Heres what you need to know.
Your battery is rated at 235 cold cranking amps. That means its designed to put out 235 amps at or above a specific temperature. Usually 32 degrees F but some times lower. What that means is it can provide a 235 amp current while maintaining 12 volts at the terminals. It will supply more amps than 235 but its output voltage will drop off rapidly. Also this output capacity is only for a short period typically 10 -20 seconds at best.
Ideally you would want a heavy piece of wire to be able to carry that current.
12 gauge copper can take short bursts of over 200 amps for a few seconds. Approximately 25 feet of 12 gauge will have a current draw of around 200 amps at 12 volts, less as it heats up. 14 gauge would be around 17 feet. And 16 gauge would be around 12 feet.
That should work well enough for magnetizing since it only takes a second to get iron magnetized if its the type that can be magnetized. Many types cant or at least wont hold their charge very long.
As far as connecting I would recommend using a heavy relay like whats used for vehicle starters or one of those old fashioned starter buttons they had on tractors. Your local auto parts store should have both for around $12.
They can take the very high currents without problems. Plus getting your system down to a simple push button or other type of switch will make it safer and easier to work with.
200+ amps going through an iron core inductor (electromagnet) will produce a very high voltage and current inductive kickback when the circuit is broken.
Use solid and adequate sized connectors and wire if possible too.