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Magnetic Levitation

What it shows:

A magnet tethered over a spinning aluminum disc levitates due to induced currents in the disc.

How it works:

As the disc spins, electrical currents are induced in the aluminum as it moves with respect to the magnet. These induced currents create a magnetic field which, in accordance with Lenz's law, opposes the field of the magnet. The magnetic repulsion causes the rider to levitate about 1cm above the disc. Lenz's law also says that the induced field will oppose the motion that causes it. The magnet therefore tugs...

What it shows:

A beam of white light incident on a giant raindrop (simulated by a water-filled round flask) produces a full rainbow of colors. As with real rainbows, one can also see that the light intensity inside the rainbow is much greater than outside the rainbow.

How it works:

A Florence (round-bottomed) flask is completely filled with water and sealed with a rubber stopper. A Beseler slide projector ¹ serves as the sunlight. The light incident on the giant raindrop is refracted, reflected, and refracted once more, back in the direction of the...

What it shows:

The photoemission of electrons from a metal surface depends on the energy of the incident radiation and not on its intensity. Knowing the energy of the emitted photoelectrons and the frequency of the incident light, you can calculate a value for Planck's constant h.

How it works:

Using a mercury source, we have at our disposal three very bright visible lines, in the blue, green and yellow (doublet), and a rich selection of ultra-violet. Our main source is a Phillips Lifeguard 1000W street lamp with its outer (uv...

Local Group of Galaxies

Lights-up-in-the-dark three-dimensional model of the local group.

What it shows:

Three dimensional model of the local group of galaxies, with a scale of 1mm = 1kpc.

How it works:

A large wooden base board forms an x-y plane from which the approximate relative positions of the local galaxies are measured. The galaxies are LEDs and (for the two biggest, the Milky Way and Andromeda) 6V bulbs, mounted atop 8mm diameter plastic tubing; the length of the tube gives a z-axis position of the galaxy. The wires from the LEDs and...

What it shows:

An object finds itself heavier and pinned against the wall of a spinning cylinder; the principle behind fairground Barrel of Fun rides and centrifuges.

How it works:

The object in such a ride experiences two forces, that of its weight and the centripetal force exerted by the barrel wall; the vector addition of these forces giving the apparent increase in weight (figure 1 ) The reaction force of the object also presses it against the wall; the increased friction force preventing it from sliding down.

The barrel in our demo is a 45cm...

What it shows:

The air inside a very large glass pipe (partially filled with a fluid) is acoustically excited into a standing wave. Once resonating, the locations of the velocity antinodes inside the pipe are dramatically made evident by the vigorous agitation of the fluid, resulting in fabulous foaming frothing fountains of fluid. The velocity of sound can also be determined by noting the resonance frequency and measuring the distance between antinodes....

What it shows:

The behavior of a substance in a non-uniform magnetic field will depend upon whether it is ferromagnetic, paramagnetic or diamagnetic. Here we test different substances to see how they are influenced by a magnetic field.

How it works:

We have a collection of samples (listed in table 1) that exhibit well the three magnetic properties. Diamagnetic substances have a negative relative permeability (susceptibility); paramagnetic substances have positive.

Ferromagnetic substances have...

What it shows:

A beam of light is distorted due to turbulent convection currents in air. This is a model of atmospheric distortion that affects seeing conditions in ground based optical and infrared astronomy.

How it works:

Turbulent air is provided by an electric stove ring, that heats the air above it as the warm earth dues to air sitting above it. The turbulent currents set up alter the refractive index of the air in a disordered and rapidly changing way. Light from a point source passing through these conditions is blurred...

What it shows:

A pulse-modulated electromagnetic signal is simultaneously displayed in the time domain (on an oscilloscope) and in the frequency domain (on a spectrum analyzer). Using ∆n for the frequency spread (uncertainty in frequency) and ∆t for the duration of the pulse (uncertainty in the time domain), the frequency-time uncertainty relation is given by ¹

∆n ∆t ≥ ¹/_4π

By progressively shortening the length of time that the carrier signal is on, the inverse relation between pulse length and spectral-energy...

Harvard Natural Sciences Lecture Demonstrations

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Demo Subjects

Key to Catalog Listings

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