The Meissner effect is the expulsion of a magnetic field from a superconductor during its transition to the superconducting state. The German physicists Walther Meissner and Robert Ochsenfeld discovered this phenomenon in 1933 by measuring the magnetic field distribution outside superconducting tin and lead samples.
In this demonstration, we have placed a yttrium-barium-copper-oxygen (123) superconductor inside liquid nitrogen (-196°C). As it is cooled below the superconducting transition temperature, the material becomes a superconductor and a perfect diamagnet, expelling the applied magnetic field. Because of this, when a magnet is held above the material it starts to levitate and hangs suspended in air. Gradually when the liquid nitrogen boils off and the superconductor returns to temperatures above its critical point, the magnet eventually loses its levitation and falls.
This demonstaration shows the semiconducting behaviour of a thermistor. A thermistor is a temperature-sensing element composed of semiconductor material which exhibits a large change in resistance proportional to a small change in temperature. Thermistors usually have negative temperature coefficients (NTC) and their resistance decreases as the temperature increases. The experimental setup consists of a thermistor being immersed in the water and a thermocouple is used to monitor the temperature of the water. The resistance change is observed through a digital multimeter which is connected to the thermistor and a hot plate is used to heat up water.
This interesting demonstration utilizes a highly evacuated electron diffraction tube to show the wave behavior of electrons. The electrons are emitted by the thermionic emission and accelerated towards target by applying a very high potential (2000-5000 V). The target is a micro meshed nickel grid on which a thin layer of graphite is deposited. The electrons being diffracted through the graphite satisfy the Bragg’s condition and produce an interference pattern consisting of two rings.
When a magnetic field is applied perpendicular to the direction of flow of charge carriers, in a semiconductor material, charge carriers experience a force in transverse to the direction of applied magnetic field and carriers flow. This effect is known as Hall effect. Being very simple and straight forward phenomena in physics, Hall effect is a fundamental principle in magnetic field sensing and have many practical applications in our daily life. This demonstration shows Hall effect in semiconductor materials and shows how n-type and p-type semiconductors can be identified.
This demonstration is specifically based upon the properties of phonons. A diatomic chain with well defined periodicity is used as a linear crystal lattice. As the excitation wave becomes comparable to the length scale of the periodicity of the lattice, the dispersion relation becomes non-linear resulting in band gap between the acoustic and optical branches of the diatomic chain. This is a computer controlled demonstration. A Hall sensor and a magnet is used to observe change in amplitude.
Table tennis balls that are glued together can be stacked on top of one another showing the two most important kinds of packing – hexagonal and face centred cubic packing. One may use balls painted in different colors to bring home the ideas more vividly. Click on the images for close-ups.