Zinc-copper-acid battery. Cut-away dry cell Batteries in series and parallel can be demonstrated.
Lemon cell - a copper and zinc nail pushed into a lemon will produce about 0.9 V on a meter. Two lemon cells hooked in series will light a small red led. Bring your own lemons.
Electrolysis of water apparatus is shown in the video below. NaOH is used as the electrolyte. The gasses are tested in the second video. Oxygen will relight a smoldering wood splint. Hydrogen causes a small pop when ignited.
The internal resistance of the Leybold multimeter on the 10 V scale can be measured with the circuit below:
The ammeter reads the entire current flowing through the internal resistance of the voltmeter, and the voltmeter reads the voltage drop across this resistance, so ri = V/i. You can check this by inserting a resistance decade box in series with the circuit and adjusting the resistance until the meter readings go to one half their former values. The decade box will then read the internal resistance of the voltmeter (assuming that of the ammeter is much smaller).
One way to measure the internal resistance of an ammeter is the circuit below.
The decade box R2 is set to zero, and R1 is chosen for an approximate full scale reading for the ammeter. R2 is then adjusted to halve this reading, so its resistance equals that of the ammeter.
ri for Leybold multimeter on 10 V scale = 34,000 ohms
ri for Leybold multimeter on 1 mA scale = 120 ohms
ri for Leybold projection meter on 1 mA scale = 100 ohms
A hot dog is impaled on nails connected directly to the 110 V AC line; alternatively DC can be used. Current passing through the hot dog will cook it in a minute or two. You can make the demonstration dramatic by putting the cooked hot dog on a bun with mustard or katsup, taking a bite, and handing it to the class to eat.
Bring your own hot dogs, buns, and mustard.
Kirchhoff's circuit laws and electrical power = Vi can be demonstrated with the light bulb board. The circuit is displayed to the class's view and voltages and currents can be measured in various places in the circuit.
A very interesting demonstration is to show a 100 watt bulb and a 60 watt bulb in parallel (as in an ordinary house circuit), and then to try the two bulbs in series (the 60 watt bulb is then brighter).
This board does not obey Ohm's law, however, since the resistance of the filaments depends on temperature, and therefore on voltage (or current). But you can demonstrate resistors in series and parallel by putting three 100 watt bulbs (or three 100 ohm resistors) in series and parallel and measure the resistance of the combinations with an ohmmeter.
A more compact version of the parallel and series light bulbs is available that uses a 12V car battery. All the light bulbs are 2.8W and can be hooked up in various configurations. The voltage and amps can be measured simultaneously using two digital multimeters.
You can wire up an ammeter or voltmeter by adding a shunt or series resistance to the Leybold 60 mV, 300 mA meter or other galvanometers. Alternatively, the Cenco multimeter has the shunt and series resistors ready to plug into a circuit that is fairly visible to the class.
You can wire up your own circuit with resistances and meters but two simple circuits using a large wirewound rheostat and demonstrating Ohm's law are a voltage divider and a current limiter.
The temperature coefficient of resistance can be investigated with a copper wire wound resistor and a carbon resistor. A small circuit with a battery, lightbulb and resistor is set up and then the resistor is put into a cup of liquid nitrogen. The change in light brightness indicates the sign of the temperature dependence. The temperature dependence of a semiconductor can be investigated by dipping a large green LED into liquid nitrogen and noting the color change.
Because any battery has an internal resistance ri its terminal voltage VT drops when current is drawn from it;
whereVo is the open circuit voltage. Measurements of VT for various i's are taken so ri can be determined. The circuit and operation are described over.
Students will be familiar with this effect from trying to start a automobile with the lights on. When the starter motor is actuated, the lights become noticably dimmer as the terminal voltage of the battery drops. A 12 V light bulb is provided in this circuit so the class can see it get dimmer as the large currents are drawn.
As of September 1979, the internal resistance of The J.C. Penney Battery was 0.02 ohms so it could deliver 600 A to a short circuit.
The circuit board for this demonstration allows the control and measurement of large currents (100 A) from a storage battery. The large currents are also used for magnetic fields of currents demonstrations.
High-Current Control Circuit
Basically the circuit routes the current from a 12 V storage battery through a series of 0.1W power resistors. When S2 is in the left position, the current passes through only one resistor, and will reach 120 A if there is no other resistance in the circuit. The single power resistor will heat up rapidly so this position must be used only momentarily. When S2 is in the right position, the current passes through the second power resistor. With S1 in the left position about 60 A will flow to a closed circuit; with S1 in the right position, the current flows through the third power resistor and 40 A flows.
The switch S3 is provided to short circuit the output for terminal voltage and internal resistance measurements.
The shunt w adjusts the Leybold 60 mV/300 mA meter to read 100A full scale at A, or 333 A full scale on its red scale when the series resistance r is switched in.
A voltmeter is hooked to V to read the terminal voltage of the battery.
Several multirange meters for measuring voltage and current in AC or DC are available including two large Leybold meters that can be used to monitor voltage and current in a circuit, and a large display digital multimeter that will measure voltage, current, resistance, and temperature.
Some professors have used the digital ohmmeter as a "lie-detector" -- a student volunteer was called up to answer embarrassing questions while holding the leads of the ohmmeter.
There are also large standing galvanometers, a model of a meter movement, and a measuring amplifier reading out onto a large meter for measuring small currents of 10-7 to 10-11 amps.