Physics demonstrations: Faraday disk

I’m prepping a new course to teach this semester: undergraduate Electromagnetism II!  I’m trying to put together some nice simple demos to illustrate principles in the class, and I’ll blog some of those that work and are interesting.

When Michael Faraday discovered the phenomenon of electromagnetic induction in 1831, paving the way for the complete unification of electricity and magnetism, he came up with a variety of experiments to demonstrate the effect.

One of them is now known as the Faraday disk, and it is very easy to construct — though I ended up buying one.  My version is shown below.


All of the parts are visible in this photo.  By turning a hand crank, one rotates a copper disk between a pair of magnets (the black disks), one generates an electrical current that runs from the outer edge of the disk to the central axis.  Wires connected to these two points runs to the red and black plugs, through which one can measure the voltage difference generated.  It isn’t a spectacular amount — I measured about 5 millivolts, max — but it demonstrates the phenomenon known as rotational electromotive force.

As we have noted, however, Faraday interpreted his disk experiment as electromagnetic induction, not rotational emf.  It turns out that he was kinda wrong — but he was also kinda right!  The explanation of this simple experiment involves some rather deep concepts in physics, and inevitably leads us to Einstein’s theory of special relativity.

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7 years of blogging!

I’ve been rather busy with my academic duties lately and haven’t had much time to update my blog with new science and fiction posts.  Because of this, I almost completely missed the fact that August 14th marked the 7th anniversary of this blog!  My first post was on August 14th, 2007, with the title “Educate or bust,” based on the name of an obscure Robert E. Howard story.  That title still captures the spirit of what I try to do here at “Skulls in the Stars,” and what I hope to continue doing in the future.

With this in mind, here’s a few teasers of posts I’m planning and working on for the next few months.  This list is partly to keep people intrigued, but also to keep me inspired!

  • A physics demonstration in which I attempt to generate “frozen lightning” in the form of a Lichtenberg figure.
  • A somewhat poetic look at the importance of early physics discoveries related to heliocentrism through the writings of its biggest advocates.
  • A history of physics post about the very first “vortex of light” discovered in the 1950s, and the strange optics that inspired its discovery.
  • An in-depth post about the most important physics research that nobody ever talks about.  (This one has been in the works for years.)
  • A “how-to” discussion about my favorite social media platform, twitter, and how to use it in a productive way.
  • The optics of impossibility!  Many things that traditionally were thought to be literally impossible in optics have now been demonstrated experimentally (usually with some important caveats).  This post will survey how these discoveries are changing the technological possibilities with light.
  • A look at one of the strangest planets in the Solar System, and probably not the one you think.
  • Some blogging about my own research into invisibility and cloaking, after I finally get the papers written and published!
  • More blog posts about the history of electricity and magnetism, particularly in the 19th century.  The first of these I’ve been looking at is the work of Heinrich Lenz.

I’ve got lots of ideas — I just need to find the time to work on them!  I’d also like to thank everyone who follows or subscribes to my blog for your interest.  Hopefully, the next seven years will be as fun as the first!

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Fred Hoyle’s “The Black Cloud”

I am utterly fascinated by active scientists who also write fiction, particularly science fiction.  There have been more of them than the average person realizes, including physicist Robert W. Wood, who co-wrote The Man Who Rocked the Earth (1915) and The Moon-Maker (1916), and astronomer Simon Newcomb, who wrote His Wisdom the Defender (1900).  I would also add to this list millionaire inventor John Jacob Astor, who wrote A Journey in Other Worlds in 1894.  The tradition continues to this day, as illustrated by my friends Blake Stacey, who wrote Until Earthset (2008), and Andrew David Thaler, who wrote Fleet (2013).

There are still more out there, I’m sure, that I have yet to come across.  This was demonstrated to me recently, when I encountered astronomer Fred Hoyle’s 1957 novel The Black Cloud.

First edition cover of The Black Cloud, via Wikipedia.

First edition cover of The Black Cloud, via Wikipedia.

I learned of this book through the always excellent Valancourt Books, who will be releasing a new edition in 2015.

Set in the year 1964, the novel focuses on the efforts of an international group of scientists as they try and save humanity from a massive black cloud that is approaching the solar system from interstellar space.  You’ll notice that I say “save humanity” instead of “stop the cloud,” because there is no stopping the cloud: it is an object of planetary scale, and the best mankind can do is anticipate its behavior using the laws of physics and attempt to plan accordingly.

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Emily Carroll’s Through the Woods

Let me say this immediately: Emily Carroll’s work is amazing.  Her work first came to my attention, as it did for many people, through an io9 post describing some of the best horror webcomics available.  The post included a link to Carroll’s story His Face All Red, which was by far the best of them, and I was hooked.  Since then, I’ve kept a constant vigil for the occasional new comic to appear on her site.

Happily, Carroll recently released a printed collection of almost all new horror comics, Through the Woods.



The collection includes five stories, four of which are previously unpublished, and the tales are bookended by an introduction and a conclusion that are equally creepy.

The five stories are:

  • Our Neighbor’s House.  Three children are given explicit instructions by their father on what to do when he goes hunting in the woods.  Of course, they don’t quite follow these instructions…
  • A Lady’s Hands Are Cold.  A young woman is forced by her father into marriage with a wealthy lord.  When she arrives at his manor, she learns that he — and the home — hold terrible secrets.
  • His Face All Red (previously published online).  A man and his older brother head into the forest at night to find the monster that is killing the village animals.  The horrors they discover, however, are completely different.
  • My Friend Janna.  Janna and her friend have made a great game out of pretending to speak to the dead.  Toying with the spirit world has terrible consequences, as they will learn.
  • The Nesting Place.  When boarding school lets out for the summer, Mabel goes to stay with her brother and his fiancée in their country home.  Rebecca — the fiancée — seems perfect, so why does Mabel feel uncomfortable around her?  And why does their housekeeper tell Mabel not to go in the woods — ever?

Carroll’s stories tap into what I can only call primal fears.  They feel like the darkest of fairy tales, and often follow a very similar structure.  The illustrations are elegant and often very subtle, conveying a sense of dread with color and shadow.  Carroll, like all great horror writers, knows that showing a monster is nowhere near as scary as hinting at it.

The book is rather short — you can read the entire thing within an hour if you like — but the tales merit repeated reading.  I’ve already read it through two times, and before it was available I read His Face All Red at least a dozen times online.

In conclusion: I very much recommend Through the Woods.  I’m looking forward to more works from Emily Carroll, and hope I won’t have long to wait.

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H.G. Wells’ stories about BUGS

Update: Added one more Wells bug story!

This short post is something of a public service.  Earlier today I saw some tweets from film critic Scott Weinberg referencing an urban legend related to the very silly 1977 Bert I. Gordon film Empire of the Ants.  I had never heard of the movie before, and I certainly didn’t know that the movie was based on a short story by the incredible science fiction author H.G. Wells!

I had never heard of the story before, though this is not surprising — Wells was ridiculously prolific, writing dozens of novels and non-fiction books over the course of his life, in addition to short stories.  Most people are only familiar with a handful of his most famous works – The Invisible Man, The Time Machine, The War of the Worlds, The Island of Doctor Moreau — though he wrote many intriguing and forward-thinking novels such as his chilling story of nuclear war, The World Set Free.

So I was curious about the story, and of course it is available to read online from in a variety of places.  Being me, however, I had to go right back to the source.  Thanks to the magic of Google books, I was able to find the original 1905 issue of The Strand magazine containing the story.  One of the great things about these Edwardian magazines is that they often illustrated the stories in question, and The Empire of the Ants is no exception.

Emboldened, I also dug up another H.G. Wells story about bugs — The Valley of Spiders, which appeared in a 1903 edition of Pearson’s Magazine.  Wells was quite famous by this point, and got quite a spectacular title image:



So my public service of the day?  To provide pdfs of the two illustrated stories!  The Valley of Spiders is quite a bizarre and fascinating tale, while The Empire of the Ants reminds me somewhat of John Wyndham’s much later novel, Web.

The Valley of Spiders

The Empire of the Ants


Update: Being obsessed with being thorough, here’s one more Wells story about a bug, though of a significantly different nature!  It apparently first appeared in the late 1800s, but I found an illustrated version of it in a 1905 volume of Pearson’s.

A Moth — Genus Novo

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Physics demonstrations: Geiger counter

Update: Fixed a couple of incorrect statements regarding cosmic rays and the radiation of uranium.  Thanks to encephalartos for the corrections!

In recent months, I’ve been diving wholeheartedly into learning how to build and design electronics.  My ultimate goal is to build a Tesla coil, but before I do, I’ve been warming up with a variety of kits and designs online.

Not too long ago, I learned that it is possible to buy a kit to build a basic Geiger counter, for only around $100!  I jumped at the opportunity and, after some minor modifications, started checking for radioactivity!



If you compare this with the image of the original kit below, you can see that I’ve protected all of the circuitry in a plastic case.  I also added an external switch as well as a spiffy drawer handle from Lowe’s.


So you probably know that a Geiger counter detects radioactivity, but how does it work — and what sort of things can you detect?  I thought I would write a short post discussing this, ending with a video showing my Geiger counter in action.

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Cloaking from earthquakes?

There has been a lot of excitement among researchers about the science of invisibility over the past decade, and a variety of designs of invisibility cloaks have been suggested since the groundbreaking 2006 papers.  I’ve talked a lot about invisibility on this blog, but I haven’t said a lot about one of the most intriguing — and overlooked — possibilities: the use of cloaking to protect objects, instead of just hide them!

What do I mean?  Well, an invisibility cloak is, in principle*, a material structure that guides light around a central space — the cloaked region — and sends it on its way as if it had hit nothing at all.  An illustration of how this would work for light rays, from the original paper by Pendry, Schurig and Smith, is shown below.


The black lines represent the light rays, being bent around the cloaked region (inner sphere).  Such a cloak, however, is not limited to rays of light; it has been demonstrated that it guides waves of light perfectly, as well.

But if a cloak can be designed to deflect light waves, it stands to reason, then why not other types of waves, or fields?  Magnetic fields could be deflected, to protect sensitive electronics within.  Water waves could be deflected, to protect offshore platforms or buoys from damaging waves during extreme storms.  Or, maybe — just maybe — seismic waves from earthquakes could be guided around vulnerable buildings!

Recent research* suggests that protecting buildings from earthquakes might be possible, at least to a limited extent.  In experimental work published in April of this year, French researchers were able to screen, or protect, a region of earth from artificially generated seismic waves.

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