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

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