I helped a friend's 10 year old son build a science fair motor: a coil of wire (20t around a pill bottle former) supported above a magnet by paper clips with the tops bent into a U. The coil has #14 copper soldered to each end, looks like this:
** ___* *___ * * **The construction is such that the #14 is rigidly attached to the coil, so it acts as the axle as well as the conductor. A D cell makes the thing spin nicely. (Usually you have to start it spinning by hand.)
The lad asked "how long will the battery last?" The problem is that, as it spins, it arcs & the current is interrupted often and with no repetitive pattern. In addition, the arcing burns the paper clip & wire which eventually stops the motor and (I assume) lowers the current draw over time as the motor runs.
He could just run the thing until the battery is "dead", where dead is some chosen terminal voltage, and note the elapsed time. That is the "science fair" answer we went with for the "how long" question. But, from a practical viewpoint, how would he measure the current? My knee jerk reaction is a series resistance to develop a voltage & charge a cap with a DMM measuring the voltage. Then, assuming worst case, he could predict a minimum run time. Is that approach valid? (I realize that with intermittent contacts and resistance changing, the battery won't discharge as quickly as worst case predicts.) Bear in mind that he is a 10 year old who needed help with this "electrical project", so he's not a computer wiz kid who can whip up an interface and take a whole bunch of samples over time, record them with a PC & spit out a prediction. :-)
A "side note" - the paper clips have been replaced by a couple of small angle irons screwed to a piece of wood. Paper clips scotch taped to the lid of a plastic container can't take the "handling" of a 10 year old boy. :-)
Ed