Posts by Bill Higgins

From the Cabinet of Physics: Better and Better Spectra
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In the past, I remarked on the Cabinet of Physics demonstration using a single-prism spectroscope to view the continuous spectrum of a hot carbon-arc light source.

Christoper Graney compared this demonstration video to another where a spectroscope reveals “emission lines” in gases heated by electrical discharges.

In today’s demonstration, we examine an improved spectroscope, comparing it to the single-prism spectroscope we have seen before. As the nineteenth century marched on, instrument-makers developed devices more precise and easier to use. Here we have a specially-shaped prism, mounted on a turntable, which gives greater dispersion to the incoming light and “stretches out” the spectrum, so that it is possible to see more detail.

In the eyepiece of the older spectroscope, the entire spectrum of light from a copper arc is visible: a continuous glow from the heated vapor, punctuated with the bright emission lines characteristic of copper.

The fancier instrument dispays only a portion of the spectrum. As the user turns a drum that rotates the prism on its turntable, we tour the spectrum. Colors parade past; first red, then orange, then green, then blue. The relative position and brightness of copper’s glowing emission lines can be seen more clearly. And markings on the drum allow the user to read off the wavelengths of features in the field of view.

One can readily imagine that quantitative work, to measure and compare spectra of various substances, is made easier by such a spectroscope. It became a valuable tool for analytical chemistry. As the technique of spectrocopy developed, as we know, it was to pave the way for new discoveries in astronomy as well.

Here’s another demonstration of an advanced spectroscope.

This one uses four triangular prisms which can rotate in a synchronized dance to produce a wide dispersion of colors. The demonstration gives a good look at the famous pair of yellow lines in the spectrum of sodium. This instrument also superimposes an illuminated scale on the field of view, so that the wavelength of a spectral feature may conveniently be read. I note that this display also advertises its maker, the instrument-shop of Jules Duboscq in Paris—which was also the source of other equipment to the Cabinet's collection.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

From the Cabinet of Physics: Riding Along with Foucault’s Pendulum
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The pendulum named for Léon Foucault (1819–1868) can be found in museums and tall buildings here and there across the world.

(Here in Illinois, there was for many years a Foucault pendulum swinging in the majestic 15-story atrium of Wilson Hall, Fermilab's headquarters building. When I worked in that building, I saw it every day. But it's no longer there; the pendulum was removed in 2010.)

For best effect, a Foucault pendulum usually is usually suspended from a long cable. At a glance, the pendulum is moving in a flat vertical plane. Each moment, the straight line formed by the cable is a line in this plane; each moment, the location of the pendulum bob, at its bottom, occupies a point in this plane.

As one watches the pendulum swing slowly, back and forth, back and forth, there is a subtle change. The plane of its movement appears to shift very slightly. Watch it for hours (or come back hours later), and a considerable movement will be apparent. A swing that started out, for example, moving north-to-south and swinging back south-to-north will, many hours later, become east-to-west.

The reason for this slow shift is this: While the pendulum is moving in an unchanging plane, the Earth beneath it is rotating, as are the walls and floor of the room.

In this video, we see a small Foucault pendulum in the Cabinet of Physics collection. It's mounted on a rotating platform. We can clearly see the behavior of the pendulum. The effect becomes more dramatic when our video camera is attached to the platform, riding along with the rotation. Then, from this new point of view, the plane of the pendulum's appears to shift relative to the platform—just as the big Foucault pendulums in museums do.

At the time the Foucault pendulum was invented, nobody really doubted that the Earth rotates. Nevertheless it remains an enjoyable demonstration, and can perhaps get students of physics and astronomy thinking about rotating coordinate systems.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

From the Cabinet of Physics: Chladni Sees Sound with Sand
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I enjoy the sort of scientific demonstrations we might call "illustrations:" they make visible something about sound, or heat, or motion that would otherwise pass unseen. Here is an example from the field of acoustics.

Out of the Cabinet of Physics come brass plates, of various shapes and sizes, each supported on a single leg at its center. An object like this can be persuaded to make a sound—one might thwack it with a spoon, perhaps, and hear it chime. In today's video, it is more effective to stroke a plate with a violin bow.

Sound is produced by motion. When stroked to produce a tone, some parts of the plate move more than others.

We see pale sand being sprinkled onto the dark plates. A sand grain that lands on a rapidly-moving portion of the plate will be jiggled, and will bounce to another location, perhaps to be bounced again. If it lands on a location that is scarcely moving, it will remain there, safe from further jiggling.

By this process, grains of sand tend to accumulate in the special locations or "nodes" that are not moving as the plate continues to vibrate with a particular tone. The spiky patterns of sand reveal the special geometry that's connected to that tone. These patterns are named for Ernst Chladni (1756—1827).

If the strokes of the bow excite a different resonance, making a higher note, the Chladni figure changes as the sand grains rearrange themselves along the new nodes.  Different shapes and sizes of plate also yield different Chladni figures. Even a paper membrane, not touched by the bow but excited only by sound passing through the air, exhibits its own Chladni pattern. Normally the processes that create sound occur invisibly, but in this demonstration, something about sound has become visible.

By the way, Ernst Chladni is renowned for his contribution to acoustics, but he might be familiar to the scientists of the Vatican Observatory for a different reason.  Chladni argued in 1794 that certain iron-rich minerals we now know as meteorites might originate beyond the Earth.  It's a startling idea, but as evidence was gathered over the next few decades, other scientists came to accept that Chladni was correct.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

From the Cabinet of Physics: The Coherer Jumps to Attention
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At the end of the Ninteenth Century, one exciting technical development was the study and application of "Hertzian waves." Physicists learned that the intimate link between electricity and magnetism allows invisible waves to be generated and, some distance away, detected.

Among the instruments in the Cabinet of Physics are quite a few associated with this revolution, which led to the "wireless" or "radio" industry. Today's video portrays the coherer, a key device in the detection of radio waves.

The coherer is a glass tube that holds iron filings or other metal. Ordinarily it is a poor conductor; when it is in circuit with the Grenet-cell battery on the right, a galvanometer detects negligible current flow through the circuit.

But if a suitably powerful Hertzian wave passes through the room, the tiny filings can be jerked into alignment. Suddenly the coherer is a good conductor, and the needle of the galvanometer is deflected.

Supplying the invisible radio wave are the supporting players in this drama, seen on the left: Leiden jars, the wheel-like Wimshurst machine that charges them, and the brass spheres of a spark gap.

The coherer is awkward to use; after each detection, one must tap the tube to jiggle the metal filings, so they again become a lousy conductor. Nevertheless, this invention enabled the first generation of wireless to move from laboratory apparatus to commercial success. I imagine that many of the Italian students witnessing this demonstration went on to careers in the new radio industry.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

From the Cabinet of Physics: Dressed for the Electrostatic Dance
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Here is a display of electrical forces that is more whimsical than profound. It may not add very much to our knowledge of physics. But I am fond of it.

A revolving Wimshurst machine, connected to two horizontal metal plates, produces an ever-changing charge on the plates, and thus a changing electric field in the space between them.

Between the plates are placed two lightweight figures made of pith, a cork-like substance. These mannequins are tiny, but they are elegantly dressed in fine 19th-century style.

When the machine is cranked, the puppets respond to the changing electric field between the plates by dancing. They hop up and down, alternately attracted and repelled by the nearby plates.

Any other small, light insulating objects would have served to illustrate this effect. But I think it's charming to watch characters dressed for a ball do the dancing.

Wimshurst machines have many uses, and have played a supporting role in several Cabinet videos we have previously featured. I imagine that typically, in a classroom lecture, this simple demonstration would be one of several electrostatic experiments the lecturer would show.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

From the Cabinet of Physics: Yanking on the Hemispheres of Magdeburg
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Here's a classic experiment involving air pressure, one of the earliest demonstrations placed online by "Florencefst," the Fondazione Scienza e Tecnica's collection of scientific YouTube videos.

The introduction is Italian; in English, it says "Hemispheres of Magdeburg," and indicates that this demonstration apparatus dates from the first half of the nineteenth century.

The top and bottom brass hemispheres are placed together. Next the bottom hemisphere, which incorporates a valve, is connected through a hose to a vacuum pump (which is not visible) and a mercury-filled pressure gauge (the glass device on the right). As the pump is operated, we see the level of mercury change, indicating that the pump is removing most of the air from within the hemispheres.

The valve attached to the bottom hemisphere is closed. Now the vessel contains a vacuum.  Well, it would be more precise to say that the air pressure within the hemispheres is much lower than the air pressure in the room outside, or that the hemispheres contain very little air.

In this state, it would take a very large force to separate the hemispheres. The total force caused by external air pressure is larger than the tiny force exerted by the small amount of air remaining within. Our demonstrator tugs on the apparatus, but is unable to pull the hemispheres apart.

But open the valve on the bottom hemisphere, and outside air rushes in.  Now the force inside is the same as the force outside, and it has become quite easy to pull the two pieces apart.

For centuries, such hemispheres have been employed to impress students with the sizable forces exerted by the invisible air around us.

Magdeburg Spheres by Gaspar Scholtz 1672 1024x448

In Magdeburg, two teams of horses try, but fail, to separate Otto von Guericke's evacuated metal hemispheres, circa 1657. Engraving by Gaspar Scholtz, 1672.

Why "Hemispheres of Magdeburg?" Because this experiment originated back in 1657, as researcher Otto von Guericke, of Magdeburg, Germany, fabricated a pair of copper hemispheres 50 centimeters in diameter. When von Guericke joined and pumped out these hemispheres, he showed dramatically that not even two powerful teams of horses could supply enough force to pull them apart. The demonstration was a spectacular sight to the onlookers in Magdeburg, and, in the centuries since, a vivid image to conjure in physics classrooms whenever the story is retold.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

From the Cabinet of Physics: Conveying Heat across Space
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Today's demonstration from the Cabinet of Physics appears at first to be about optics but, in a surprising twist (SPOILER ALERT), turns out to be about heat.

A curved mirror, a paraboloid, reflects the light of a candle flame to another curved mirror some distance away. The candle sits at the focal point of the first mirror. The second mirror brings the light to its own focal point; hold a white card at this precise spot, and you'll see an image of the candle flame.

Now the candle is replaced by a collection of burning lumps of charcoal. Across the room, a piece of tinder is placed at the focus of the second mirror. To the eye, the coals seem much dimmer than the candle flame. The mirrors are gathering not only the visible light, but also the more copious invisible infrared light emitted by the coals.

The unseen radiant heat is energetic enough to ignite the tinder. Soon it too is glowing red, and smoke is rising. It's a nice example of the close kinship between light and radiant heat.

Perhaps the novelist H. G. Wells had a demonstration like this in mind in 1897 when, in War of the Worlds, he imagined Martian invaders incinerating the English landscape with "heat rays."

This apparatus reminds me of a story. The physicist Theodore Taylor was responsible for a most peculiar application of focused heat radiation. At an observation post in a Nevada desert on 5 June 1952, Taylor was waiting for the test of a nuclear bomb he had designed.  He took a small parabolic mirror and attached a rig, made out of wire, that could hold the tip of a cigarette at the mirror's focal point.   He aimed the mirror carefully at the distant weapon. At the end of a countdown, intense visible and infrared light engulfed the test site. Miles away, Taylor withdrew his Pall Mall from the mirror and took a puff. He had invented the first nuclear-powered cigarette lighter.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

From the Cabinet of Physics: Joule, Electricity, Heat, and Light
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The English physicist James Prescott Joule (1818 – 1889) spent a lifetime working to understand the behavior of energy in its many forms. Today the Cabinet of Physics explores the Joule Effect, which concerns the connection between electricity and heat.

Joule showed that the heat energy created by electrical current passing through a circuit is proportional to the the square of the amount of current, and also proportional to "resistance," a measure of the ease with which current passes through a circuit.

Copper is a pretty good conductor, but nonetheless a copper wire has some resistance, and will heat up if we send many amperes of current through it. In today's video the copper wire gets hot enough to give off a dull red glow. It's not a very useful light source, but it's a hint toward the invention of an incandescent light.

Of all metals, silver is the best at conducting electricity in a circuit like this one,* so if we compare a piece of silver wire to a piece of platinum wire having identical dimensions, the silver wire will have a lower resistance, and the platinum wire will have a higher resistance.

This is made vividly apparent in today's demonstration. In the chain of linked sliver and platinum wires, the same amount of current passes though each segment, yet the platinum gets hot enough to glow, whereas the silver, having lower resistance, is much less hot, so is invisible in the darkened laboratory.

The Joule Effect is harnessed directly in such devices as toasters, electric heaters, and incandescent bulbs, which intentionally produce heat and light. In devices which put electrical currents to other uses, designers must still deal with the consequences of Joule heating; computers, for example, are intended to handle information, but many computers develop enough heat in their operation that they require fans to provide airflow to keep them from overheating.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments. The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

*Superconductors are an exception, but they require extremely cold conditions. Also, superconductivity is a Twentieth Century discovery, as yet unknown at the time most of the instruments in the Cabinet were being used to teach physics.

From the Cabinet of Physics: A Vocabulary in Iron
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A family of fascinating mechanisms appears in today's double-barreled video from the Cabinet of Physics. Each illustrates a clever method of transforming rotational motion in direction or speed.

Some students of Tuscany would become technicians who operated, maintained, and repaired the machinery of the Industrial Age. Others would become engineers who might design factory equipment, steamships, drawbridges, or locomotives. They observed these "kinematic models" to become familiar with the many mechanisms—some common, some exotic—in use within contemporary machines. In a sense, they were learning a kind of iron vocabulary. A designer could reach into this collection of ingenious mechanisms for a solution to a problem. Or, having studied such a great variety of mechanisms, an inventor might have the insight necessary to devise a new one, adding a new "word" to the language of machinery.

In the first video, my favorite is the pair of quadrilateral gears appearing at 1 minute 22 seconds. A square gear! And a cloverleaf!

A student of astronomy, viewing the elliptical gears at 1 minute 4 seconds, may be reminded of the planetary orbits Johannes Kepler described, moving around the Sun slowly, then quickly, then slowly, then quickly again, in a mesmerizing fashion.

In the second video, I particularly admire the mangle wheel seen at 1 minute 55 seconds. It's an ingenious way to reverse the direction of a wheel on each cycle.

I have remarked before that we often see curators in these Fondazione demonstrations turning handles. Today is another abundant example. The Cabinet of Physics seems to contain more cranks than a convention of the Flat Earth Society…

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

From the Cabinet of Physics: Sharing a Flea Together
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We recently saw the projector based on the Duboscq arc lamp. Today's video introduces a nifty optical attachment: a microscope. Intense light from the projector passes through a slide containing a prepared specimen. A system of lenses focuses a magnified image of the specimen upon a screen.

Using a microscope is ordinarily a solitary activity. Only one person sees what's on the slide. This device allows a teacher and students, in a darkened room, to examine specimens together. Not only is this valuable in teaching about the specimens on the slides, but it can also be helpful in training students how to see when they later use conventional microscopes.

In 1665, during the early days of microscopes, the English scientist Robert Hooke prepared an elaborate book of drawings, Micrographia, that would introduce readers to the newly-visible world of the very small. The showpiece of the book was Hooke's masterful drawing of a flea.

HookeFlea01

Robert Hooke's drawing of a flea, studied under a microscope. His classic 1665 book Micrographia was the first book published by the Royal Society.

In the centuries since, in textbooks, Hooke's famous flea has often introduced students to microscopy. So it seems fitting that in Florence, the slide we see projected is a flea. It's traditional!

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

From the Cabinet of Physics: In the Days before PowerPoint
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If you want to project images on a screen, you need two things: A bright light source, and an optical apparatus to direct and focus the light through the image. Three things, if you count the semi-transparent slide bearing the source image.

All three may be seen in today's demonstration from the Cabinet of Physics in Florence. We begin with the Duboscq arc lamp, introduced in 1851 (many years before Edison's incandescent light bulbs appeared). An electric arc across the gap between two carbon rods can produce an astonishingly bright light. But it can be erratic and diminish in brightness as the carbon is consumed. Léon Foucault and Jules Duboscq devised a feedback mechanism to adjust the gap and keep the lamp's brightness steady.

This reliable and intense light source is the heart of a "lantern" whose lenses can project an image of the arc on a white screen some distance away. A glass slide can be inserted into the focal plane of the optical system. Suddenly the tiny picture on the slide appears, much magnified, on the screen.

Such projectors became a boon to education and entertainment. Artwork or photographs could be shared witha roomful of people. A lecturer could transport an audience to the Piazza San Marco in Venice, as we see here.

Though the less-cumbersome incandescent light came along later in the 19th century, arc lamps remained useful for a long time afterward in specialized applications such as projectors or theatrical lighting. When I began lecturing, Kodak-style slide projectors with very bright incandescent bulbs were the standard tool. Eventually video projectors became common, and transparent film was replaced by video signals generated inside a laptop computer. Microsoft's PowerPoint software became as ubiquitous as Kodak projectors once had been.

The ability to show pictures to an audience remains a great blessing, and speakers—not least those who lecture about astronomy—should be thankful for it. The greatest challenge of my speaking career came shortly after the Cassini spacecraft delivered the Huygens probe to Saturn's largest moon. In the room where I was to speak, the projector's bulb had failed, and no replacement was to be found. Yet The Show Must Go On. I found myself describing the surface of Titan orally for the next hour. I got through it, with the help of adrenaline. But the experience made me more grateful to Messrs. Foucault and Duboscq and all of their fellow developers of display technologies.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.

From the Cabinet of Physics: Many Ways to Look at Centrifugal Force
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Today, in exploring the effects of centrifugal force, we see no fewer than seven gadgets demonstrated. Our curator does a lot of cranking.

1. As a spinning glass globe rotates faster, colored liquid within it rises to form a belt or ring around its middle.

2. An array of pendulums starts out hanging vertically, but the pendulums spread out as rotation speeds up.

3. A spring-loaded sphere, arranged to slide on a horizontal bar, moves outward as the bar spins, compressing the spring.

4. Two flexible strips of metal form two circular hoops, outlining an imaginary sphere. As they are spun, they are centrifugally deformed. We see that the shape they outline becomes less like a sphere and more like a pumpkin.

This demonstration makes a point of interest to planetary scientists: Many planets—Earth and Jupiter are good examples—bulge slightly at their equators, and are flattened at their poles, because of their rotation. The effect is more subtle than the dramatic bulging we see in the spinning hoops, but perhaps witnessing this apparatus helped students visualize an important piece of geophysics.

5. As glass vessel rotates, the flat surface of colored liquid within it changes in shape. The liquid's surface becomes a bowl shape, low in the middle, high at the edges of the glass—a paraboloid.

Since a parabolic shape is ideal for applications in acoustics, radio, and optics, such as making the mirror of a reflecting telescope, and an accurate parabolic shape is not easy to make, many an engineer has contemplated the surface of a spinning liquid and daydreamed.

In the 1980s at the University of Arizona's Steward Observatory, J. Roger P. Angel found a practical way to build a huge spinning furnace. He cast large telescope mirrors from molten glass. The speed of the furnace's rotation and the diameter of the mold determined the shape of the resulting paraboloid. Then the furnace kept spinning as the glass was allowed to cool. The glass solidified into the desired shape, ready for testing and polishing.

By this means, in 1985 Professor Angel's team produced a mirror 1.8 meters in diameter. This became the primary mirror of the Vatican Advanced Technology Telescope (VATT) on Mount Graham in Arizona. So astronomers of the Vatican Observatory may have a special reason to smile when they see the surface of a spinning liquid. Angel's group went on to cast mirrors for many more telescopes, including four 8.4 meter behemoths for the Giant Magellan Telescope array.

6. As it rotates, a dangling metal ring responds to forces that leave it spinning in a horizontal plane.

7. With an apparatus similar to example 6, a loop of chain is spun. Centrifugal effects force the chain into the shape of a circle, and also bring its motion into a horizontal plane.

These seven demonstrations flash by in less than four minutes, but they establish that the Cabinet of Physics was well-equipped to present students with the phenomena of centrifugal motion… as long as the person doing the demonstration had a sturdy cranking arm.

The Foundation for Science and Technics, or Fondazione Scienza e Tecnica, of Florence, Italy, has made available many videos exploring the Cabinet of Physics, a large collection of antique scientific demonstration instruments.  The Foundation's homepage may be found here, and its Youtube channel, florencefst, here.