I took part in a competition of my college's electronics club to build an electromagnet under £100, with the strongest and cheapest one winning.
Thanks, Jessie xx
Go to the botton 1/2 of this page and look at their BB experiment, it looks like with more amp/turns more BB's were lifted.
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With 35 amps available at 12 volts You can figure the minimum resistance of the wire in your coil. Use ohms law, E=IxR, rearrange to E/I=R so, 12volts / 35Amps = .34 ohms. Lets say use use a #20 wire, it has 10.15 ohms of of resistance per 1000ft or .01 ohm per foot. We want .34 ohms so .34/.01 = 34 ft If you use #10 wire with 1 ohm per 1000ft then; .34 / .001 = 340 ft Now, you need to maximize turns within the physical size of your magnet while keeping your current at 35 amps. The next thing to consider is the magnetic circuit. With just a rod in the center you have a long path with a large air gap. The magnetic field has to find its way from one end of the rod back around to the other end. Most ot this is through air with no concentrating affect. If you add the proper magnetic conductor (iron?) on the inside and the outside of the coil you reduce the resistance of the magnetic circuit. The form often used is shown here;
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Basically a cup with a rod in the center, that way the only magnetic circuit to be completed is at the front of the magnet. Notice they have one with 4700 lbs of pull using only 60 watts. The form factor is 10" dia and 2.5" tall, that may be usefull.
I don't know what it is your lifting so if you need just a rod, here's a little info about that.
Look at page 122 of this google book. You want Max amp/turns and a conical tip.
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Summarize what you have learned and someone will correct you. Good luck, Mike