Experiment Sunday update: Electric Circuits

[Note: Each week my geeklet and I have "experiment sunday", a brief and casual exploration of hands-on science and engineering.]

This week’s experiment was a great success. That isn’t to say that it went off without a hitch; the hitches made for more valuable learning than the experiment itself. We set out to make a simple electric circuit, but what we ended up doing was troubleshooting and learning about unstated constraints.

The experiment was a series of steps from The Science Book Of Electricity (Gulliver Books, 1991). The book has a few simple experiments like this one, described in general terms with lots of photos. There’s no theory and not much in the way of details, but the descriptions were enticing enough to catch my eight-year-old geeklet’s interest without any pushing from Dad.

The first step was to make a simple circuit using a light-bulb holder, bulb, two lengths of wire, and a battery. Trivial stuff, we lacked some of the necessary materials. The hardware store had everything we needed, but our parts were lost among all the variations of bulbs and wiring available. (Lesson: a simple parts list contains a lot of assumptions.) We eventually picked out a basic bulb holder, a set of 4W night light bulbs, and three feet of coated copper wire.

On returning home, we set to wiring up the circuit. Stripping the wire was easy with a pair of scissors, but the wire itself was so thick as to be unwieldy. (Lesson: there’s wire, and there’s wire.) With some work, we were able to screw two lengths of wire onto the bulb holder. The geeklet taped one wire end to the negative end of a battery, and I touched the other wire to the positive end. Let there be light? Well, no. (Lesson: it probably won’t work the first time.)

Now came the fun part: troubleshooting! How did our setup differ from the book’s description? The geeklet noticed a caveat in the book: “The battery must be the same voltage as the bulb.” What did that mean, though? The battery’s voltage was printed helpfully on the side: 1.5V. The bulbs, though, were listed as 4W. (Lesson: units are important.) I talked a little bit about the difference between current and voltage, and we looked more closely at the package of bulbs. No hint of voltage listed anywhere. Hmm. A quick check with Mama (who recently had to buy bulbs for one of her projects) yielded the clue we needed: a bulb’s voltage is often listed on its base.

120V. A bit of a difference, then. What to do? Improvise! (Lesson: improvise!)

We scrounged through the tool box to see if there were any other bulbs. We found a krypton bulb for a flashlight, which was listed (now that we knew what to look for) as 3.6V. Closer, but how to make up the difference between a 1.5V battery and a 3.6V light? This gave me a chance to talk about serial vs. parallel circuits, and how batteries in series will add their voltage together. 3 batteries at 1.5V made 4.5V, which should be close enough to make the bulb light up.

The geeklet found and taped together three D cells, which made an impressive battery of batteries. We dropped the krypton bulb into the bulb holder—a loose fit, but it closed the circuit if placed carefully—and wired up the rest again. Let there be light? Yes! (Lesson: persistence pays off.)

The next step was to wire in a pair of thumbtacks, spaced slightly apart on a bit of cardboard. These allowed us to bridge the gap with pins, coins, cloth, buttons, and other things that may or may not carry electric current. A tester! (Lesson: even our improvised monster circuit met the requirements of the experiment.) Once that principle was shown, we used one of the handy current-carriers (a paperclip) to fashion a contact switch. The geeklet showed off the completed circuit and switch to Mama, and we talked about applications like telegraphs and signal lights.

Writing this up, I just now realize that we re-invented the flashlight using (essentially) flashlight parts. Oh well, the process was the important thing.