Hey jamonwindeyer AKA the physics guru, thanks for helping me out with this prac!
I'll try to explain the practical a bit more in-depth, so:
A bar magnet is placed on an electronic scale. Two retort stands are placed on opposite sides of the scales that are connected to two boss heads. A conducting wire will be placed between the two retort stands, by hanging them through the boss heads (with high tension so as to ensure an accurate 90 degree angle). This conducting wire is connected to a variable resistor, an ammeter and a transformer to complete a circuit. (Note: i made it sound like one wire, when its actually quite a few). Once a current runs through the conducting wire, as the magnetic field and direction of the current are perpendicular to each other, a maximum force will be experienced by the conductor. This will either increase or decrease the mass of the magnet as recorded by the electronic scale. By varying the resistance, and hence the independent variable, the current is changed. From the results obtained, we must calculate the value for magnetic field strength and hence graph Force vs Current. Should be a linear relationship, however whether it is linear or not gives me material to talk about reliability, validity and accuracy. I hope this helps you visualise the practical 
I just got a few questions on the errors and improvements you mentioned:
- By power loss in the transformer, would this affect the practical i am performing as the current through the conductor will always be measured?(Sorry i forgot to mention that we have an ammeter in this practical in my previous comment)
-Would placing a magnet on an 'electronic' scale affect how it functions and maybe distort the value for the weight of the magnet by meddling with the electrical components of the scale?
-As the magnet is placed in a stationary position and is not moved, would this be considered as conducting the experiment in a 'constant' magnetic field?
Thanks Jamon!
Ahhh okay, I'm with you now, I follow! A seriously cool prac, do you know what sorts of differences you're expecting in the mass of the magnet? I'd imagine even a large force on the wire will not do much at the magnet end, I kind of want to try this myself now aha
Okay, so with your clarifications:
- Yes, my transformer comment is actually invalid. The way you describe it, that transformer is there to both convert an AC signal to a DC signal (so, it must also contain a rectifier) and to step up the current to accentuate the motor effect. In any case, no error there if you are measuring with an ammeter through the conductor. You could add that an ammeter will cause a (negligible) change to the circuit, the equivalent of putting another tiny resistance in series with the variable one.
- Oh, I like it, definitely possible! But I'd wager that, unless it malfunctions completely, the difference would be negligible. Good thinking though!
- Not really, so basically the fact that you'll be using a bar magnet (or similar) means every point on the wire will experience a slightly different magnetic field strength, due to a slightly different angle with the field and distance from said magnet. So, the magnetic field you find would likely be the average strength of the field across the wire. It could throw some decently sized errors.
Beyond this, you can mention things like intolerances in the resistor and wiring (the wiring will have a small resistance, the variable resistor will likely have a 10% error either way). Even inaccuracy in acceleration due to gravity, a tilt in the scale,, rounding error, etc. For reliability, just keep saying repetition, that's what they'll want to hear
For accuracy, you nailed it, the more linear the better (remember to ignore outlier results). I hope this helps a bit!
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