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A Brief History of the Discovery of Electricity
Who discovered electricity? It wasn't Benjamin Franklin.
Let's go way back in time to search for the first electric appliance. Perhaps it could have been the magic lamp of Aladdin fame. It's easy to imagine that wealthy people in ancient times may have had lamps carved from amber that produced mysterious effects when rubbed, leading to legends of genies living in the lamps. However, the Aladdin story did not reach the Western World until 1709, and even then, the lamp itself had no special powers, only the genie living inside it.
Another candidate is the Electric Donkey Bottom Biter, a device first mentioned in later versions of the legends surrounding King Arthur's quest for the Holy Grail. Unfortunately, this device does not appear in any earlier medieval sources and is almost certainly a modern embellishment.
To find the real first "electric appliance," we have to go back to around 600 BC, when Thales of Miletus, a Greek mathematician and philosopher, made the earliest known reference to a portable, hand‑held electric lint remover.
All levity aside, by about 600 BC, it was already known that rubbing amber
(fossilized tree sap) with wool caused it to attract dust, chaff, and lint. In
later centuries, this property was called electricity, after the term
electricus, coined from the Latin word for amber, "electrum." Today, we call it
static electricity—electric charge at rest. You can also produce static
electricity by rubbing glass with silk, or by running a plastic comb through
your hair. Modern plastics are excellent at generating static charge. However,
electricity remained little more than a curiosity until the 18th century,
Guericke makes an electric generator
In 1650, Otto von Guericke, a German scientist, inventor, and politician, built an electric generator from a sulfur ball. Placing his hand on the rotating ball would produce electricity just as rubbing amber with wool did. Later, it was found that a hollow glass sphere worked as well.
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Figures V and VI from Otto von Guericke's treatise New Experiments (such as they are called). |
Gilbert names electricity
William Gilbert, an English physician, physicist, and natural philosopher, did the earliest modern research on the properties of amber. In 1600, he published a treatise titled On the Magnet and Magnetic Bodies, and On That Great Magnet the Earth. Gilbert noted that the properties of amber were different from the magnetism of lodestones.
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William Gilbert demonstrating his experiments to Queen Elizabeth. |
To differentiate between these two, he introduced the Latin term, electricus, meaning "like amber." He was also the first to use the term electrical force.
The gilbert is an obsolete unit to measure magnetic force.
Gray discovers "electrical fluid"
In 1729, British astronomer Stephen Gray experimented with electricity generated by rubbing a glass tube with silk. He noticed that a cork used to keep the inside of the tube dry and clean became electrified with the glass. Cork could not be electrified by itself. When he stuck a fir stick into the cork, the electricity extended to the end of the stick. He extended the electricity further with a length of oily hemp packing thread. The next day he extended the reach from his balcony to the courtyard below. He found that the electricity would travel around bends and was unaffected by gravity. Since electricity could be made to travel from one place to another, Gray speculated that electricity is a fluid. This gave rise to the term electrical fluid, which was used up to the early 20th century to refer to electricity.
Desaguliers names conductors and insulators
Gray discovered that some materials carried the electricity better than others and some not at all. John Desaguliers, a French-born British natural philosopher, clergyman and engineer—a member of the Royal Society and assistant to Isaac Newton, whom Gray had corresponded with—called these materials conductors and insulators.
Du Fay theorizes two types of
electricity
In 1736 Charles Francois du Fay, a French chemist noted that electricity made by rubbing amber with wool appeared to have opposite properties to electricity made by rubbing glass with silk. Two pieces of electrified amber would repel each other. Likewise, two pieces of electrified glass would do the same. However, an electrified piece of amber would be attracted to an electrified piece of glass. He called them resinous and vitreous electricity: resinous because amber is fossilized resin and vitreous because vitrum is Latin for glass.
Von Kleist and Musschenbroek invent
the capacitor
In 1745, Ewald Georg von Kleist, a German lawyer, Lutheran cleric, and physicist, tried to store electrical fluid in a medicine bottle filled with alcohol with a nail through the cork. After attempting to store electricity from a friction generator (such as Guericke's), he received a substantial shock when he touched the nail with one hand while cradling the bottle with the other. Pieter van Musschenbroek, a physics and mathematics professor at the University of Leyden in the Netherlands, tried to repeat von Kleist's results using a glass of beer. He found that it only worked when cradled in his hand, rather than set on an insulating resin block. William Watson, an English physician and scientist, found that wrapping the glass with metal foil worked better than holding it in hand. He substituted water for beer, a jar for the glass, and a metal post with a metal ball on top for the nail (the jar would mysteriously lose its electricity if the post had sharp edges). Eventually, the water was replaced with metal foil inside the jar, with a metal chain connecting the central metal post to the inner foil. It is called a Leyden jar. Modern versions of this device are called capacitors.
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Franklin demonstrates that there is only one type of
electricity
Around 1750, Benjamin Franklin, an American statesman and scientist, and
William Watson noted that when a Leyden jar was electrified (charged as Franklin
called it), it contained resinous electricity on the inside and vitreous
electricity on the outside or vice versa. When they connected a metal wire from
the inside to the outside, both charges would disappear. They thought this would
happen if the two types of electricity were the same substance, but with
different amounts of "abundance" (Watson labeled the greater abundance positive
and the lesser abundance negative). When the inside and outside of the Leyden
jar were connected, the electricity flowed like a fluid from the greater
abundance to the lesser abundance until they equalized. However, it was
impossible to tell with contemporary technology whether the fluid was flowing
from the inside to the outside or from the outside to the inside; you could not
determine whether vitreous or resinous electricity was the greater abundance.
Franklin proposes an experiment
By this time, it seemed obvious that lightning was a form of electricity.
Small electrical sparks produce tiny flashes and sharp clicks; lightning
produces a gigantic flash and a deafening boom. To most observers of the 18th
century, lightning simply looked like a much larger version of the sparks they
could create in the laboratory.
In 1750, Franklin wrote a series of
letters proposing experiments to draw electricity from clouds. His idea was to
place a small shed—something like a sentry box—beneath a tall, sharp‑tipped
metal rod that extended high enough to extract the charge from passing low
clouds. The rod would be connected to a Leyden jar at the base to store the
collected electricity. This would demonstrate that clouds themselves carried
electrical charge.
Franklin also hoped the experiment would settle the
question of which type of electricity had the higher abundance, vitreous or
resinous. It was wrongly assumed that lightning traveled from cloud to ground.
Therefore, Franklin and others reasoned that the cloud must hold the greater
abundance. Whichever type of electricity was collected in the Leyden jar would
represent the greater abundance.
Dalibard beats Franklin to the punch
Thomas-François Dalibard was a French scientific writer, editor, and
translator who had translated some of Franklin's letters into French. He was the
first to use the word “pressure” (French pression) to describe electricity with
a greater or lesser abundance. In May of 1752, Dalibard performed Franklin’s
proposed experiment using a 40-foot-tall metal rod attached to a stone building
on a high hill. He successfully extracted electricity from a cloud and stored it
in a Leyden jar. Two months later, unaware that Dalibard had scooped him,
Franklin also extracted electricity from a cloud with his famous kite
experiment.
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Some historians doubt that Franklin actually performed his kite experiment. Franklin never explicitly wrote that he performed it. He only mentioned that the experiment had been done, and only in the third person. The first detailed account was published by Joseph Priestley years later, but there is no evidence that Priestley had firsthand knowledge of Franklin’s actions. Much of the skepticism comes from the fact that the experiment (as popularly described, not as Franklin actually described it) would almost certainly have been fatal. Several people who attempted to repeat Franklin’s kite experiment were killed by lightning strikes. However, Franklin never proposed performing the experiment during a thunderstorm. In his original writings, he argued that all clouds contain electricity, so experimenting during a violent storm was unnecessary and reckless. The shed in his proposal was meant only to protect the experimenter from rain, not lightning. Modern demonstrations support the idea that strong fields can be detected even without storm clouds: in one MythBusters segment, more than 1,000 volts were extracted from the air in clear weather.
Nevertheless, neither Franklin nor Dalibard determined whether the electricity extracted from the clouds was vitreous or resinous. The best the historical record tells us is that Franklin arbitrarily labeled the charge produced by rubbing glass with silk “positive,” and the charge produced by rubbing resin or ebonite with wool “negative.” From then on, he preferred to call them “plus” and “minus” charges.
The Franklin is an obsoleter measure of electrostatic charge.
Cavendish almost invents Ohm's Law
In 1781, Henry Cavendish, best known for discovering hydrogen, determining
the composition of water, and measuring the density of the Earth, described the
relationship between electric pressure and the flow of electric fluid (electric
current). His work was a precursor to Georg Ohm’s work. Cavendish’s methods were
crude. To represent what we today call resistance, he used different lengths of
wire. He made his measurements by observing (feeling) the effect of electricity
on himself. Cavendish was reclusive and didn’t seek society's accolades,
publishing only work he considered complete and important. His unpublished works
show that he anticipated much of the work later credited to better-known
scientists. He never bothered to publish his work on electricity, so he remains
virtually unknown in the field. He would probably have remained unknown if James
Clerk Maxwell hadn’t published Cavendish’s work in 1879. Unlike many who came
after him, Cavendish has no standard unit of measure named after him. However,
the method he used to measure Earth's density is called the Cavendish
Experiment.
Galvani almost invents the battery
Also in the 1780s, Luigi Aloisio Galvani, an Italian physician, physicist, biologist, and philosopher, observed the muscles of a dead frog twitching as he dissected it with metal tools. Trying to consistently reproduce the effect, he hung a frog’s leg from an iron railing and inserted a brass hook into the spinal nerve. When the hook contacted the railing, the muscles twitched. Galvani assumed the nerve stored electricity like a Layde jar. Touching the metals together completed the circuit, allowing the nerve to discharge into the muscle, which contracted in response. He concluded that animals possess their own intrinsic form of electricity, which he called “electricità animale,” or animal electricity. There is no unit of measure named after Galvani. However, today we have the Galvanometer (a type of current meter), we galvanize steel to make it rust-resistant, and the production of electric currents by chemical action is referred to as galvanic action.
Volta paves the way for the modern world
In 1800, Count Alessandro Volta, an Italian physicist and chemist, repeated
Galvani's experiments. He discovered that for the experiment to work, the tools
had to be made of different metals. He thought that the electricity might be
generated from the salty fluids in the frog's body acting on the dissimilar
metals. To test his theory, he constructed a stack of alternating zinc and
copper disks. Each pair of disks was separated from the others by heavy paper
disks that were soaked in brine. The stack of disks produced electricity.
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The more disks he put in the stack, the more electrical pressure was produced. This “XE "History:electric pile"electric pile,” as he called it, allowed sustained electrical currents instead of the short bursts produced by static electricity.
Volta's invention was a turning point in modern history. Volta's pile allowed Orested to discover the relationship between electricity and magnetism. Only after that could the generator be invented and electricity be created and distributed around the world. Many historians say that the battery,[1] as we call the modern version of Volta's pile, allowed the development of portable electronics. This is a gross understatement. Without the battery, modern electronics as we know them could not have been developed. Even today, with a few exceptions, all electronic equipment requires a battery or simulated battery to operate. Most appliances that plug into the power grid/mains (other than electric motors, resistive heaters, etc.) have power supplies that simulate one or more batteries. Without the battery, electricity would have remained a curiosity suitable for little more than parlor tricks and would not have ushered in the modern world.
The unit of electrical potential (pressure), the volt, is named after Volta.
Oersted demonstrates electromagnetism
In 1820, Hans Christian "Hans Christian Oersted (Ørsted in Danish), a
professor at the University of Copenhagen, demonstrated the relationship
between electricity and magnetism. Oersted subscribed to the philosophy that
all forces of nature were connected and thus thought there must be a
connection between electricity and magnetism. Unfortunately, he was unable
to develop an experiment to explore that relationship. During a lecture
demonstrating heat produced by electric current (using Volta's electric
pile), he noticed a nearby compass moving when he completed the circuit.
After the lecture, he recreated the demonstration in private and observed
that, when held near the wire while current was flowing, the compass needle
tended to align perpendicular to the wire. When he reversed the current
flow, the compass needle flipped. This proved that when an electric current
flows in a wire, a magnetic field surrounds the wire.[2]
To his surprise, the magnetic field surrounded the wire in a circular
manner; he expected that such a field would radiate outward from the wire
(thus causing the compass needle to point to the wire). His discovery led to
the invention of the electromagnet, the electric motor, and the electric
generator.
The Oersted is an obsolete unit to measure magnetic field
strength.
Ohm develops Ohm's Law
In 1827, Simon Ohm,[3] a German physicist and mathematician established the mathematical relationship between electrical pressure, current, and resistance. Before Ohm, experimenters were handicapped by the irregularities of Volta’s pile, making precise, repeatable measurements difficult. Nevertheless, it appeared that, under steady electrical pressure, cutting the length of a conductor exactly in half didn’t exactly double the current. Instead, they observed the resulting current to be lower than a linear relationship would predict. This led many researchers to believe that the relationship between electrical pressure, resistance, and current was non-linear.
Ohm's testing apparatus (incorrectly labeled as a torsion balance)
Ohm performed his own experiments to clarify the exact relationship. First of all, he found that he obtained consistent, repeatable measurements using thermocouples immersed in ice water as the source of electrical pressure. He measured current with a magnetized needle suspended over the wire, the whole apparatus oriented such that the needle was aligned with the Earth’s magnetic field. He observed the minute deflection of the needle when current was applied using a microscope. Still, the current didn't exactly double when the wire length was cut precisely in half, etc. However, he found that subtracting a constant factor from the calculated resistance yielded linear results. He determined that the constant was the thermocouples' inherent resistance, which added to the resistance of the wire being tested. This is how Ohm succeeded where others failed. Today, the relationship between voltage, current, and resistance is known as Ohm's Law.
The unit of electrical impedance (of which resistance is one example) is named after Ohm.
Henry and Faraday discover induction,
invent the telegraph
In the early 1830s, Joseph Henry, an American physicist who served as the
first Secretary of the Smithsonian Institution, and Michael Faraday, an
English scientist, experimented with the electromagnetic effects of coils of
wire wrapped around iron bars. When the coils are connected across a
battery, the resulting magnetic field is concentrated because the coils are
close together, with the iron further amplifying it.
Henry noticed
that when he broke the circuit, a spark would jump across the gap between
the switch contacts. He first explained this saying the current had a kind
of “momentum,” like a moving train that would be hard to stop. This would
force the current across the gap. Later, he realized that when he broke the
circuit, the collapsing magnetic field around the coil of wire induced its
own current in the wire. This self-induced current forced the electricity to
jump the gap.
Around the same time, Michael Faraday placed two coils
side by side (or wound one on top of the other) and passed current through
one of them. He discovered that when he broke the circuit, stopping the
current in one coil, a galvanometer attached to the other coil would briefly
show some current. In this case, it was the moving magnetic field produced
by the first coil that induced the current in the second. This phenomenon is
now called mutual induction.
In reality, Henry and Faraday each
independently discovered self- and mutual induction at about the same time.
Despite Faraday publishing first, the scientific community decided to split
the credit, giving Henry credit for discovering self-induction and Faraday
credit for discovering mutual induction.
Most historians now credit
Henry with inventing the electric telegraph. Henry's telegraph consisted of
a battery, a switch, a sufficient length of wire, and an electromagnet (one
of the above coils) that moved a clapper to ring a bell. He used it to send
signals from his office to his home while he was a professor at Princeton.
The unit of inductance is named after Henry, and the unit of capacitance,
the farad, is named after Faraday.
Morse's telegraph
By 1837, electric XE "telegraph"telegraphs were being built across Europe. These were various devices that used electromagnets to move pointers to letters of the alphabet. The operator at the transmitter would move his pointer or pointers to a letter and the receiver would mimic the transmitter. That year, XE "History:Morse, Samuel "Samuel Morse, an American painter, developed a system that used a cumbersome mechanism in which long and short notches were cut into a piece of wood. This was drawn over a switch, closing and opening the circuit for long and short intervals. The receiving end had an electromagnet that pushed an inked wheel against a strip of paper when the circuit was closed, making long and short marks on the paper. Morse and XE "History:Vail, Alfred "Alfred Vail—his machinist assistant—developed a code of dots and dashes to use with the apparatus to send messages.
Although the Morse apparatus worked, it proved unwieldy and unnecessary once operators discovered they could tap out the code by hand and decipher it by ear.[4] Morse soon replaced the transmitter and receiver with the now-familiar key and electromagnetic sounder. The Morse code with the key-and-clicker mechanism became the dominant electric telegraph system worldwide.
Morse's improved telegraph transmitter
Another key part of the Morse/Vail system that was widely
adopted was the use of a single wire between stations, with the Earth as the return
wire to complete the circuit.
Kirchhoff's Laws
In the 1840s, Gustav Kirchhoff was a student of Franz Neumann, an early pioneer in electromagnetic theory, who trained Kirchhoff in rigorous investigative methods. Kirchhoff clarified how charge and energy behave in electrical circuits. The principles he described, later called Kirchhoff’s current law and Kirchhoff’s voltage law, show how current divides at junctions and how voltage changes around a loop. These ideas, as important as Ohm’s law in circuit analysis, provided a general framework that engineers soon relied on as telegraph and power networks expanded. Kirchhoff would later make major contributions far beyond electricity, including foundational work in spectroscopy and thermal radiation.
Swan, Edison, et al. invent the light bulb
In the mid‑1800s, inventors began searching for a practical form of electric lighting. Passing an electric current through a wire heats it, and with enough current, the wire glows. The challenge was to harness this effect in a durable, reliable lamp.
Joseph Swan with his light bulb
The main obstacle was that a glowing wire. Depending on its material, it would quickly melt or burn up. By 1878, both Joseph Swan in Britain and Thomas Edison in the United States had developed workable incandescent lamps. Each used filaments made of materials that could withstand very high temperatures, enclosed in glass bulbs from which the air had been removed to prevent oxidation. Edison eventually adopted a carbonized paper filament and designed a bulb that could be manufactured economically in large numbers.
Maxwell predicts what we now call radio waves
In the early 1860s, James Clerk Maxwell developed a mathematical theory showing that electric and magnetic fields are linked. In 1864, he predicted that these fields should travel through space as waves, and that visible light is one such electromagnetic wave. His equations also implied the existence of much longer, invisible waves. This idea was not widely accepted at the time, but Maxwell was later proven correct.
The obsolete CGS unit of magnetic flux, the maxwell, was later named in his honor.
Braun invents first solid state devices
In 1874, German physicist Karl Ferdinand Braun discovered that certain crystals would conduct electrical current more easily in one direction than the other. This effect appeared when a fine metal contact touched the crystal, making an imperfect connection, creating what later became known as a “cat’s‑whisker” detector. Braun’s crystal rectifiers were soon used to detect radio waves.
Braun's cat's whisker radio detector
He patented the crystal rectifier in 1899. These devices, now called point‑contact diodes, were the earliest solid‑state predecessors of the transistor.
Hughes invents the microphone
In the early 1870s, British‑American musician and inventor David Hughes developed a device whose electrical resistance changed in response to sound waves. It used a loose electrical contact (often a carbon or metal point resting lightly against another surface) so that vibrations altered the pressure at the contact and thus its resistance. Hughes called this device the microphone. He chose not to patent it, declaring it “a gift to the world.”
Hughes' carbon microphone
Bell and Watson invent the telephone
In the 1870s, several inventors were working on devices to convert sound
into electrical signals and back again, attempting to create an electric
telephone (the word “telephone” already existed for speaking tubes and what
we would now call a megaphone). These early experimenters could transmit
tones, buzzing, and some vowel‑like sounds, but none could transmit
intelligible speech.
Around this time, Alexander Graham Bell was
experimenting with his own system. His transmitter used a diaphragm attached
to a needle partially submerged in diluted acid and powered by a battery.
His receiver used a diaphragm attached to a coil of wire placed near a
permanent magnet; changes in current through the coil caused the diaphragm
to vibrate. Each end of the system had a transmitter and a receiver. Like
their contemporaries, Bell and his assistant Thomas Watson were unable to
convey intelligible speech.
On March 10, 1876, while Bell was
adjusting the liquid transmitter, he accidentally spilled acid on himself.
Instinctively, he called out, “Mr. Watson. Come here. I want to see you.”
Watson rushed in and reported that he had heard Bell’s words clearly through
the receiver in the next room.
Bell and Watson did not record
exactly how the system was wired or what happened at that moment. However,
the historical consensus is that the circuit remained intact after the
spill, and Bell’s voice was picked up not by the liquid transmitter but by
the electromagnetic receiver in Bell’s room, which was still connected and
acted as a transmitter. Up to that moment, no one had realized that such a
receiver could also function as a microphone. Bell and Watson abandoned the
liquid transmitter and demonstrated a working telephone using identical
electromagnetic instruments at both ends. This system required no battery,
since speech vibrations induced the current directly.
Bell's telephone transceiver
Credit for the invention of the electric telephone has long been controversial. Many others were working on similar ideas and later claimed priority. But after hundreds of lawsuits and more than a century of historical research, most historians agree that Bell and Watson were the first to transmit intelligible speech electrically.
Edison improves the carbon microphone
In 1877, Thomas Edison patented his version of the carbon microphone. Edison’s transmitter used loosely packed carbon granules held between two metal electrodes, with the front electrode acting as a diaphragm that vibrated in response to sound waves. As the diaphragm moved, it changed the pressure on the granules, causing their electrical resistance to vary. This produced a clean, linear modulation of the battery current, yielding a much stronger electrical signal than earlier transmitters.
Edison’s carbon microphone was quickly adapted to Bell’s telephone system, which had previously relied on battery‑less electromagnetic transmitters that generated only weak signals. With carbon transmitters, powered by a battery added to each of Bell’s electromagnetic receivers, telephone conversations became loud enough and clear enough for practical use. Early telephone repeaters mechanically coupled an electromagnetic receiver to a carbon microphone to boost the signal.
Hughes discovers radio waves
In 1878, David Hughes discovered that an induction balance (essentially an early metal detector) produced faint clicks in a telephone across the room whenever one of the telephone’s battery connections was slightly loose.
Hughes' radio transmitter and receiver
Familiar with Maxwell’s theories, Hughes believed he had detected Maxwell’s predicted invisible electromagnetic waves. But when he demonstrated the effect to the Royal Society, they dismissed it as ordinary induction, already known from the work of Henry and Faraday. Today, we know that the loose connection acted like a primitive semiconductor junction, turning Hughes’s telephone into what we would now call a crystal receiver.
Hertz proves radio waves exist
In the late 1880s, Heinrich Hertz, a German physicist, succeeded in producing Maxwell’s invisible electromagnetic waves in the laboratory.
Schematic of Hertz' transmitter and receiver
Familiar with Maxwell’s theories, Hughes believed he had detected Maxwell’s predicted invisible electromagnetic waves. But when he demonstrated the effect to the Royal Society, they dismissed it as ordinary induction, already known from the work of Henry and Faraday. Today, we know that the loose connection acted like a primitive semiconductor junction, turning Hughes’s telephone into what we would now call a crystal detector.
Laming and Stoney discover and name the electron
Marconi develops commercially viable radio broadcasting
Early Marconi transmitter
His equipment used rotating switches and spark gaps to create radio signals that were mostly just radio noise. However, by turning the transmitter on and off with a telegraph key, he was able to send usable information. In 1897 he formed The Wireless Telegraph & Signal Company in the United Kingdom, which became the Marconi Company.
Tesla and Westinghouse design and build the first hydroelectric power station
Ontario Power Plant at Niagra Falls
The advantage of alternating current is that it can be stepped up to extremely high voltages efficiently through transformers (Invented by Henry and Faraday). They are just coils of wire wrapped around each other and iron cores. The high voltage can deliver power over long distances efficiently where it is stepped back down to safer voltages through more transformers. This was the beginning of modern electrical power distribution. The unit used to measure magnetic field strength is named after Tesla.
Thomson isolates the electron, shows the direction of electrical current
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| A Crooke's tube, showing a Maltese Cross and its shadow. The electrons are coming from the small end of the tube toward you and to your left. The positive electrode is below the cross. The electrons streaming from the cathode (negative electrode) cannot make the turn to the anode (positive electrode) and strike the cross and the end of the tube. The electrons that miss the cross strike the glass, causing the glass at the large end to glow. |
Contrary to everyone's educated guess, Thomson showed that electricity flowed from the negative side to the positive side. By this time the labels of positive and negative electricity were firmly ingrained in the scientific lexicon. Thompson showed that electricity flows from negative to positive rather than from positive to negative as previously thought. Thompson also showed that the electrical flow could be restricted to the point that individual particles could be detected. Not knowing that these particles had already been predicted to exist and had already been named "electrons" (by Laming and Stoney), Thompson called these particles electric corpuscles.
Fleming invents the vacuum tube (electric valve)
Fleming's first diodes
A metal cylinder, called the plate, surrounds the glowing filament. If a high voltage is placed between the filament and the plate—negative to the filament and positive to the plate—electrons will flow from the filament to the plate. However, if the voltage is reversed, electrons are repelled from the plate back to the filament and there is no electrical flow. Like Braun's crystal rectifier, the diode acts like a check valve allowing electrical flow in only one direction.
Alexanderson vastly improves on Marconi's radio transmitters
Alexanderson electromechanical radio transmitter
These had a significant advantage over Marconi's transmitters in that they produced oscillations (variations of voltage or current at regular intervals) at a single frequency. Marconi's transmitters produced radio noise of many frequencies at once.
We can use light to illustrate how Marconi's and Alexanderson's transmitters differed. Marconi's transmitters produced the equivalent of white light. Remember, from elementary school science, that white light contains all the colors of the visible spectrum. You can look at white light through different colored filters, and you will still see the light, except tinted to the color of the filter. If you look at white light through a red filter, you will see red light. If you look at the same white light through a green filter, you will see green light. If you send a message, by flashing the light (using Morse code), you will get the same message through either filter.
Alexanderson's transmitters produced the equivalent of a single color. Let's say one transmitter produces only red light, another only green light, yet another only blue light, etc. Now let's say we want to send messages to different people in a room by flashing theses colored lights (using Morse code as above). Just to complicate things, let's say has their backs are to the lights, so they can't look at the individual lights. All they can see is the light bouncing off the walls and ceiling. We also need to send all the messages at the same time. Can you see the problem? It will be impossible for anyone to distinguish his or her message from the chaos of flashing colors.
Now let's say that the people who are to receive the message from the red light have red filters. Looking through the red filters, they will see only the red light. Likewise, the people receiving the green message have green filters. The others have blue filters. Distinguishing the messages is now easy. Electronic filters can separate radio waves in the same manner.
Two Marconi transmitters cannot operate anywhere near each other at the same time. You cannot separate one signal from another. Multiple Alexanderson transmitters can be used near each other as long as they rotate at different speeds, thus creating radio signals at different frequencies.
Fessenden transmits voice over radio
Fessenden AM transmitter
Talking into the microphone caused the power output to vary, changing its characteristics to match the voice characteristics. This is called amplitude modulation (AM). It worked much like Bell's telephone except over radio instead of wires.
Rutherford and Bohr develop a new model of the atom
Rutherford-Bohr model of an aluminum atom
The Rutherford-Bohr model is now mostly obsolete but is still useful to model how energy is held by electrons. The electrical and chemical properties of any element are primarily determined by the electrons in the outermost orbital shell, known as the valence shell.
De Forest invents amplifying vacuum tube
Early triode vacuum tubes
This "control grid" is given various amounts of negative charge and controls the flow of electrons. With the triode, a small change in voltage on the control grid can cause a large change in electrical current between the filament and the plate. This allowed the development of fully electronic amplification (as opposed to electromechanical amplifiers based on the carbon microphone).
All-electronic radio transmitter (no moving parts)
GE engineers show Marconi the transmitter that will make his obsolete
Marconi's transmitters were soon outlawed because a single Marconi transmitter would monopolize a large geographic area. The new oscillator-based transmitters could operate at different frequencies. Filters on a radio receiver could reject the unwanted signals. Multiple radio broadcasts could cover the same geographic area by operating on different frequencies (see the discussion on Marconi's and Alexanderson's transmitters above).
Armstrong invents modern radio receiver
Farnsworth invents all-electronic television (no moving parts)
An early Farnsworth-type television receiver
The intensity of the beam would be varied to "paint" an electronically produced picture on a layer of material that glowed when hit by the electrons. He demonstrated the first all-electronic television, with no mechanical spinning disks or oscillating mirrors, in 1927. Vladimir Zworykin is often credited with inventing electronic television for RCA. However, Farnsworth had demonstrated his system to Zworykin while negotiating with RCA to license his inventions. When Farnsworth refused to sell his inventions to RCA for a flat fee, Zworykin copied much of Farnsworth's system. Patent interference suits brought by RCA against Farnsworth were found in favor of Farnsworth.
Armstrong invents FM radio
Bardeen and Brattain invent the transistor
In 1947, John Bardeen and Walter Brattain at AT&T's Bell Laboratories made a device with two gold contacts on a germanium crystal. It was essentially one of Karl Braun's point contact diodes with two contacts very close together. A small change in electrical current from one contact to the crystal caused a large change in the current from the other contact to the crystal. This solid-state device (a single piece of material, not containing a vacuum) became known as the transistor. It amplified electrical signals like the vacuum tube triode but had many advantages over it (i.e., no hot filament, no high voltage, smaller, cheaper to mass produce). William Shockley is often credited with inventing the transistor. However, he participated little in the development other than being Bardeen's and Brattain's boss.Fairchild Co. invents the integrated circuit
First integrated circuit
Today, ICs are made with millions of transistors and other devices on a single silicon wafer (chip). This has led to miniaturization and low cost that was unthinkable before.
Biard and Holonyak invent the light emitting diode (LED)
In 1961, James R. Biard and Gary Pittman discovered near-infrared light coming from a tunnel diode. They developed this into an efficient infrared emitting diode. In 1962, Nick Holonyak Jr. invented a visible red LED while working for General Electric. These solid state light are very efficient and long-lasting. Early LEDs were best suited as indicator lights. Red and Green LEDs were available for many years before Shuji Nakamura invented high brightness blue LEDs in 1994.
 Light emitting diodes |
Any color of the visible spectrum can be created with red, green and blue LEDs. Most white LEDs use a deep violet LED to illuminate a phosphor that converts the violet light into white light. The cost of high wattage white LEDs has dropped dramatically in recent years and they are becoming the choice for area illumination.
This history is certainly incomplete, but it covers the basic principles and inventions that are the basis for most electronic equipment today.
Video Lecture 1
A Brief History of the Discovery of Electricity
| Vocademy |