I found an article about mystery circuit boxes several years ago in The Physics Teacher. I’ve recently had the opportunity to actually use it in a lesson!
Martha Lietz (the author of the lesson I based mine off of) uses rewired bathroom beauty bar lighting in order to teach her students about series and parallel circuits in reverse. Basically, students are given a strip of lights and unscrew the light bulbs one at a time to deduce how the circuit is wired.
I decided to take it one step further and use mystery circuits to assess students’ knowledge of circuits using various wirings of circuits in the different beauty bars in order to evaluate how well students understand the differences and effects of series and parallel wirings. You can view my lesson plan here.
One of the greatest parts of this demonstration is that the materials are easy to find and relatively inexpensive. I found the beauty bar at Lowes (or Home Depot) for around $13. I also had to by a wire cutter/stripper and extra wire nuts. Plus, you can use them over and over again!! I recommend using 40 watt rated bulbs or less because they get hot rather quickly.
One of my mystery circuits was the one designed by Lietz, and I’ve included pictures here. Can you figure out how it’s wired?
Picture One: All bulbs are screwed in tightly. Bulb one is the brightest, bulbs two and three are the dimmest but shine to the same intensity, and bulb four has a brightness right in the middle of the others. NOTE: It is important to be sure that a bulb’s brightness is due to the power it receives from the circuit and not simply a reflection of light from the bulb next to it. This was the major point of confusion for students with regards to bulb two.
Picture Two: Bulb one is unscrewed and all bulbs go out.
Picture Three: Bulb two is unscrewed. Bulbs one and two are the brightest and bulb three goes out.
Picture Four: Bulb three is unscrewed. Bulbs one and two are the brightest and bulb two goes out.
Picture Five: Bulb four is unscrewed. Bulb one dims slightly. Bulbs one, two, and three all shine at the same intensity.
Using the information and pictures provided above, can you figure out how the circuit is wired??