Sorry for being overly wordy- its just what I have read on the forum about magnets so far the people seemed to be leaving out required details and had to clarify a bunch later, so i'm trying to provide as much info as possible. Ok i'm trying to make a project to make adjustable resistance using magnets, but i'm just entering the realm of magnets. I want to have a sliding mechanism with an adjustable electromagnet on one end thats fixed and a permanent magnet on the sliding end. Poles facing each other would the same so that resistance would be formed. For size of the electro magnet is limited to 1in length and 1inch diameter ideal size (so fairly small) and possibly could extend it another .5in and also increase diameter another inch. The permanent magnet is limited to an even smaller size (particularily thickness). The maximum distance between each magnet is going to be about 3in open air. I want to power the electromagnet by a rechargeable battery pack. I would like to be able to set this up with as strong a maximum resistance as possible and be able to adjust it down. Permanent magnet- totally lost here. Where is a good source that will show me what level if magnatism each type of rare earth magnets are capable of? Where would be a good place to buy them? Electromagnet- from what ive read so far about the subject, I will need to wrap a core with wire, and that magnet wire is going to give me the most power. More wraps and smaller wire will make the magnet stronger, and that solid core is better than multiple strands. I should not have too many layers of wraps around the core as it will become counter productive, and that 3-4 layers is ideal and that the direction of the wraps must be the same. However im unsure of how to make these layers? do you wrap it around the entire length of the core than bring it back and start again? I have gotten conflicting infornmation about the wire, but it seems that the wire should still be insulated? Also I will need a resistor in the circut, and a potentiometer to be able to adjust the strength of the magnet. I know I need to make sure I don't exceed the wattage rating of the resistor (im assuming the potentiometer too?). So I would need to calculate current with voltage and resistance and than use that and voltage to calculate wattage. So is there a better core than soft Iron? What type of battery pack should I use? Right now i'm thinking about Ni-Mh because it seems more stable- li-po and li-ion's are a bit scary though the size and power of the batterys is apealing. How long could I expect the battery pack to last with high miliamp hours? Can I expect a decent amount of adjustability with the electromagnet? How much maximum resistance can I expect. would adding an insulating layer around the coiled wire result in stronger poles of the magnet?