hi,
As you have 8 of 'D' cells, why dont you just use one cell, get the magnetic values you are looking for,
then use 2 cells, then 3 cell... etc.
This will give you 7 'magnetic values', enough to plot a simple graph.
To do an empirical 'magentic value' test, use the electro magnet to pick up various metallic weights,
use the kitchen scales to get their weights.
If you have a voltmeter/ammeter you could measure and plot the results for each cell added.
If you have an old car 12v lamp, this connected in series with the electromagnet this will limit the current to some lower value,
again plot the results.
A standard potentiometer will just burn out, it cant carry the current.
I would suggest a well plotted graph of voltage against the weight picked up, would be a good answer for a school science project.
Are you winding your own magnet/coils?