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Table of ContentsResistance Wires -- one board containing five wires of different lengths, areas, and materials. Each of the wires is hooked in turn across a battery and the current through the wire is shown on a large ammeter. The dependence of resistance on length, area, and composition of the wire can be shown.
Ohm's Law -- uses the resistance board described above, but in this demo voltage is also varied, so that the dependence of current on voltage and resistance can be shown.
Series/Parallel Resistance -- two identical resistance wires on a board are hooked across a battery, either singly, in series, or in parallel, and the resulting current for the different hookups is measured with an ammeter.
Rheostat -- a slide-wire or coil rheostat can be used to discuss the principles of a variable resistor.
Decade Box -- commercial decade resistance box.
Sample Resistors -- various types, values, and power ratings.
I2R Losses --a nichrome wire, an iron wire, and a copper wire are hooked up in series with a Variac. A small paper rider is wrapped around each wire. As the voltage is increased, the wires begin to heat up in order of decreasing resistance. Although the current (and thus I2) is the same for all wires, the wire with the greatest R (nichrome) heats up first and burns its paper rider. Increasing the voltage (and current) further makes the iron wire burn the paper and makes the nichrome glow red-hot. The copper barely gets warm.
Galvanometer as Ammeter and Voltmeter -- a 500 mA galvanometer is mounted on an acrylic board for use on the overhead projector. Plug-in resistor modules convert it to a voltmeter with either 5 or 25 volts full scale, or an ammeter with 0.1 or 1 amp full scale. The resistances of the meter movement and all module elements are printed on the boards, and use of the overhead allows the students to see clearly the manner in which the resistors are connected to the meter circuit.
Loading by a Voltmeter of Insufficient Resistance -- voltage levels in a simple series circuit are examined first with a voltmeter of high resistance, then are dragged down by simultaneously measuring the voltage with a poor (approx. 600 ohms) voltmeter.
Neon Lamp -- a neon lamp does not conduct below 80V, but passes current easily above that voltage, and will continue to carry current down to about 60 volts once the current has been started
Carbon and Tungsten Lamps -- data are taken on current vs. voltage for two lamps, one with a tungsten filament and the other carbon. Resistance increases with increasing voltage (temperature) for the tungsten lamp, but decreases for the carbon lamp.
Diode -- meters show voltage and current through a diode hooked in series with a load. Voltage can be continuously changed from positive to zero to negative to show the “one way valve” action of a diode.
Heated Wire -- a resistance wire in series with a small lamp and an ammeter is heated by a gas flame. The resistance of the wire increases with rising temperature, causing the current to decrease and the lamp to dim.
Coil in Liquid Nitrogen -- a lamp in series with a resistance coil barely glows at room temperature, but when the coil is dipped in liquid nitrogen its resistance decreases and the bulb lights brightly.
Superconductor -- a liquid N2 cooled ceramic superconductor (Yttrium Barium Copper Oxide, YBa2Cu3ox) loses all resistance to electrical current. It will float a small rare-earth magnet (Meissner Effect), and can also be turned into a “permanent” (until it warms up) magnet due to internal currents that do not die out. This can be shown using a small compass to demonstrate the magnetic field from the superconductor.
Series/Parallel Resistance -- two resistance wires on a board are hooked across a battery, either singly, in series, or in parallel, and the resulting current for the different hookups is measured with an ammeter.
Series/Parallel Light Bulbs -- two sets of three light bulbs, one hooked up in parallel and the other in series, will clearly show the different current in the two circuits by the relative brightness of the bulbs.
Sum of IR Drops -- three large variable resistors are hooked in series with a battery. The voltage drop across each resistor is measured with a projection voltmeter, and the sum of the IR drops is found to equal the battery voltage. The battery voltage may be varied.
Conservation of Current -- current entering and leaving a circuit node can be read on three ammeters, and compared to show that current coming in is always equal to current going out of the node. Can also be used as a two-loop circuit to show the applications of Kirchoff's laws to analyze current flow in a slightly complex circuit
Resistance Cube -- twelve identical resistors are soldered together to form the edges of a cube, whose resistance can be measured across an edge, a face, or across the entire cube from corner to corner.
Slide-Wire Potentiometer -- classic highly accurate voltage measuring device.
Wheatstone Bridges -- devices which measure resistance by balancing voltage drops over two different paths, one of which includes the resistor to be measured. Three types are available: (1) With Lamps - a demo item to show the principle but not make actual measurements, (2) With Slide-Wire - to make actual measurements, and (3) Commercial Bridge - a show-and-tell item.
Opposing EMF's -- batteries are hooked together backwards to show the subtractive effect of opposing EMF's.
I2R Losses --a nichrome wire, an iron wire, and a copper wire are hooked up in series with a Variac. A small paper rider is wrapped around each wire. As the voltage is increased, the wires begin to heat up in order of decreasing resistance. Although the current (and thus I2) is the same for all wires, the wire with the greatest R (nichrome) heats up first and burns its paper rider. Increasing the voltage (and current) further makes the iron wire burn the paper and makes the nichrome glow red-hot. The copper barely gets warm.
Voltage and Current in House Lines -- a bank of ten lamps and heaters are hooked up in parallel to 120 VAC. A switch on each allows them to be turned on and off independently. As each element is added to the circuit, the voltage and current flowing into the circuit are displayed on projection meters to allow calculation of power consumption. A fuse and/or circuit breaker may be added to the circuit to show the function of such safety devices, and the current may also be run in on an iron wire which will overheat and burn a paper rider if the breakers are bypassed and the “safe” current limits are exceeded. A good way to teach students not to stick pennies in fuse sockets!
Fuses -- see above
Circuit Breakers -- ditto
Hallway Relays -- switch the room lights on and off to let the class hear the large hallway relays opening and closing.
Battery Effect -- demonstrates different voltages developed between various pairs of dissimilar metals in an acid solution. Two “windmills”, each with four arms (Zinc, Iron, Lead, and Copper) rotate so that any selected pair of metals dips into the acid trough at the bottom. Voltage developed between the metals is displayed on a projection voltmeter.
Simple Lead Storage Cell -- two lead plates in sulfuric acid may be charged with a battery eliminator, then discharged through a small bell. The longer the cell is charged, the longer the bell rings.
Electroplate Battery -- copper and carbon electrodes in copper sulfate will act as a battery (as shown on a voltmeter), but if the carbon is subsequently plated with copper, battery action ceases. Demonstrates that two different materials are required for battery action.
Copper Plating -- one copper and one carbon electrode in a glass container of copper sulfate are supplied with current from a battery eliminator to copper plate the carbon.
Human Battery -- Cu and Zn electrodes attached to a galvanometer and touched to the tip of the instructor's tongue produce a current.
Internal Resistance (Terminal Voltage) -- voltage from two sets of dry cells (old and new) are measured for an open circuit and a closed circuit with a load. Both sets show a voltage on an open circuit. For the fresh set of cells, voltage does not drop significantly when the load is added, but for the old set of cells with high internal resistance the voltage drops to practically zero.
Opposing EMF's -- batteries are hooked together backwards to show the subtractive effect of opposing EMF's.
Standard Cells -- cells of accurately known voltage (show and tell).
Sample Batteries -- various types and capacities.
Conductivity of Solutions -- conductivity of different solutions (tap water, distilled water, vinegar, sugar, salt, etc.) is measured with a 110V lamp and probes - the brighter the lamp, the higher the conductivity.
RC Charging Curve on Scope -- an oscilloscope is hooked across a capacitor which is in series with a battery and resistor. When the switch is closed the capacitor begins to charge and the assymptotically rising voltage is seen on the scope. Releasing the first switch and pressing another discharges the capacitor, with the resulting discharge curve shown on the scope. Circuit elements are laid out on a vertical board for easy viewing and analysis.
RC Circuit Analog -- a flat disc tied to the end of a spring is submerged in a large glass jar of water. Pull up suddenly on the end of the spring and the disc will rise in the water, quickly at first then more slowly as it approaches equilibrium, just as does the RC circuit described above. The spring is analogous to the capacitor in the RC circuit - pulling on it suddenly is the same as applying a sudden voltage - and the resistance of the water to the motion of the disc is the analogue of the electrical resistance in the circuit. The distance that the disc moves is the analogue of capacitor charge.
Relaxation Oscillator -- capacitor, resistor, and 90V battery are hooked in series, with a neon bulb in parallel with the capacitor. Capacitor charges to about 80V (the breakdown voltage of the neon bulb), then discharges through the bulb and begins the cycle again. The capacitor voltage curve can be displayed on an oscilloscope.
Emergency Flasher -- commercial emergency flasher uses a 9V battery to flash a neon lamp at approximately 2 Hz. (Battery voltage is stepped up electronically to provide the 600 volts needed to flash the tube).
Strobe Light -- commercial strobe light with variable frequency flash.
Diode -- meters show voltage and current through a diode hooked in series with a load. Voltage can be continuously changed from positive to zero to negative to show the “one way valve” action of a diode.
Diode as Rectifier -- a diode rectifier circuit mounted on a vertical board can be probed at different points with an oscilloscope to see the effect of diode rectification on an AC voltage. Filtering elements at the output end of the circuit may be switched in and out to demonstrate the smoothing of ripples in the DC output.
Gain of Transistor -- transistor circuit mounted on a vertical board amplifies input current. Both input and output current are displayed on projection meters, and amplification level can be changed.
Sample Printed Circuit Boards, IC's, Components, etc. -- various.
Thermionic Emission (Edison Effect) -- current across a tube diode is measured as applied voltage is increased, and it is found that no current will pass until the cathode has been heated by the cathode coil. Demonstrates the basic principle of the tube diode.
Tube-Style Radio Circuit -- old style radio circuit layed out schematically shows the function of various parts of the circuit.
Large Radio Tubes -- 30cm tall amplifier tubes may be shown with a modern transistor to show advancement of the technology.
Photoelectric Effect (Electroscope) -- bright light from an arc lamp strikes a metal plate mounted atop a negatively charged electroscope. Scope discharges quickly if the plate is zinc, less quickly for aluminum or copper. Will not discharge if a plate of glass is held between the light and the zinc (cuts out UV), or if the electroscope is positively charged.
Solar Cells -- small bank of solar cells hooked to a milliammeter produce current, increasing with brighter light.
Photomultiplier Tube -- show and tell.
Electroluminescent Panel -- solid-state 1 by 3 inch panel glows with a pale green light when energized by the proper voltage.
Thermocouple -- current registers on a milliammeter when this large thermocouple junction is heated in a flame.
Infrared Thermopile -- small thermocouples in series at the focus of a reflector react to radiant (IR) energy by producing current.
Thermopile with Light -- a small thermopile lights a bulb when heated over a flame.
Thermistor -- resistance of this electronic component decreases dramatically with rising temperature.
Piezo-Electric Sparker -- a thumb-squeezed piezo crystal sparker that can be used to charge an electroscope.
Piezo Crystal with Neon Lamp -- finger-tapped crystal develops sufficient voltage to flash a neon lamp.
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