## Saturday, March 03, 2007

### Melted meter—what to do?

Analog meters. Very old-school, I know. But for teaching circuit concepts to students, I think they're great. Needle rides up and down like the one in a speedometer. Circuit physics is not very "naked-eye," so I think it's good to use a meter that has a familiar look and feel. Some teachers prefer digital meters and they have their reasons. To each his/her own.

I use DC voltmeters (0-10 V) and dual-range DC ammeters (0-1 A, 0-5 A), each one less than \$20. In one lab, I have students connect each meter in series and in parallel in succession. They decide which is connection more useful for each meter. Series for the ammeter and parallel for the voltmeter, as fortune would have it.

Though the meters are, um, "inexpensive," they hold up pretty well under the conditions of the introductory laboratory. My students are generally very good about treating equipment well. And I do what I can to encourage that behavior.

But things do happen. The meter pictured above is an ammeter. (Click the image for a larger view.) It had been connected to a variable DC power supply in parallel to the circuit whose characteristics were to be measured. Using voltmeter test lead wires. In a lab whose instructions included, in ALL CAPS, the admonition that the test leads were to remain connected to the voltmeter at all times and were never to be connected to the ammeter.

This was our third lab using the ammeters, and the second one using the power supplies.

What to do about the students who melted the meter by not following instructions? What would you do?

Anonymous said...

I read this, realized my group used the voltmeter leads in the ammeter at one point, panicked that our group broke it, and then noticed the big "B" on the melty one. Looks like I'm in the clear.

But my lab answers have got to be all wrong.

Stevie Ray said...

Intall an in-line fuse at one side of the power source. You control the max. current - not the students.

Dean Baird said...

Thanks, Stevie Ray. The supplies have a circuit breaker for their own safety. It was this model (ah, CENCO...).

I want to have access to 5A if I can get it (for magnetism labs). The meters are capable of handling 5A when properly connected (negative terminal and +5A terminal). But as I recall, the lab group in question had the test leads connected to both positive terminals of the ammeter.

Anonymous said...

I collect a \$5 "insurance" fee at the beginning of the year. When things break, I check to see if they paid their insurance. If not, their parents get a bill if the student was negligent. If they did, I just replace the equipment. I usually have enough \$ left over to buy some new cool stuff.