In the 19th century, James Clark Maxwell’s theory explained the inter-relation of electricity and magnetism. It also predicted that electromagnetic waves should exist. In the 1880s Heinrich Hertz worked in the laboratory to produce these waves, and to measure their properties. The Cabinet of Physics can demonstrate experiments similar to some of Hertz’s. The transmitter is a spark gap driven by an induction coil; the receiver includes a Marconi-style coherer (remember the coherer?). The coherer is part of a circuit containing a battery-powered bell. So when waves from the transmitter arrive at the receiver, the coherer transitions from being a bad conductor of electricity into being a good conductor, and we hear the bell ringing. It keeps ringing until a sharp whack returns the coherer to its original state. In today’s video, the curators first use this setup to show that interposing a sheet of copper reflects away the waves from the transmitter. The receiver’s coherer detects nothing, and the … Continue reading →
About Bill Higgins
William S. Higgins is a radiation safety physicist at Fermilab involved with the transport of high-energy particle beams. He frequently writes and speaks about spaceflight, astronomy, and the history of science. A graduate of Notre Dame, he lives in Aurora, Illinois.
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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 … Continue reading →
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 … Continue reading →
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 … Continue reading →
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 … Continue reading →
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 … Continue reading →
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, … Continue reading →
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 … Continue reading →
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 … Continue reading →
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 … Continue reading →
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. In the centuries since, in textbooks, Hooke’s … Continue reading →
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 … Continue reading →