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Leaning Ladder

Small ladder against wall with variable load and inclination.

What it shows: 

A small ladder is leaned against a vertical wall. A weight can be moved up succeeding steps to find when the ladder will slip and fall down. The forces holding the ladder in equilibrium are in consideration here.

How it works: 

A short (4 foot) step-ladder leans against the wall. The reaction of the wall may be considered horizontal, tangential friction being negligible (the edge of the ladder is covered with a sheet of...

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Conductivity of Water

What it shows:

Pure water is an electrical insulator. But provide an ionic compound in the form of salt, and you complete the circuit.

How it works:

A simple circuit with the mains supply connected to a 15W light bulb and two copper sheet electrodes (figure 1). The electrodes are placed in a 1500ml beaker containing distilled water. Distilled water is a very good insulator, with an autoionisation of 1:10-7 (the proportion of molecules in H3O+ + OH- form) it has a resistance of...

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Fiber Optics

What it shows:

Light is transmitted by a bundle of optical fibers and/or a coiled length of plastic rod, regardless of the twists and turns in the path it must negotiate. Total internal reflection keeps the light confined.

How it works:

A HeNe laser is used as the source of light. The bundle of optical fibers consists of a very large (but unknown) number of individual glass fibers measuring 0.05 mm (0.002") in diameter. About 30 cm of the bundle is exposed at the end while the rest of the length is protected by a rubber sheath....

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Thermal Expansion

Brass ball doesn't fit through brass ring until ring is heated.

What it shows:

Most solids (see Comments) expand when heated due to increased atomic and lattice vibrations. In this demo, a brass ring expands when heated to let a previously too small a ball pass cleanly through.

How it works:

The apparatus consists of a brass ring on a handle (figure 1), attached by a chain to a brass ball. Demonstrate that the ball is too large to pass through the ring, then heat the ring over a blue Bunsen flame for about a minute. The...

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Earth's Magnetic Field

OHP representation of lines of force using bar magnet and iron filings.

What it shows:

The magnetic field lines of the Earth can be represented by the field lines of a bar magnet.

How it works:

The Earth's magnetic field is basically a magnetic dipole. It can therefore be represented to first approximation by the field of a bar magnet. The shape of the field lines can be highlighted by the sprinkling of iron filings, or by the use of plotting compasses. The latter method has the advantage of showing the variation of dip...

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Vector Sum of Forces

Magnetic blackboard mechanics with spring balances and weights.

What it shows:

Solve a problem in composition of concurrent forces by graphical or trigonometric means.

How it works:

The blackboard mechanics set1 includes apparatus to demonstrate most of the common laws of statics and some dynamics. The pieces of apparatus are held on the blackboard by magnets and, although not large, are fairly visible in the lecture hall. A booklet with a few dozen suggested demonstration experiments is included in the set.
...

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Siphon

What it shows:

A siphon is a device that allows the transfer of a fluid from one reservoir to a second at a lower level even though the first part of the journey is up-hill.

How it works:

A siphon is effectively an inverted U-tube with unequal length tubes. The asymmetry means that there is a pressure difference between the ends;

at the upper reservoir: p1 = P - ρgh1
at the lower reservoir: p2 = P - ρgh2
(where P = atmospheric pressure)

so p1 > p2 if h2 > h...

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Bird on a High-Voltage Transmission Line

What it shows:

Why doesn't a bird sitting on a high-voltage wire get electrocuted? This demonstration addresses that question and serves as a model of the situation.

How it works:

The important concept conveyed is that there needs to be a voltage difference across a conducting medium for current to flow through the medium. In this situation the conducting medium is a bird sitting on a high-voltage wire. The voltage on the wire is the voltage of the whole length of wire with respect to the ground. Although the bird on the...

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Frustrated Total Internal Reflection

What it shows:

In quantum mechanics, it is possible for a particle to tunnel through a potential barrier because its wave function has a small but finite value in the classically forbidden region. Here we use FTIR as an optical analog of this quantum mechanical phenomenon.

How it works:

A 45°-90° prism will deflect a beam of light by total internal reflection. When two such prisms are sandwiched back-to-back and pressed together, the air-glass interface can be made to vanish and the beam then propagates onward undisturbed. This transition, from...

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