by the Boom



A nice way to allow students to see the results of combining resistors in series, parallel, and compound circuits, is to use standard clear glass 120-volt light bulbs as well as electrical meters. This way, they can see the changes in voltage drops by the intensity of the filament glow.

Screw a sturdy double-pole knife switch to one end of a board about one meter long. Attach to the switch the leads of a cord that has a plug on one end. This will enable you to activate your circuits by the switch rather than by pulling the plug.

To the rest of the board, attach five or six surface sockets in a row.

Radio Shack supplies bags of inexpensive connectors (with Roach clips at each end) that are ideal for making your circuits.

(My students informed me that the reason that they are called "Roach Clips" is that "Pot Holder" was already taken).

Acquire five or six clear glass light bulbs of the same wattage and several bulbs of different wattage.

If you want to do quantitative measurements, you'll need AC meters for at least 120 volts, and 10 amps.

Now you're ready for electric action, and do not include yourself in any of the circuits.

Set up first several series circuits with the same wattage bulbs, then include some different sized ones, note the intensities of the bulbs and measure the voltages drops and amperages in the circuits. See if the sum of the voltage drops equals the applied voltage.

Set up parallel combinations and likewise measure the voltages and amperages. And see if the total amperage is the sum of the individual ones.

Set up compound circuits and do likewise.

Calculate wattages by multiplying voltage drops by amperages.

Another interesting measurement is the difference between the resistance of a light bulb when the filament is cold and when it is white hot.

To do this, determine the resistance of a single bulb by measuring its voltage and amperage and using Ohm's Law to find its resistance. For the cold bulb, use a battery as your voltage source, and for white hot, use the line voltage on the bulb. The resistance is very much higher at higher temperatures.

After comparing the two resistances for the same bulb, ask why an electric appliance such as a stove, a heater, or a toaster, will warm up to red hot but not keep heating further and melt down. It is because the resistance increases with temperature until it finally opposes further heating (the inverse feedback system).

To demonstrate the action of a fuse, cut a very thin (couple of millimeters wide) piece of aluminum foil about three or four centimeters long, fold over the ends a bit to increase their strength, and clip the ends to two connectors. Being sure your switch is open, and connect the roach clips to the switch. Stand by to execute the fuse... Throw the switch and watch it blow! Neat action! (Be sure you know where your circuit breaker is located as you may have to re-set it).

Return to Demos Page