Demos: 5B-01 Ohm's Law Strip
This apparatus can be used to demonstrate Ohm’s Law as well as Electrical Resistance (dependence of resistance upon length, cross sectional area and material of a conductor). Six wires, each 1 meter long are attached to a display strip with 4 mm sockets in which you can attach an ammeter or voltmeter. Five of the wires are made of Constantan, the sixth one is made of brass. The Constantan wires have diameters of: 1 mm; 0.5 mm; 0.7 mm (two); 0.35 mm. The brass wire has a diameter of 0.5 mm. The wires are mounted on the display strip such that current and voltage measurements can be compared between the wires and at defined points along each wire.
Directions: Power is supplied to the circuit by means of a series of one to four D Cell (1.5 volt) batteries. Connect the ammeter leads to the two ends of the wire to set up a current in the wire. Then use the voltmeter leads to tap the voltage at various points along the wire. You can choose between the two large display meters or two multi-meters on the document camera to display voltage and amperage.
Suggestions for Presentation: Although this is called an Ohm’s Law Strip, it might be well to address the issue of what Ohm’s Law means. If you are simply using the relationship V = IR, this is not technically Ohm’s Law, rather a relationship that defines resistance, R = V/I. Ohm’s Law is restricted to a linear relationship between V and I.
To optimize the effect of this demo, use combinations of wire type and wire length. To test whether Ohm’s Law applies here, clip one ammeter lead at the center of the wire. This should reduce the resistance to one-half and with the voltage the same, the current should be twice as much. By comparing voltages and currents of different diameter, type or length wire you can also demonstrate the factors that determine Electrical Resistance.