It's a combination of the current through the coils, coupled with the number of turns (ampere turns in fact). It's a fine balancing act at that too.

Too many coil windings = too high an inductance, which means the coil presents resistance to higher frequencies feeding into it. The higher frequencies are the thinner strings, less ferrous material in them, so they actually need more drive

Too few windings = not enough magnetic force presented at the strings...only way to compensate is to cranck the current up, but that saps the battery.

Re round vs square coils. Square would be better (as it goes, I'd already considered them...in fact, you wouldn't believe the amount of McSpank Grey Matter processor 'cycles' I've dedicated to this little project!), but my simple coil winder (a drill chuck) cant cater for winding anything other than circular shapes! In fact longer rectangular coils would be better still...slightly overlapping one another so no dead spots when bending strings etc.

Quote Originally Posted by Ross77 View Post
Ah. I see. sounds like a good project.
Is it the no.of turns and length of wire that makes it work? If so you could use a small square bobbin to reduce the wasted space between the coils and then you can drill them to suit the size of the magnet. no need for a fancy holder and hopfully reduce the height.
Not sure I fully grasp what you're getting at when you say (re square bobbins) "drill them to suit the size of the magnet" nor how could this proposale "reduce the height" .....I'm all ears, could you put more meat on the bone?