Maybe I shouldn't have started this blog now, not with everything that's been going on.

In my electricity and magnetism lab last Wednesday we had an experiment on the construction of ammeters and voltmeters using galvanometers, something David has been reminding me about since the start of the term even before he saw the manual we were using.

In fact he was there when I gave them the short quiz at the start of the session (at the end of his mechanics lab session next door, where they performed the Atwood’s machine experiment using the new set up the students from last term submitted and this is a run on parenthetical clause).

He was probably surprised that I just dictated the questions to them instead of cutting up small sheets from paper with the questions printed on them. He was also probably surprised at how general the questions were (how many measuring devices are used in this experiment, how many circuits are used) instead of the specific ones I made for the mechanics lab last term after which he modeled his types of questions.

I, personally, was surprised that, even though I had used these types of questions earlier in the term, the students were still guessing instead of preparing by reading the experiment.

Overall the experiment was a success though. We had to adjust from the procedure because it assumed we were using a 500-microampere galvanometer when in fact we had 50-milliampere ones available. I also decided to give different full deflection ampere and voltage values to each of the three groups.

The ampere value I gave, for one, had to be larger than 50 milliamperes otherwise the computed shunt resistance would be negative. It also had to be a certain range otherwise the shunt resistance they would get would also be less than one ohm, which would be very difficult to measure exactly without connecting different valued resistors in parallel.

In fact, the shunt resistance everyone got was less than ten ohms. The smallest resistor we got though, was ten ohms. Fortunately we had ten-ohm potentiometers handy, so I just asked them to connect one of those to the ohmmeter and turn the knob until they got the desired resistance.

I was also surprised that the galvanometer resistance they all consistently measured was only 1 ohm. This meant that the multiplier resistance they computed for constructing a voltmeter was 199 ohms, 399 ohms and 249 ohms. For this one I told them to get several resistors that would add up to these values, and connect them in series.

As far as I could tell the largest difference between the standard and constructed measurements they had was 30 amperes and 34 amperes respectively.

For their next experiment in magnetic fields I asked them to bring per group one solenoid and 150 to 200 grams of iron filings.

In succeeding posts I’ll talk about yesterday’s electricity and magnetism lecture on the new chapter, the Friday foundation day activities, and the College of Computer Science students doing their undergraduate thesis on astronomy software giving their oral defense.