Help making an Electromagnet

[mgiddy911]

I am working on a physics project, and I need to build a strong electro magnet. I have previously made many real weak ones simply by coiling wire around a nail and conecting it to a AA battery(1.5V). Can anyone help me make a stronger version. I tried using a powersupply from an old computer, it plugged into a 120 V wall outlet and coverted AC to DC with outputs +5V, -5V, +12V and -12V. so I took the 12 and -12 leads and stripped them and connected them to a wire coiled around an iron pipe. however the powersupply then wouldn't kick on, as if a safety feature was preventing it, as soon as I disconnected one of the wires it would kick on, but then i didn't have an electromagnet. My idea was to use the 12 volt leads and transform them to high voltage low current to be safer, but for now I was just using the 12V as is. ONe idea my dad gave me was that possibly I need some resistance for the powersupply to work, so adding some resistors in series before the wire coil may work?

So I need some instruction on making a much stronger electromagnet.
Any help or weblinks would be appreciated

[mgiddy911]

could someone atleast give me some insight into what makes an electromagnet stronger, some of the things i know are the number of turns of wire in the magnet, as well as the curent or voltage applied to it.
The reason i wanted to do it without just using batteries is because i didn't want to just have to connect 20 batteries together to get a high voltage. Maybe using the computer powersupply isn't possible however, again I cut the wires of one of the outputs that would connect to a hard drive, there are +12V, +5V, -5V, -12V. I connected one end of the wire coil to the +12 and one to the -12, am i going about this wrong, is there a different way i should be using this power supply, should I be connecting one end to the +12V and grounding the other?

Any help would be great, this project is becomming evermore tedious