular notion, he proposed that muscles and nerves possessed an intrinsic animal electricity similar to the charge stored in a Leyden jar.

[00:01:38]To experiment further with animal electricity, Galvani connected a frog's nerve to a long metal wire rising to the top of his house and waited during a thunderstorm. During four lightning flashes, he observed what he called not small contractions occurring at the instant of the lightning and clearly before the sound of the thunder. This seemed to suggest that whatever was causing the contraction propagated directly from the lightning to the frog.

[00:02:03]To reconcile animal electricity with atmospheric effects, Galvani imagined a universal electrical atmosphere filling space.

[00:02:11]To quote Luigi himself, "The electrical atmosphere hit and pushed and vibrated by the spark is that which brought to the nerve and similarly pushing and commoting some extremely mobile principle [music] existing in nerves excites the action of the nervio-muscular force."

[00:02:28]Galvani lacked the modern concept of electromagnetic waves, but his explanation already linked three [music] key ideas: a distant electrical event, an invisible disturbance propagating through space, and the triggering of an electrical reaction in a secondary object at a distance. [music] These three key ideas were conceptually quite close to how the phenomenon would come to be understood in the coming century.

[00:02:53]In 1820, Danish chemist and physicist Hans Christian Ørsted showed that an electrical current deflects a magnetic compass needle, revealing that electricity and magnetism are not separate phenomena, but are directly linked. I won't go into much detail on that discovery here, and I have covered it in another video that I will link in the description. That discovery, however, opened a new direction in research in electricity. If currents can create magnetic fields, then perhaps magnetic fields might in turn create electrical effects.

[00:03:23]Michael Faraday took up this problem in the 1820s and 1830s.

[00:03:27]At the Royal Institution, he systematically explored how magnets, coils of wire, and currents interacted, searching for a way to convert magnetic events into electricity. In 1831, he found that a current is induced in a closed conducting loop whenever the magnetic environment of that loop changes over time. Moving a magnet toward or away from a coil, switching a nearby current on or off, or rotating a conductor within a magnetic field all produced transient currents [music] that appeared only while the change was taking place.

[00:03:58]From this realization, he inferred that induction was not an instantaneous action at a distance between discrete bodies, but a process that unfolded [music] through a region of space between them. To describe what filled that region, Faraday introduced the idea of lines of magnetic force, a way of representing how magnetic influence occupies space.

[00:04:18]Iron filings sprinkled around a magnet arranged [music] themselves in curved patterns, and Faraday treated these curves as physical lines that showed the direction in which a magnetic [music] force acts on each point. He extended the very same idea to electric charges and currents, speaking of electric lines of force and conceiving of space as filled with continuous fields rather than empty gaps between objects.

[00:04:41]Faraday was the first to regard these iron filing lines as more than a visualization tool, and he made them his working model for how electrical and magnetic effects could be transmitted through a medium. This field-based view was a breakthrough experimentally, but lacked the mathematical foundation to back it up. Faraday himself claimed he wasn't the best at math, and instead of trying to unite his theory with mathematics, stuck to experimentation and left the theory for future scientists. When that mathematical foundation came though, it completely revolutionized how people saw electromagnetism and electrical action at a distance.

[00:05:17]Throughout the 1830s and 1840s, American physicist and inventor Joseph Henry investigated how brief electrical discharges could produce induction at record-breaking distances. He used Leyden jars as charge reservoirs and connected them to long coils of wire, essentially constructing circuits with capacitance and [music] inductance.

[00:05:37]By placing steel sewing needles inside secondary coils, he could detect currents indirectly. If a charge magnetized the needle, an induced current must have flowed through the coil. In 1842, while experimenting with his setup, Henry noticed an interesting anomaly.

[00:05:53]When the Leyden jar was discharged through the primary coil, the magnetized [music] needle in the nearby secondary coil was not always pointing the same direction after the discharge. The compass needle would also change directions when the length of the spark gap was changed.

[00:06:07]This behavior made absolutely no sense if the discharge were a single surge of current. A single pulse should always magnetize the needle in the same direction.

[00:06:16]From these observations, Henry inferred that when the jar is discharged into a wire, the charge overshoots electrical equilibrium and then rebounds, [music] shuttling back and forth between the inner and outer coatings in a rapidly diminishing series of rebounds.

[00:06:31]Each reversal of current could contribute a separate step in the magnetization process, explaining why the direction of the needle depended on characteristics such as the length of the spark [music] gap.

[00:06:41]To explore how far these effects could reach, Henry extended the distance between the primary and secondary circuits. In one set of experiments, a single spark from an electrostatic machine was discharged into a circuit of wire in an upper room, while a separate circuit containing a magnetizing spiral and needle was placed in the cellar roughly 30 ft below with two thick floors and ceilings between them. Even under these conditions, the induced current was strong enough to magnetize the needle in the lower circuit. Henry then turned his attention to atmospheric electricity. Using the tin-covered roof of his house as a large inductive plate, Henry soldered a wire to the roof, ran it down into his study, coiled a section of the wire, placed a needle inside the coil, and continued the wire to a metal plate sunk in a deep well to ground it.

[00:07:30]During thunderstorms, distant lightning flashes induced currents in this long circuit, and the needle in the indoor spiral became magnetized by flashes up to approximately [music] 8 mi away. He found that electrical disturbance from the lightning was also oscillatory. The discharge first passed from the roof to the well, [music] then reversed direction, and continued back and forth with diminishing intensity until equilibrium was restored. However, Henry interpreted these phenomena within a framework of induction and spreading motion in a luminiferous ether, not as independent [music] electromagnetic waves traveling through empty space. Although his experiments demonstrated long-range inductive effects that closely foreshadowed radio transmission, the leap to electromagnetic waves traveling through space and the revolution that would succeed it would have to wait for new theory to catch up and change the perspective of this experimental data.

[00:08:22]In 1853, British mathematician and physicist William Thomson, later to become known as Lord Kelvin, translated Joseph Henry's experimental picture into a precise mathematical model. He treated a Leyden jar discharge circuit as a system with three key properties: inductance, capacitance, and resistance, essentially an LCR circuit. In this model, the charge on the capacitor plates and the current in the wire obey the same kind of differential equation that describes a mass on a spring. They are both damped harmonic oscillators.

[00:08:55]Kelvin showed that if the circuit's resistance is small enough compared to its inductance and capacitance, meaning if R squared is less than 4L over C, then the discharge does not simply die away in one direction. [music] Instead, the system undergoes a damped oscillation. The charge and current reverse direction repeatedly with an amplitude that gradually decreases over time. From this analysis, he derived an expression for the natural resonant frequency of the oscillation, determined entirely by the inductance L of the circuit and the capacitance C of the jar.

[00:09:27]In modern notation, the oscillation frequency is equal to 1 over 2 pi times 1 over the square root of L times C.

[00:09:35]This means that for a given coil, shortening the wire to reduce L raises the frequency, while lengthening it lowers the frequency. Similarly, reducing the jar's capacitance also lengthens [music] the frequency and vice versa.

[00:09:48]What Henry had merely inferred from his experiments with needle polarities and spark behaviors, Kelvin now specified numerically as a resonant [music] frequency of an LCR circuit when the resistance is small.

[00:10:00]Kelvin's formula ultimately [music] showed later experimenters how to adjust inductance and capacitance to push oscillations into an area of frequency that they wanted. Around the same time, Kelvin was mathematically defining the behavior of electrical oscillations in circuits. Scottish physicist and mathematician James Clerk Maxwell was working on the spatial and dynamical structure of electromagnetic fields.

[00:10:23]Heavily inspired by Faraday's lines of force, Maxwell formulated a set of equations that described how electric and magnetic fields are generated by charges and currents and how they influence each other. A key step in formulating these equations was to modify Ampere's law by adding what he called the displacement current term, which defines a current produced by a time-varying electric field rather than by moving charges.

[00:10:47]This ensured that the equations remained consistent even in scenarios where no electrons flow, aka in circuits with capacitors.

[00:10:56]From this complete set of equations, Maxwell derived a wave equation for both the electric field and the magnetic field in empty space.

[00:11:04]The solutions of these wave equations describe transverse waves in which changing electric fields generate changing magnetic fields and vice versa propagating together through space.

[00:11:15]The speed of this wave emerges from the constants that characterize electric and magnetic effects in a vacuum, the permittivity and permeability, and is expressed mathematically as c = 1 over the square root of μ₀ ε₀.

[00:11:29]Maxwell then went one step further and plugged in values for these constants measured in prior experiments. Upon doing so, he found the smoking gun to electromagnetism.

[00:11:40]The calculated wave speed c was approximately 300 million meters per second, which was the known experimental value for the speed of light.

[00:11:49]From this, [music] he concluded that light itself is an electromagnetic disturbance and exists in a range of frequencies of the waves predicted by his equations. Kelvin's and Maxwell's calculations addressed different sides of the same emerging picture.

[00:12:03]One analyzed how an electrified circuit can behave as a damped oscillation with a resonant frequency set by its inductance and capacitance. The other showed that time-varying electric [music] and magnetic fields form waves that can propagate through space at the speed of light. Merging the two theories shows that Joseph Henry's circuit serves as a source of electromagnetic waves radiating energy away at a frequency [music] determined by the circuit. The stage was now set for a groundbreaking discovery [music] that would come about two decades after Maxwell's equations were published.

[00:12:36]The first person to come very close to building a transmitting and receiving system was a Welsh-American inventor named David Edward Hughes.

[00:12:44]This moment came in 1879 when Hughes already had a great reputation and was well-known for his work on telegraphy and for developing a sensitive carbon-based microphone.

[00:12:53]While experimenting in London with an induction balance, a device using coils and alternating currents to detect metal, he noticed something unexpected.

[00:13:03]Crackling sounds were being detected by a microphone some distance away even when there was no source of sound in the room.

[00:13:09]He traced the effect to poor electrical contacts in the microphone's circuit, which produced tiny sparks whenever the current changed abruptly. Hughes gradually realized that he could treat the induction balance as a kind [music] of transmitter and the microphone as a detector and that by adjusting and separating the devices, he could send detectable signals through walls and across nearby streets.

[00:13:32]By 1880, Hughes was demonstrating this effect as a form of wireless signaling over dozens of meters.

[00:13:38]On February 20th of that year, he presented the phenomenon to a committee of the Royal Society that included Thomas Henry Huxley and the society's president, George Gabriel Stokes.

[00:13:49]Hughes believed that he had found a new form of aerial transmission. Stokes, however, argued that the effect was simply ordinary electromagnetic induction and that nothing fundamentally new had been discovered.

[00:14:01]Hughes, despite his initial excitement, accepted the criticism and rather than pressing the point in print, largely abandoned further exploration of the wireless aspect of his work. Looking back, [music] many judged that Hughes actually had indeed generated and detected radio waves using the spark-like contact in his [music] microphone as a detector nearly a decade before the next instance I'm about to discuss.

[00:14:24]But because he lacked a theoretical framework, did not provide quantitative wave measurements, and accepted the criticism from other experts, his results did not enter the mainstream until after the results published [music] by German physicist Heinrich Hertz.

[00:14:38]Hertz approached the problem from quite the opposite direction of Hughes.

[00:14:42]He actually started with Maxwell's equations and deliberately asked whether electromagnetic waves could be produced [music] in a controlled laboratory setting. During the years of 1886 to [music] 1888 working in Karlsruhe, he constructed a spark gap transmitter [music] consisting of two metal rods facing each other with a small gap between them.

[00:15:03]This transmitter was connected to an induction coil that supplied high-voltage rapidly alternating currents.

[00:15:09]Each time a spark appeared in the gap, the metal structure behaved just as Henry's did. Charge surged back and forth at a very high frequency radiating electromagnetic waves into the surrounding space.

[00:15:22]For his receiver, Hertz used a simple but sensitive device, a loop of wire with a small spark gap.

[00:15:28]When the receiver was placed in the vicinity of the transmitter, tiny sparks appeared across the gap in the loop synchronized with the much longer sparks in the transmitter.

[00:15:38]Hertz then carried out a series of experiments to test whether this invisible influence behaved like a wave, in which he ultimately found overwhelming evidence for.

[00:15:47]Firstly, he showed that the signal could be reflected from large conducting surfaces.

[00:15:52]By arranging the transmitter and receiver with metal reflectors, he observed regions where the sparks were strong and other [music] regions where they nearly vanished forming standing wave patterns similar to those seen with light or sound. Measuring the distance between the nodes and the antinodes in these patterns [music] gave him the wavelength of the radiation.

[00:16:11]Knowing both the wavelength of the radiation and the frequency of the discharge circuit, he calculated a propagation speed that turned out to be the speed of light.

[00:16:19]He also demonstrated that these waves refract and polarize and the receiver responded differently to depending on its orientation relative to the transmitter.

[00:16:28]These results, published between 1887 and 1889, provided the first experimental confirmation of Maxwell's prediction that electromagnetic disturbances propagate as waves at the speed of light. Hertz finally showed, after a century of experiments with electrical action at a distance, that oscillating currents in a circuit produce electromagnetic waves that can be reflected, refracted, polarized, and interfered with just like visible light.

[00:16:54]When Hertz published his results in 1887, he dubbed the waves electric waves, but the public quickly changed their name to Hertzian waves to honor him.

[00:17:04]Over time, probably around the year 1910, was when the change to the term radio waves was made.

[00:17:10]>> [music] >> As you can probably see at this point, the discovery of radio waves was not merely made by one man, [music] but took a series of both experimental and theoretical developments by many scientists to reach [music] a definite answer to a question that spanned over a century.

[00:17:25]All of these people deserve [music] at least some form of credit as our knowledge of radio waves wouldn't be possible without all of their efforts.

[00:17:33]And about a decade after Hertz's findings, an Italian engineer named Guglielmo Marconi would take radio waves even a step [music] further.

[00:17:42]Not only would he put radio waves into practical use, but he would also spark a revolution and change communication forever. But that is a story for another day.

[00:17:53]If you enjoyed this video, please consider liking and subscribing. Click here if you want to see more scientific progress made during this time period.

[00:18:00][music] Thank you for watching and I will see you in the next video.