F=BIL Relationship with coils Explanation?

[Kognito]

Homework Statement

There's no specific problem here so I'd say this template doesn't specifically apply. I'm doing a piece of coursework investigating F=BIL and electromagnets and I'm having some trouble. I think the issue here is the data that I've collected so bear with me.

I've been making use of the motor effect (as per: http://bit.ly/YNEZVh) to create a downwards force that I can measure with a balance underneath the magnet. The magnetic field from the horseshoe magnet there has been the same throughout, I've simply varied current up and down to give several readings. My other variable has been to coil the wire that the current flows through with several quantities of turns, giving me a number of sets of data.

What I'd expect to see is that the downwards force that is created should increase with current, which it does, and it should increase with the number of turns, which it does. What I don't understand however is what B does in the equation when I'm adding turns to the coil?

For Force to increase whilst L is increasing (assuming I stays constant), does B stay constant?

Homework Equations

F=BILThanks in advance

EDIT: I should also add, I'm plotting 1/I against F/L to give a value for B. What shape would you expect this to give with a simple varied current to create the force, all other variables remain constant? I've ended up with a downwards curve trending towards zero on both axes, does that sound about right?

 

[rude man]

Why are you plotting F/L against 1/I? B = F/IL so you should plot F/L against I to see if that ratio remains constant over changes in I and L.

B should stay constant unless your permanent magnet's magnetism is changed by application of I. If that happens, B would and therefore F/IL should rise somewhat with I and/or L.