Finding an appropriate digital multimeter

Extech MN36 digital multimeter

I'm introducing my general physics class to electric circuits with an experiment.  I generally find that the idea of voltage and current doesn't really sink in until students have had a chance to measure these quantities in circuits with which they can play.  (I also find that there is little correlation between my best problem solvers and my best circuit hooker-uppers.  Circuits experiments give an opportunity for success to some otherwise weak students.)  So this week, my class will do the simplest circuit experiment one could ever devise:  keeping a constant battery voltage, graph the current through a variable resistor as a function of the resistance. 

For years, though, I could not do this experiment effectively.  The ammeters -- really multimeters -- in my classroom wouldn't measure a wide range of currents, and would blow a fuse if they measured too much current.  You can certainly trust introductory students to blow an ammeter's fuse, no matter how much preventitive instruction and warning you give.

I really wanted a meter which would measure currents as small as a few microamps, and as large as a few milliamps.  Then we can use the standard available power supplies that provide 5-15V with tens of kilohms of resistance.

Why are these values important?  Most cheap resistors are rated at about 1/4 watt... so to keep the resistor from getting hot and ruined, the voltage squared divided by the resistance for any individual resistor must be less than 0.25 watt.  Even with as much as 15 V across as small as a 1 k resistor, the power is acceptable.  That calculates to a maximum current of 15 milliamps... and my preference isn't to come too close to this maximum. 

The meters I bought 10-15 years ago weren't this sensitive to current.  Their sensitivity was in the 1 mA range.  But last summer I searched google shopping under "digital multimeter."  I found a number of reasonably priced meters that would measure currents as small as 1 μA!  I've pictured one, the Extech MN36, which was listed at $17 on Amazon.  No price guarantees, and I've never used this particular meter -- I'm just showing an example of what I found.  Look around, there are gazillions of meters, many of which will serve your needs.

When you're looking for a class set of multimeters, DON'T look at science supply stores!  Their prices are inflated, and they don't necessarily give you a good selection.  Look around at electronics retailers.  Make sure to check the specs to see that the resolution for DC current is in the 1-10 microamp range.  If you have to, buy four this year, and four more next year, etc. 

Once you have the meter, all you need is a huge pile of 5-200 kilohm resistors -- which can be found from electronics stores in bulk for 1-2 cents apiece -- and some battery holders.  You can add more expensive power supplies, breadboards, or "resistance substitution boxes" later if you want.  But don't let cost or procurement be an obstacle to simple and fun electronics experiments.