Wednesday, February 08, 2012

Connecting wire continuity test?

There are seven lab activities we complete in our exploration of current and circuits. All of them require connecting wires. And we continue using connecting wires in our study of electromagnetism.

I'm fond of connecting wires ("alligator cords," "jumper leads") that have alligator clips on both ends. I'm also fond of miniature bulbs (flashlight bulbs) in miniature sockets. We put three flavors (2.5-V, 6.3-V, and 14.4-V) of mini-bulbs to good use.

But getting back to the wires. The alligator clip connecting wires that we've been using for many years (Radio Shack and Sargent Welch) are more about "cheap low-cost" than they are about "durable." Many go bad every year. When they do, they lose continuity; a gap suddenly exists, usually between the copper wire and the clamped-on alligator clip.

A wire can fail at any time. When it does, it's not obvious to physics learners that the wire has failed. This can lead to frustration. I scurry about as best I can, troubleshooting circuits as students ask for assistance.

If I suspect a bad wire, what's the best/quickest way to check it for continuity while the lab is in progress and other groups may be waiting for help? I'm not sure. Here's what I've been doing:

I connect the suspect wire to a Genecon and crank. A good wire will offer noticeable mechanical resistance. A bad wire will offer no more mechanical resistance than would occur if the Genecon's leads weren't connected to anything.

I've found that you have to "bounce" the wire a bit while cranking the Genecon to find pesky intermittent failures. It's fairly quick and painless, and doesn't require a meter of any sort. Better still, it garners quizzical, "What the heck is he doing?" looks from students. Later, they will use the Genecons, too. Then I'll be able to ask them how my continuity check worked.

Still though, I'd be willing to pay double (or more) for alligator cords/jumper leads that were built to last.

In any case, what's your preferred method of continuity testing during a lab?


Mr. Gigante said...

I just use my multimeter in the continuity test mode. It has an audible signal when there is continuity so it is very quick and easy to check a large number of wires in a short amount of time.

Len said...

I love playing wih the Genecon generators, but they, too, suffer from a certain cheapness -- the plastic gears are prone to breaking after a while.

Other things we've done:
Once, when I was lighting a lightbulb with a Genecon, some members of the class asked me to crank hard enough to blow the bulb. I insisted they pay for the bulb first, and we raised enough money to buy several. So I cranked. The bulb got brighter and brighter, and when it burned out, the Genecon suddenly "let go" and became much easier to crank.

I later turned this into a demo, showing that I could tell when a student completed the circuit by the resistance felt while cranking. We also speculated that a large generator might suffer some trouble if the circuit it was attached to was suddenly open. I gather they have fail safe mechanisms to prevent the generator from self-destructing.

I also hook one Genecon up to another to show that one can act like a generator while the other is a motor. By counting the number of times the motor crank turns and comparing it to the number of times the generator crank was turned, we can calculate an efficiency for the process. We also see what effect fast vs. slow cranking has on the efficiency.

Len said...

Ah, forgot to answer the question... I usually hook the wires up to a multimeter and check the resistance. I bounce them around, too, to check for intermittent breaks. Not as fun as using the Genecon, but maybe a little faster.

John Millard said...

Clip the alligators to the leads of a multimeter set to measure resistance. If the meter reads something close to zero, the wire is good. If the meter reads "infinity" (usually seen as "1-" the wire is discontinuous.

Dean Baird said...

My DMM show "OL" (over limit, I suppose) for an open circuit and will show 0 for a good wire. It must be that when I started these labs, I didn't have a reliable DMM or my DMM didn't have a resistance meter. Who knows?

I do enjoy the curious looks from students and their questions when I crank on a dangling wire and pose quizzical. Of course, I don't immediately answer these questions—better to let them marinate for a spell. It enhances the appreciation of the answer when it is finally revealed.

The DMM is likely more efficient in some ways, but it's more likely to come across as a "magic box" whose inner secrets are not so accessible.

Now that I've got a good DMM, I'll use it when I'm testing wires during my prep period or after school.

And Len, I've had pretty good luck with Genecons. I tell students to limit their crank speed to no more than 3 rps. That's a pretty high whine, mind you. But as long as they avoid jerkiness (always a good approach), the Genecons should hold up well.

We do many of the same Genecon activities you describe. Great minds!

Stevie (EMF-er) Ray said...

If you let students turn the crank as you alternately connect and re-connect a good wire as they toil away, they'll get an unforgettable kinesthetic experience of what generators "feel" each time you turn on a light or plug a device into a wall outlet.