Category: History of Science

The invention of the Leyden Jar marked a significant moment in this history of electrical engineering. The Leyden Jar can be thought of as the first electrical capacitor – a device that stores and releases electrical energy.

The Invention of the Leyden Jar

During the 18th century the mysterious phenomenon of electricity was becoming a hot topic among learned men of science. Electricity could only be created and observed in the moment. One of the mysteries to be solved was whether electricity could be stored for later use and how to accomplish it. The invention that solved the mystery became known as the Leyden jar, named after the city of an early inventor of the device.

The Leyden Jar is typically credited to two individuals, who independently came up with the same idea.  In Germany, Ewald Georg von Kleist was experimenting with electricity.  He was attempting to store electricity with a medicine bottle filled with water and a nail inserted through a cork stopper.  He charged the jar by touching the nail with an electrostatic generator, and he assumed that the glass jar would prevent the electricity from escaping.  While holding the glass jar in one hand, he accidentally touched the nail and received a significant shock, proving that electricity was indeed stored inside the jar.  

Von Kleist experiments were not well known and around the same time another man experimenting with electricity named Peter van Musschenbroek, of Leyden, Netherlands, also stumbled upon the same invention.  Musschenbroek’s device was much like von Kleist’s jar.  It consisted of a glass jar filled with water that contained a metal rod through a cork sealing the top of the jar.  The outside of the jar was coated with a metal foil.  When an electric charge was applied to the metal rod it was found that electricity could be stored in the jar.  Unfortunately for the person touching the metal rod, a significant shock was received. As Musschenbroek recorded what happened when he first touched the rod:

Suddenly I received in my right hand a shock of such violence that my whole body was shaken as by a lightning stroke…. I believed that I was done for.

It didn’t take long until Musschenbroek’s Leyden jar being used and improved by others.  In 1746, the following year, the English physician William Watson improved the jars storage capacity by coating both the inside and outside with metal foil.  Also that same year, the French physicist Jean-Antoine Nollet discharged a Leyden jar in front of the French King Louis XV.  During the demonstration, Nollet arranged a circle of 180 Royal Guards, each holding hands and he passed the charge from the Leyden jar through the circle.  The shock was felt almost instantaneously by all members of the Royal Guard, to the delight of the King and his court.  Demonstrations such as this brought widespread attention to the exciting new field of electricity.  

Impact and Legacy of the Leyden Jar

Prior to the invention of the Leyden jar, electricity could only be observed and experimented at the moment it was created.  The Leyden jar changed this by allowing scientists to store electrical energy and use it when needed.  Researchers could now conduct various experiments related to electric discharge, conductivity, and other electrical phenomena.  As an added bonus, the jar was easily transported, especially compared to the electrostatic generators of the day.  The jars could be linked together to provide additional storage capacity.  These abilities contributed to the growth of knowledge in the field of electrical science.  

For one example, the American scientist and statesman Benjamin Franklin famously used the Leyden jar during his kite experiment in 1752.  In that experiment, Franklin flew a kite during a lightning storm in an attempt to prove that lightning was a form of electricity.  He attached a metal key to the end of the kite, and the key was then connected to a Leyden jar.  Despite some popular accounts of the experiment, lightning likely never struck the kite directly or else Franklin would have been killed. However, he was able to observe that the Leyden jar was being charged, thus proving the electric nature of lightning.  

The Leyden jar is also considered the first electrical capacitor, which today is a fundamental component in modern electric circuits.   The invention of the Leyden jar laid the groundwork for the development of more sophisticated capacitors.  While they briefly feel out of use after the invention of the battery, the basic idea of the Leyden jar capacitor found a renewed use at the end of the 19th century in modern electronic devices, albeit in a much smaller form. 

Overall, the Leyden jar played a pivotal role in the early explanation and understanding of electricity.  Its impact can be seen in the subsequent development of electrical technology and science.  

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The neutron was discovered by the British physicist Sir James Chadwick in 1932, marking a pivotal moment in the understanding of atomic structure. It was, in a way, the culmination of a series of scientific investigations of the subatomic particles of the atom that spanned several decades. The identification of the neutron provided answers to questions about the mass of the atom, ultimately leading to important developments in nuclear physics.

Developments that Led to the Discovery of the Neutron

In 1897, J. J. Thompson discovered the electron, a subatomic particle with a negative electrical charge. This discovery provided the evidence that atoms were composed of smaller particles. Two decades later, Ernest Rutherford discovered the proton, a subatomic particle with a positive electrical charge. Rutherford proposed a model of the atom with a dense, positively charged nucleus at its center, orbited by negatively charged electrons. However, this model presented a problem. The positively charged protons in the nucleus should all repel each other, causing the nucleus to burst apart. Yet, this did not happen as the nucleus is obviously stable, and the reasons for this stability were not understood at the time. The existence of a neutral particle was postulated by Rutherford as early as 1920.

The Discovery of the Neutron

This discovery of the neutron was the culmination of a series of successive experiments, worked out by several scientists in the late 1920s and early 1930s. In Germany, Walter Bothe found that beryllium exposed to alpha particles produced a new form of radiation. Bothe attempted to explain this sradiation in terms of gamma rays, because it was not deflected by either electric or magnetic fields. All known particles at the time (electrons and protons) contained a charge.

Taking this information a step further, the French husband-and-wife team of Irene Joliot-Curie (the daughter of Pierre and Marie Curie) and Frederic Joliot reported results from an experiment in January 1932 that led to to neutrons discovery. In the same vein as Bothe, their experiment involved the bombardment beryllium by alpha particles. They noticed that the radiation could eject protons from hydrogen-rich substances such as paraffin.  This was puzzling because gamma rays should not have enough energy to be able to knock out protons in this manner. In other words if this unknown radiation was indeed gamma rays then the law of conservation of energy was being violated.

Back at the Cavendish Laboratory in Cambridge, James Chadwick, a college or Rutherford, quickly became interested in these results.  Chadwick and Rutherford had been working on and off over the past decade in identifying the missing neutral particle suspected to be in the atomic nucleus.  This background allowed Chadwick to move quickly.  He also conducted a series of experiments where he bombarded light elements, such as beryllium, with alpha particles.  He noticed the same radiation being emitted which was not deflected by electric or magnetic fields.  However he interpreted the results differently than the others who conducted similar experiments. His observations led him to correctly conclude that the radiation was composed of uncharged particles. He used the laws of conservation of momentum and conservation of energy to calculate that the neutron has a mass similar to that of a proton.  He presented this as evidence of a new subatomic particle, which he detailed in a paper published in 1932 and named the neutron.  For his work he was awarded the Noble Prize in Physics in 1935.  

Impact of the Neutron’s Discovery

The discovery of the neutron was a key piece of the puzzle that allowed scientists to understand the binding energy of the atomic nucleus.  It had enormous impacts in both applied and theoretical physics.  

The discovery of the neutron explained the missing mass in atomic nuclei.  This in turn helped to explain the existence of isotopes – variants of elements with the same number of protons but different atomic weights – and therefore different numbers of neutrons in the atomic nuclei.  It also advanced the understanding of radioactive decay processes.  

The most important impact of the discovery of the neutron was in nuclear physics.  The neutron – a particle without an electric charge – was the crucial component in the development and study of nuclear fission, which occurred in 1938 by Otto Hanh and Lise Meitner.  The development of nuclear fission was quickly applied the development of nuclear energy and weapons.  The neutron plays the key role in the chain reactions that occur in both nuclear reactors and atomic bombs.  It is probably fitting then, that James Chadwick was placed as head of the British team that work on the Manhattan Project that produced the world’s first atomic bomb. 

Beyond nuclear fission, the discovery of the nucleus aided in our understanding in the nuclear processes that power the stars through the process of nuclear fusion.  The study of this process had advanced our understanding on the origins and evolution of the elements.  

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It has been famously stated that if Albert Einstein hadn’t published his theory of special relativity, someone else likely would have within the decade. But if Einstein hadn’t published his theory of general relativity we would still be waiting on that discovery to this day. The theory of general relativity, proposed by Einstein in 1915, is one of the most profound and revolutionary achievements in the history of science and transformed our understanding of space, time, and gravity.

Background on the Theory of General Relativity

For nearly two centuries, Isaac Newton’s law of universal gravitation stood unquestioned and unchallenged in the field of physics.  However, around the end of the 19th century some tiny, yet vulnerable, kinks in its armor were beginning to emerge.  Physicists noticed it was unable to explain certain phenomena, such as the orbit of Mercury.  They were also struggling to reconcile classical mechanics with new observations of the nature of light and electromagnetism.   A relatively unknown physicist at the time named Albert Einstein provided the solutions to these vexing problems.

Einstein’s journey towards his theory of general relativity began in 1905 with the publication of his theory of special relativity.  Special relativity primarily focuses on objects moving at a constant speed.  It ignores acceleration, or objects affected by gravity, which Einstein determined are essentially the same thing after he formulated general relativity.  

Between 1907 and 1915 Einstein worked extending of his theory of special relativity to include gravity.  The mathematical complexity of this task proved enormous, and so Einstein had to learn various advanced mathematical techniques to complete his ideas.  He worked with other mathematicians the help him understand the underlying math needed to formulate his ideas. 

Space, Time, and a Sliver of Curvature

In November 1915, Einstein presented his revolutionary ideas on general relativity to the Prussian Academy of Science. His theory describes how mass and energy curve the fabric of spacetime, effectively producing the force of gravity.  Therefore, a major implication of general relativity is that it redefined gravity from a force acting at a distance to a description of gravity as a geometric property of spacetime. The core of general relativity is the field equations, which explain the geometry of four-dimensional spacetime to the distribution of mass and energy within it. These equations are known as the Einstein field equations.  

General relativity also implies an equivalence principle.  This states that the effects of gravity are locally indistinguishable from the effects of acceleration. In other word, someone in a closed space cannot determine whether they are feeling the effects of gravity or acceleration.  A classic example is the elevator thought experiment.  Imagine you are in an elevator with no windows.  If you drop an object it falls to the floor at an acceleration of 9.81 m/s^2 due to earth’s gravity.  However, you could also be in distant space, far away from any gravitational force accelerating at a constant rate of 9.81m/s^2.  In the second scenario when you drop an object it will also fall to the floor at an acceleration of 9.81m/s^2 – the effects of the two scenarios are indistinguishable.  

Testing and Acceptance

General relativity has been repeatedly verified by observation and experimentation.  One of the first key confirmations of general relativity came in 1919 during a solar eclipse expedition by Sir Arthur Eddington.  To goal of the expedition was to measure the apparent shift of the stars near the sun during the solar eclipse.  Indeed, the observed shift matched the predictions made by general relativity.  

Another significant test of general relativity was the perihelion precession of Mercury’s orbit.  Mercury’s orbit exhibits a small deviation in the orientation of its elliptical orbit over time, a phenomenon known as perihelion precession.  Prior to general relativity, astronomers struggled to account for the advance of this phenomenon.  Newtonian classical mechanics failed to explain the discrepancy.  However, when Einsteins equations of general relativity were applied to this problem, they yielded an answer that matched the observed rate of precession.  This successful prediction, along with the results of Eddington’s experiment, provided crucial empirical evidence for the validity of general relativity.  General relativity also predicts other phenomenon such as gravitational redshift and time dilation, both of which have been confirmed through experiments as well.

As empirical validation rolled in, general relativity gradually began to gain acceptance within the scientific community.  By the latter half of the 20th century, general relativity had become fully accepted and became one of the two twin pillars of physics – along with quantum mechanics.  The acceptance of general relativity can be viewed as a watershed moment in the history of science, marking the transition from the Newtonian worldview to that of a relativistic worldview.  

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The existence of a form of light other than visible light was inconceivable prior to the 19th century. However, in nature there exists a continuum of radiation waves, all traveling at the same speed of light, with a nearly infinite possibility of wavelengths and frequencies. This is the electromagnetic spectrum, and the first new form of light was unexpected discovered in 1800 by the British scientist William Herschel.

The Discovery of Infrared Rays

William Herschel is best know for his discovery of the planet Uranus but his interests also extended beyond the visible universe. Herschel was fascinated by the nature of heat and light and his involvement in this area of study lead to the discovery of a completely new and unexpected form of light.

In 1800 Herschel conducted an experiment where he directed sunlight though a glass prism to create a spectrum of visible light. He then measured the temperature of the different colors of light, where he noticed the temperature increased as he moved from the violet to the red end of the spectrum. Then in a moment of true scientific curiosity, he took his experiment one step further. He decided to measure the temperature just beyond the red light, but where no sunlight was visible. This region showed the highest temperature of all, and this led Herschel to conclude that there must be some form of invisible light present that we cannot see. He named this invisible radiation infrared, from the Latin ‘infra’ meaning “below” – or in this case below the red in the spectrum.

The importance of this discovery can hardly be overstated. It added an entirely new dimension to our perception of the universe and to this day has had significant practical applications in a variety of fields such as astronomy, telecommunications, healthcare, and environmental science. It also suggested that there may be other forms of light yet to be discovered.

A Continuous Spectrum of Electric and Magnetic Fields Oscillating Together

Shortly after Herschel discovery of infrared rays, the German chemist Johann Wilhelm Ritter discovered another of invisible rays. Ritter was obviously inspired by Herschel’s discovery of infrared rays, and he decided to experiment at the opposite end of the spectrum, beyond violet. Ritter conducted his experiment by also focusing sunlight through a glass prism to create a spectrum of colors. He noticed that silver chloride, a chemical known to darken when exposed to sunlight, darkened faster when exposed to the region beyond violet light. He had discovered what would later become known as ultraviolet rays, from the Latin ‘ultra’ meaning “beyond” – in this case beyond violet in the spectrum. The discovery of ultraviolet and infrared rays further prompted the discovery of other types of electromagnetic rays.

In the 1860s the British physicist James Clerk Maxwell revolutionized the world of physics with his electromagnetic theory. He developed four mathematical equations, known as the Maxwell Equations, that described how electric and magnetic fields interact. These equations unified the previously separate fields of electricity and magnetism. His new theory suggested that light was a form of electromagnetic wave, and that it was just a smaller part of a much broader spectrum. This spectrum is known as the electromagnetic spectrum. In 1867, Maxwell predicted that there should be wavelengths of light longer than the infrared rays discovered by Herschel.

It took another two decades, but in 1887 the German physicist Heinrich Hertz confirmed Maxwell’s predictions when he discovered radio waves in his laboratory. Light with wavelengths of a shorter length were soon discovered next. In 1895 Wilhelm Roentgen accidentally discovered what became known as X-rays while experimenting with cathode ray tubes. It took another two decades until scientists were able to determine that these X-rays were indeed another form of light. The last part of the spectrum to be discovered was gamma rays. The French physicist Paul Villard discovered these in 1900 while he was studying radiation emitted by radium.

Far Reaching Technological Impacts

The discovery of the electromagnetic spectrum has had far reaching impacts on 20th century technology and beyond. The development of radio and television technology was predicted on the understanding of radio waves. In similar fashion, the invention of radar during World War II was possible due to the understanding of microwaves.

From science and technology to medicine and daily life, the impacts of the electromagnetic spectrum are almost too vast and numerous to mention. But to name a few – radio and television broadcasting, mobile phones and the internet, X-rays used in healthcare, space telescopes and satellite-based sensors are just a few of the many examples of the areas in which electromagnetic radiation have had an impact on civilization.

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Plate tectonics is a scientific theory that explains the movement of the Earths surface and many of its most prominent geological features. It is responsible in forming the deepest trench in the ocean to the tallest mountain on land – and in the ocean as we’ll soon see.  The uppermost layer of the Earth is called the lithosphere and is composed of large rocky plates that are on top of a lower molten layer of rock called the asthenosphere.   A convection current is generated between the two layers, causing the plates to glide at a rate of a few centimeters per year.  While it may not seem like much, given enough time this process has formed many of the geologic features of our planet.  It has resulted in the formation of the Himalayas and the separation of the South American and African continents. 

History of Plate Tectonics

The theory of plate tectonics is a relatively new idea, however fragments of ideas can be found in earlier times. In 1596 cartographer Abraham Ortelius observed that the coastlines of Africa and South America could be fitted together like pieces of a jigsaw puzzle.  He speculated that the continents may once have been joined, but have since been ripped apart by earthquakes or a flood.  Creationist have ceased on this idea to “prove” the existence of Noah’s Flood.  It does provide for an arresting image, as Richard Dawkins points out, of South America and Africa racing away from each other at the speed of a tidal wave. In the 1850s there was a noticeable correlation of rock type and fossils discovered in coal deposits across the two continents.  1872 saw the mapping of the Mid-Atlantic ridge. 

In the early 20th century a few versions of a continental drift began making an appearance but there was one that stood out in its influence and the development of the Earth sciences. In 1912 Alfred Wegener published two papers where he proposed his controversial theory on Continental Drift.  He suggested that in the past all landmasses were arranged together into a supercontinent that he called Pangaea (meaning “all lands”).  However he did not have a geological mechanism for how the landmasses drifted apart and the idea was met with much skepticism first. Over a period of decades Wegener continued to amass evidence for his theory and continued to promote his model of continental drift. He gathered evidence from paleoclimatology to provide a further boost to the idea. He noticed glaciations in the distant past occurring simultaneously on continents that were not connected to each other and were outside the polar region. Unfortunately, while in Greenland on an expedition for some of this data, Many of his specific details have turned out to be incorrect but his overall concept that the continents were not static and did move over time has been proven correct.

During the middle of the 20th century more evidence began to support the idea that the continents did move. Huge mountain ranges were being mapped on the ocean floor. It was assumed by both supporters and opponents of continental drift that the sea floor was ancient and would be covered with huge layers of sediment from the continent. When samples were finally obtained it showed all of this was wrong. There was hardly any sediment and that the rocks were young, with the youngest rocks being found near the ocean ridges. In 1960 the American geologist Henry Hess pieced all of this evidence together in his theory of sea-floor spreading. The ocean ridges were produced by molten lava rising from the asthenosphere. As it rose to the surface the magma cooled, forming the young rocks, and spreading the ocean floor in conveyor belt like motion through the slow process of convection. While some parts of the Earth were creating new oceanic crust and spreading the continents apart, other parts were doing just the opposite. Along the western edge of the Pacific for example, thin layers of oceanic crust were being forced beneath the thicker layer of crust, being driven down into the mantle below. This explains the presence and high frequency of earthquakes and volcano’s in places like Japan.

By the beginning of 1967 the evidence of continental drift and sea-floor spreading was quite strong and ready to be assembled into a complete package. That year Dan McKenzie and his colleague Robert Parker published a paper in Nature introducing the term plate tectonics for the overall package of ideas that describe how the Earth’s plates move and the resulting geological features it creates. To this day many details are still being flushed out but this moment marked a revolution in the Earth Sciences. It is relevant now only in geology but also in its synthesis with the evolution of life on Earth.

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