The Mystery of the Torpedo Fish and the Concept of Voltage

A “shocking” fish

The common torpedo, or eyed electric ray, is a species of fish found in the Mediterranean Sea and the eastern Atlantic Ocean. It has an unusual capability of stunning its prey, with what we now understand to be an electric shock.

Common torpedo
Image: Roberto Pillon. CC-BY 3.0.

The torpedo was known to ancient Greeks and Romans who believed it to possess magical and medicinal properties. Fishermen who were unfortunate to catch a torpedo sometimes felt as if they had been struck hard, and at other times felt a numbness in their arms. The torpedo’s “shock” is painful to a human, but not dangerous.

But is it electrical?

The study of electricity made rapid progress in the mid 1700s. In the latter half of the century, electrical scientists inevitably became interested in the torpedo’s shock. They were familiar with the electrical shock from a Leyden jar (the first capacitor). The torpedo’s shock seemed eerily similar.

Leyden jar
Image: Public domain

However, despite the similarity, several scientists refused to accept that the shock from the fish was due to electricity. There was one nagging problem. The Leyden jar always produced a spark when it was discharged, but there was no spark when the torpedo struck.

Cavendish distinguishes between “quantity” and “degree”

Henry Cavendish, perhaps the most enigmatic character in 18th century science, was the one to crack the problem. He correctly reasoned that the key to understanding the torpedo’s shock was to distinguish between the “quantity of electricity” and the “degree of electrification”.

Henry Cavendish
Image: Public domain

Cavendish painstakingly demonstrated this distinction using multiple Leyden jars of different sizes. He showed that a big jar and a small jar, when charged to the same degree as shown by an electroscope, gave shocks of different strengths. The bigger jar gave a stronger shock for the same “degree of electrification”. Yet they both produced sparks of the same length.

In other words, he argued that the size or even the occurrence of a spark was independent of the strength of the shock.

Simulating the torpedo’s shock

Some other scientists might have stopped at this theoretical explanation, but not Cavendish. He set about trying to recreate the torpedo’s shock artificially, without the spark.

Cavendish connected several Leyden jars together to form a battery of jars with a very large effective capacity. He then charged his battery of jars to a very low degree of electrification, as indicated by his electroscope. When he touched the jars, he got a strong shock. And as he had expected, there was hardly any spark. It was so tiny that it could be seen only through a magnifying glass.

Not satisfied with just the battery of jars, Cavendish went even further. He made an artificial torpedo fish using wood and leather, immersed it in wet sand and hooked it up to his battery. He wanted to recreate all the conditions of the torpedo’s strike, not just the shock without the spark.

A schematic diagram of Cavendish’s artificial fish
Image: Public domain

Cavendish invited a group of distinguished electrical scientists including Joseph Priestley to come and try out his artificial torpedo. The guests played with the new equipment and were very impressed. Even those who had been disinclined to believe the torpedo’s shock to be electrical were convinced.

Stepping stone for the quantification of electricity

The new concepts that Cavendish introduced perhaps seem vague and not well defined. It is true that his “degree of electrification” is some distance away from the mathematical definition of electric potential and potential difference (or voltage), which had to wait till the early years of the 19th century.

Still, the distinction between “quantity” and “degree” was an important first step towards the equation Q = CV (capacitance formula) that every physics student would know today.

Reference

  1. Henry Cavendish. An Account of Some Experiments to Imitate the Effects of the Torpedo by Electricity. Philosophical Transactions of the Royal Society 1776.
  2. James Clerk Maxwell (edited). The Electrical Researches of the Honourable Henry Cavendish.
  3. J. L. Heilbron. Electricity in the 17th and 18th Centuries.

Ostwald’s “Electrochemistry: History and Theory”

Hardbound English translation published in 1980 by Amerind Publishing Co., New Delhi

Today I thought of writing about a rare history-of-science book that I have in my collection. Wilhelm Ostwald, after whom the nitric acid manufacturing process is named, wrote an exhaustive history of electrochemistry in 1896. The book was in German, and was titled Elektrochemie, ihre Geschichte und Lehre (Electrochemistry: its History and Teaching).

I came to know about Ostwald’s book from Hasok Chang’s Is Water H2O?, where it was repeatedly cited in discussing the controversy over how to interpret the electrolysis of water. Only the original German edition was available on the Internet Archive. After a bit of searching, I stumbled upon a used copy of an English translation on Amazon’s US store, and immediately bought it.

When I got the two-volume edition, I was intrigued to learn that it was translated into English by an Indian, N. P. Date. It was published for the Smithsonian Institution and the National Science Foundation by a company called Amerind Publishing Co. in New Delhi. The publishing company does not seem to exist any more, according to online records.

What the book covers

Volume 1

The first volume begins with the early history of electrochemistry in the mid 1700s. Back then, scientists had observed chemical effects of static electricity such as the formation of oxides of nitrogen on passing an electric spark through air, and the decomposition of water.

However, the history of electrochemistry truly begins with the work of Luigi Galvani, who is best known for his legendary experiment on making a dead frog’s leg twitch electrically. The debate between Galvani and Volta, whether the frog’s leg moved due to “animal electricity”, led to the invention of Volta’s “pile” – the first electrochemical battery.

Not only was the pile the first continuous source of electric current, but its invention spawned a fresh controversy over how it worked. Volta and his followers believed that the current originated from the simple contact of dissimilar metals, while another group of scientists believed that a chemical reaction was the cause. The resolution of this question would take more than 50 years and spur new discoveries in electrochemistry.

The first volume closes with the pioneering work of Michael Faraday which systematised and quantified the study of electrochemical reactions. The gist of this work would be known to students of grades 11 and 12 as Faraday’s Laws of electrolysis.

Volume 2

The second volume enters a more modern period of research that includes the development of new types of electrochemical cells, the application of quantitative concepts such as conservation of energy to electrochemical reactions and the measurement of electrochemical potential.

It also tracks the development of a new theoretical framework to explain the conduction of electricity in electrolytes, culminating in the pathbreaking work of the Swedish chemist Svante Arrhenius whose theory of electrolytic dissociation is accepted even today.

***

Ostwald’s text is a rare work that exclusively focuses on this important branch of science that straddles physics and chemistry. Hopefully, the book will be made more accessible in the future, either as a reprint or in the Internet Archive’s digital library.

A Window into the Stomach: Dr. Beaumont’s Experiments with the Gastric Juice

William Beaumont’s experiments on his patient Alexis St. Martin’s stomach in the 1820s are probably well known. St. Martin’s gunshot wound left him with a hole in the stomach that wouldn’t close. Moreover, it formed a fistula with the skin as the wound healed, which meant that he was left to live with a direct opening into his stomach.

Dr. Beaumont tried to heal the wound but when it wouldn’t, realised that he had a unique opportunity to study what happened to food inside the stomach. What followed was a decade of weird experiments that involved extracting the gastric juice, analysing half-digested food taken from the stomach, dipping small pieces of different kinds of food directly into the stomach and so on. Dr. Beaumont showed that the process of digestion was chemical in nature, and not just a mechanical breaking up of food.

Dr. Beaumont published his findings in a book titled Experiments and Observations on the Gastric Juice, and the Physiology of Digestion in 1838.

This is a story from the history of science that’s beautifully used in the class 5 NCERT Environmental Studies textbook to introduce students to the human digestive system. I vaguely knew the story earlier, but have got the opportunity to revisit it given that I regularly flip through the NCERT textbooks as part of work.

Recently, I read a bit more about the doctor’s experiments and discovered that there is a lot more that is fascinating, than just the experiments. There is a darker subplot to the story, that involves the human element of subjecting a person to such experiments.

St. Martin recovered from his injury and was capable of doing physical labour again. Dr. Beaumont got the illiterate man to sign a contract to perform domestic chores and become a guinea pig for the doctor’s experiments. After two years of discomfort at having foreign objects thrust into his stomach, St. Martin managed to escape to his native Canada in 1825. There, he got married and started living a normal life.

Dr. Beaumont did not give up, and tracked down St. Martin in 1829. He was successful in convincing St. Martin to go back with him, with the promise of housing and employment in return for allowing him to continue the experiments.

In 1833, St. Martin and his family left Dr. Beaumont for good, and they would never meet again. Though it was a path-breaking study, it remains a question of ethics why Dr. Beaumont never performed the simple procedure of sewing up the fistula. St. Martin lived to the end of his days with a hole in his stomach.

References

  1. From Tasting to Digesting. NCERT Environmental Studies Class 5.
  2. William Beaumont. Experiments and Observations on the Gastric Juice and the Physiology of Digestion.
  3. George Burden. St. Martin and Beaumont.
  4. Lissa Edwards. The Gruesome Medical Breakthrough of Dr. William Beaumont on Mackinac Island.

The Circular Reasoning of “Verifying” Ohm’s Law in School

The experimental set-up

A common experiment done in high school is to verify Ohm’s law. Ohm’s law, or rather one part of it, states that the current through a resistance is directly proportional to the difference in potential (voltage) between its two ends.

The diagram below represents the circuit that is often used for verifying this relationship. A fixed resistance R is connected to a battery in series with a variable resistance (rheostat). A voltmeter is connected in parallel to R and an ammeter in series.

The resistance of the rheostat is varied to get different sets of values of voltage and current which may then be plotted on a graph to show that they are proportional.

Do voltmeters actually measure potential difference?

The fallacy arises from how voltage is measured. The analog voltmeters commonly used are actually modified galvanometers in which a magnetic needle is deflected depending on the strength of the current. In fact, voltmeters make use of Ohm’s law to convert the needle’s deflection to a voltage reading in the first place!

Digital multimeters work in a different way, but they too depend on current that’s drawn from the circuit, and their calibration too likely involves the application of Ohm’s law at some point.

It is therefore apparent that it makes little sense to “verify” Ohm’s law using measuring instruments that are themselves designed on the basis of the same law.

Axiom for Ohm, Hypothesis to be tested for Kohlrausch

What Georg Simon Ohm did was to define a new quantity called potential difference for a circuit element. Electric potential is essentially an electrostatic quantity and Ohm used it to explain what happened in a circuit with a continuous current.

Ohm himself never intended to try to measure this potential difference. However, Rudolf Kohlrausch did exactly that, a quarter of a century later.

Kohlrausch used a special, sensitive electrometer to directly measure the electrostatic potential difference between the ends of a resistance and showed that it was indeed proportional to the current.

His work was published in 1849 as a paper titled Die elektroskopischen Eigenschaften der geschlossenen galvanischen Kette (The Electrostatic Properties of a Closed Galvanic Circuit). Unfortunately, an English translation of the paper does not seem to be available.

Reference

  1. Nahum Kipnis. A Law of Physics in the Classroom: the Case of Ohm’s Law
  2. R. Kohlrausch. Die elektroskopischen Eigenschaften der geschlossenen galvanischen Kette. Annalen der Physik vol 78

Eunice Newton Foote: the Woman who Foresaw Global Warming

A short paper titled “Circumstances affecting the Heat of Sun’s rays” appeared in the 1856 edition of the American Journal of Science and Arts. It was only a page and a half long, and had a brief and crisp description of three experiments.

Effect of density

The first one studied how the warming of air by the Sun’s rays was affected by its density. Two identical containers, one with rarefied air and the other with condensed air, were kept side by side in the sun.

The author, Eunice Newton Foote, found that the temperature of the condensed air rose significantly more (to 110 oF) than that of the rarefied air (88 oF).

Effect of humidity

In the second experiment, Foote used one container with humid air and another with dry air and saw that the former became more warm.

Warming of different gases

The final experiment described in the paper compared the warming of different gases as compared to air. Foote noted that a jar filled with carbon dioxide (or “carbonic acid gas” as it was known then) got heated much more than one with common air.

She postulated that a greater proportion of carbon dioxide in the atmosphere could cause an increase in temperature, effectively predicting global warming as we know it today.

Unknown until 2010, Celebrated in 2018

Foote’s work was not widely recognised until as recently as 2010, when Ray Sorenson, a retired geologist stumbled upon her paper. Sorenson realised that Foote’s paper preceded by three years the work of John Tyndall, who had been previously credited with recognising the greenhouse effect of carbon dioxide.

In 2018, 162 years after her landmark paper, Eunice Newton Foote’s work was commemorated in a symposium at the University of California Santa Barbara. A charming short film titled Eunice was also released in the same year to honour the forgotten scientist.

References

  1. Wikipedia article on Eunice Newton Foote. https://en.wikipedia.org/wiki/Eunice_Newton_Foote
  2. New York Times. Overlooked No More: Eunice Foote, Climate Scientist Lost to History.
  3. https://thinkprogress.org/female-climate-scientist-eunice-foote-finally-honored-for-her-contributions-162-years-later-21b3cf08c70b/
  4. American Journal of Science and Arts 1856.

Nahum Kipnis on the 7 Benefits of a Historical-Investigative Approach to Science Teaching

In the introduction to his book Rediscovering Optics, physics educator and history of science scholar Nahum Kipnis lucidly summaries what he considers to be the main benefits of a historical-investigative approach to science teaching – an approach in which historical experiments are repeated and the findings reconstructed by the students. I paraphrase them below.

1. Doing something “real”

Most laboratory work in school can be described as “cookbook experiments”. There are pre-determined procedures to be followed with expected pre-determined outcomes. The students are assessed on how well they can follow the procedure and analyse the data obtained. It has very little in common with a real scientific investigation.

Recreating a historical experiment, on the other hand, places the activity in a context. The students can be introduced to the question that was being investigated by scientists in the past, and can carry out the experiment as if they were trying to find the answer themselves. Reliving an important event in the history of science may also be exciting to many students.

2. As smart as Galileo and Newton!

Students tend to ask questions and harbour preconceptions that are often remarkably similar to the ones early scientists did. In repeating historical experiments, the students can also independently arrive at conclusions similar to those put forward by these scientists. This could enable students to see themselves as scientists, and boost their self-confidence and interest in learning science.

3. Doing, not watching

This is not specific to the historical-investigative approach, but generally students engage better if they themselves carry out an experiment rather than simply watch it demonstrated by the teacher. In this approach, however, it becomes imperative since the students are trying to find an answer to a question on their own.

4. The scientific method in practice

By carrying out a series of experiments and deducing scientific concepts independently, the students are enacting a microcosm of the discoveries made in the past. This would give them a taste of how new knowledge is constructed by the interaction of theory and experiment.

5. “Obsolete” theories can help in understanding

Many old theories are based on simple models. This makes them more easily accessible than modern theories to students who are introduced to a new topic for the first time. The students can analyse and critique them while they make sense of the phenomena under study.

For example, while teaching electrostatics to class 6 students, I have found it helpful to completely sidestep the modern knowledge of subatomic particles and use the 18th century conception of two electrical fluids – resinous and vitreous.

Teachers need not feel that they are teaching something “wrong” by tapping into the potential of the old theories. History of science tells us that no theory is final.

6. Qualitative, not quantitative

Until the mid-1800s, most scientific experiments were qualitative. Many of the quantities and units that we obsess over in high school science were yet to be precisely defined. Several important physical laws were derived from rather crude measurements.

In a historical approach, this opens the door for setting up a much wider variety of experiments than the standard ones, using commonly available materials. Aspects of experimental work like precise measurements and estimation of error which are given a lot of attention in the modern labs are of much less consequence for early experiments.

7. Treasure trove of stories

History of science is full of fascinating stories that are sure to captivate students, whether they are interested in science or not. They also reveal the messy human side of science, which is totally ignored in a conventional approach that focuses on the mastery of modern scientific knowledge.

Ampere’s Alternate Interpretation of Electromagnetism

A. K. T. Assis is a Brazilian scholar who has written some intriguing books on physics and the history of physics. He has very generously made all these books available for free access on his website – https://www.ifi.unicamp.br/~assis/books.htm.

I have been flipping through the book Ampere’s Electrodynamics by Prof. Assis. It requires a much closer study, but I wanted to write about one thing that stood out for me.

All this while, I had thought of the development of electromagnetism as a linear timeline. I had thought that starting with Oersted’s experiment, other scientists had added bit by bit to the understanding of electromagnetic phenomena, until Maxwell synthesised all the knowledge into one single theory. This book showed me that the actual history was very different.

It seems that there were many differences of opinion between Ampere and the other prominent scientists of the time regarding how to interpret the phenomena. The following snapshot from the book’s Contents pages promises an enticing discussion of these scientific controversies.

The most remarkable aspect of Ampere’s theory was that he imagined all magnetic phenomena as arising from electric currents. In this, almost all his contemporaries disagreed with him vehemently.

For Ampere, there was no ‘magnetic field’ – it was simply a manifestation of an interaction between currents. He hypothesised that there were circular electric currents inside magnets and inside the Earth, which we know today to be true.

The following muse (edited for readability) from Ampere’s writings is a striking example of the role of imagination and creativity in science:

“Suppose we had known that a magnetic needle is influenced by an electric current into a position perpendicular to the wire before knowing that a magnetic needle points to the geographical north. Then, would not the simplest idea and the one that would occur immediately to anyone who wanted to explain it be that there is an electric current inside the Earth?

Andre Marie Ampere

Assis’ book also includes a complete English translation of Ampere’s classic work written in 1826, Theory of Electrodynamic Phenomena, Uniquely Deduced from Experience.

I hope to write more on this when I do manage to study this book in detail.

When Young Faraday’s First Discovery Led to Charges of Plagiarism

Two Faradays and two electromagnetic discoveries

Michael Faraday is one of the most well known names in science, probably even for people outside science. He made two landmark discoveries which led to the invention of the electric motor and the electric generator — two devices which we benefit from, every single day of our lives. But equally legendary is Faraday’s fairytale journey from being a bookbinder’s apprentice to the assistant of Britain’s biggest scientist of the time, Humphry Davy.

The relationship of Faraday and Davy — master and servant, professor and student, and eventually and inevitably rivals — is a fascinating story in itself. A story that reveals a lot about not just the two individuals but also the quirks of English society at the time.

The two discoveries in electromagnetism referred to above were not made together. In fact, they were separated by a decade. And they were made by two different Faradays. The first by a 30-year old lab assistant with his head brimming with ideas and craving for an intellectual outlet; the second by a 40-year old established scientist, elected to the Royal Society and successor to the illustrious position previously occupied by his mentor.

It is the first discovery that is the subject of this article.

Electromagnetic Rotations

In the summer of 1820, Danish scientist H. C. Oersted had discovered that a magnetic compass needle is deflected when kept near a current-carrying wire. News of this momentous discovery soon reached all parts of Europe, and it reached Humphry Davy too. Faraday got the opportunity to learn about the phenomenon assisting Davy in his recreation of Oersted’s experiment, and it captivated him.

By now, Faraday had been in the service of Davy and the Royal Institution for over eight years. From the very first days, Davy had recognised the brilliance of Faraday’s intellect. As Faraday assisted Davy in the laboratory, the two would engage in conversations that played a big part in Faraday growing into a scientist in his own right.

The deflection of the magnetic needle was a hot topic of discussion that year. Faraday had probably been a spectator if not a participant in the conversations Davy had with his friend William Hyde Wollaston on the subject. Wollaston thought that it must be possible to somehow produce continuous motion using magnets and electric currents, but had failed in his attempts.

After his work hours, Faraday played with wires and batteries and magnets. One day, he put together a strange apparatus. He fixed a long magnet in the middle of a cup and filled the rest of the cup with mercury. Above the magnet and the cup was a support from which a piece of wire was hanging freely, with one end dipped in the mercury.

Faraday connected one terminal of a battery to the hanging wire through its support, and the other terminal to the mercury in the cup. The moment the circuit was completed, the hanging wire began going around the magnet, in circumambulation. Faraday had got what he was looking for — continuous electromagnetic rotations!

Announcement and indignation

This should have been the moment when Faraday became a celebrated scientist, but it wasn’t.

Faraday excitedly called on Wollaston to tell him about the electromagnetic rotations, but Wollaston was out of town. Faraday knew that he had discovered something extremely important, and couldn’t wait to communicate it to the world. He wrote a 16-page article titled “On Some New Electro-Magnetical Motions” and naively sent it to the Quarterly Journal of Science for publication.

Within days, all hell broke loose. Members of the scientific community accused Faraday of stealing Wollaston’s ideas while the latter was still working on them. Faraday was also condemned for not acknowledging the knowledge he gained by assisting Davy in his experiments.

Davy did not overtly criticise Faraday. However, his silence when he could have doused the raging controversy suggests that he felt the criticism was deserved. Wollaston himself was graceful, and invited Faraday to come and discuss his new experiments. Wollaston must have realised that his own ideas were somewhat different from Faraday’s electromagnetic rotations.

Into an exile from electromagnetism

Faraday received the support of scientists in continental Europe, and he quickly grew in stature. However, in London, he was still Davy’s servant.

Over the next few months and years, Davy kept Faraday busy with many non-scientific tasks. Even in science, Davy assigned work in other areas of research that made sure there wasn’t much time for Faraday to build on his work in electromagnetism. Faraday’s diary entries of the 1820s reveal that he hardly did any electromagnetic experiments for the rest of the decade.

Still, Davy couldn’t stop the emergence of Faraday as a scientist. In 1823, Faraday’s name came up for election as a Fellow of the Royal Society. Davy staunchly opposed the nomination, and for the first time publicly condemned Faraday for his indiscretion of a couple of years earlier. Faraday, however, had the support of a majority of the members including Wollaston, who clearly did not hold any grudge against him.

Nevertheless, it was only after the death of Davy in 1829 that Faraday could truly get back to the work that he was destined to do. This time, in 1831, Faraday would go on to discover how electric currents could be produced by the motion of a magnet and a wire. Even today, most of our electrical energy is produced using this principle uncovered by him.

Reference

James Hamilton. A life of discovery : Michael Faraday, giant of the scientific revolution

Electrostatics in Middle-School – Part 3: Early Electrical Knowledge in the Classroom

Read the previous parts if you haven’t: Part 1 Part 2

In the previous article, I briefly introduced the major developments in the knowledge of electricity up to the first half of the 17th century. So, what lessons and activities for the classroom can be designed based on this?

I intend to describe one possible approach here. I leave it to the creativity and imagination of readers who may be teachers to think of better ways. I hope you’ll come back and share your ideas in the comments if you get inspired by this and come up with other ideas to try out!

From plastic ruler to amber

Every student would invariably have rubbed a plastic ruler on hair and observed tiny paper bits flying towards it. I would tell my students that people in ancient Greece had done the same thing with amber, as a starting point for the narrative. Although amber itself is a rare and expensive material and difficult to come by, some students would have read about it and would be eager to share with the class what they knew about it.

Is it magnetism?

It might seem like a trivial thing, but asking students whether they think that this attraction happens due to magnetism is a good way to get them warmed up for scientific inquiry. The students will be forced to articulate why the attracting power of the rubbed plastic ruler cannot be magnetism. And if you’re lucky, you may even have one student in class who asserts that it is magnetism and states her reasons — then it won’t be a trivial discussion any more!

Depending on how the debate unfolds, there may be opportunities to get the students to design simple experiments to check if a rubbed plastic ruler has any properties of a magnet.

  • Does a rubbed plastic ruler have poles? Are the paper bits attracted more strongly to the ends as in the case of iron pieces to a magnet?
  • Does the rubbed plastic ruler have any directionality? Does it align in the north-south direction if suspended by a thread?

Which other materials become attracting when rubbed?

The students can then repeat William Gilbert’s experiments to find out which materials around them get this attracting power when rubbed. I would get the students to make their own simple versorium by hanging a pencil from their desks with a thread. They would then pick up every substance they could find around them and check whether they can make the versorium rotate when rubbed. Some students might start exploring the question of whether the material used for rubbing matters.

Gilbert’s versorium
(Image: Public domain)

This activity is great fun, but it also quickly reveals the difficulties and limitations of scientific inquiry in the classroom. What the students will learn from the activity depends a lot on what materials they have with them to test with the versorium.

Ideally there would be several materials that are electrics and several that are non-electrics. However, this is not so easy in practice. Apart from most plastics like polythene, PVC, etc. there aren’t too many common materials that easily become charged on rubbing. In very dry weather, you can succeed in charging candle wax, a rubber eraser and sometimes paper — enough to detect with a versorium.

One question I was faced with was whether I should choose the set of materials I wanted the students to experiment with. I could make sure that there were different kinds of electrics and non-electrics in the set. But wasn’t that cheating? Wasn’t that in a way pre-determining what the students would “discover”? Could I even say that the students were “discovering” anything?

This is a complex question and I’ll share my thoughts on it in a later post. Here, it would suffice to say that in such situations, I’ve almost always tilted towards a pre-determined outcome — mostly due to my own personal discomfort with not knowing how a class will unfold!

Attraction and repulsion

One last experiment that can be done by the students based on this part of the story is on electrical repulsion. This can be demonstrated the first time, in case the students do not stumble upon this phenomenon on their own in the limited time they have.

It is actually the same experiment with the plastic ruler, but using small bits of aluminium foil instead of paper. With bits of aluminium foil, repulsion happens in a far more noticeable way when the pieces hit the ruler. Sometimes you can even get a foil piece to dance up and down between the ruler and the table!

Students can be asked to form their own hypotheses about why the aluminium foil pieces sometimes rebounded.

Electrostatics in Middle School – Part 2: Early Knowledge of Electricity

Read Part 1 if you haven’t.

Amber and Lodestone

Since antiquity, people have been fascinated and mystified by rare materials with special properties. Two such materials were amber — a fossilised resin from extinct pine-like trees — and lodestone — an iron-containing mineral that we now recognise as permanently magnetic.

The ancient Greek (4th century BCE) knew about the property of amber to attract light objects like feathers when rubbed. They also knew that lodestone could attract small bits of iron. Amber and lodestone were often mentioned in the same breath, because their powers to attract objects were strikingly similar.

An ant preserved in amber
Image: Anders L. Damgaard. CC-BY-SA

Jerome Cardan, a 16th century Italian mathematician and physician, was one of the first to make a clear distinction between the action of amber and lodestone. He noted that rubbed amber attracted any light object while lodestone attracted only objects made of iron. He also observed that the lodestone’s attraction was the strongest at its ends (poles) while amber seemed to have no such polarity.

Such distinctions between amber and lodestone remained scattered until the end of the 16th century. It was William Gilbert — Queen Elizabeth’s physician — who first studied electrical phenomena independently. His book De Magnete (“On the Magnet”) was published in 1600. The book was mostly about magnetism, but included Gilbert’s experiments on electricity. It is ironic that electricity was discussed only as a part of one chapter on magnetic attraction, yet it established the study of electricity as a separate branch of science.

William Gilbert demonstrating electrical experiments to Queen Elizabeth.
Image: Public Domain

Not just amber

Perhaps Gilbert’s greatest contribution to the study of electricity was his discovery that not just amber, but a large number of other substances too showed similar attractive powers when rubbed. His list included precious stones such as diamond, sapphire, carbuncle, opal and amethyst, artificial materials like glass, minerals like fluorspar from the mines, fossils like belemnites, sulfur and hard sealing wax.

Gilbert coined a new term for such materials which displayed the amber effect — he called them “electrics“.

The first electrical instrument

Gilbert also invented a handy little device to identify whether a substance was an electric or not. It consisted of a thin metal needle suspended such that it could rotate freely. If a rubbed electric was brought near one end, the needle promptly turned towards the electric. He called it the ‘versorium’ – Latin for ‘turn-about’.

Gilbert’s Versorium
Image: Public Domain

The versorium enabled Gilbert to detect even electrics with very weak attracting power, because the needle needed very little force to rotate on its support. Before the versorium was invented, an electric could be identified only if it directly attracted small objects, which needed a stronger force.

Do electrics repel too?

Thus far, all the studies on electricity only mentioned the power of attraction. In 1629, an Italian scholar named Niccolo Cabeo first reported that when a light object was attracted to an electric, often it rebounded to a distance of several inches after touching the electric. This seems to have been the first observation of electrical repulsion. Cabeo found this strange behaviour hard to explain, and the rebounding remained a mystery.

No significant progress in electrical research took place for almost another 100 years. Electricity continued to arouse the curiosity of scientists during this time, and several intriguing experiments were carried out. However, given that my primary purpose is to lay out a convenient narrative for teaching, I choose to skip them here.

***

In the next article, I will examine the lessons and classroom activities that can be designed based on the early discoveries discussed so far.

Reference

Duane Roller and Duane H. D. Roller. The Development of the Concept of Electric Charge: Electricity from the Greeks to Coulomb. Harvard Case Histories in Experimental Science – Volume 2