Re: Electromagnet field strength vs current

     Good question!  Any real magnetic material has parameter specifying 
its "magnetic hardness" called the coercivity.  The field applied must be 
larger than the coercivity in order to magnetize it very much.  The field 
is given by 

                   H = 4 Pi N I/(10L)

where N is the number of turns, I is the corrent in Amps, and L is the 
length of the coil in cm.  With N = 100 and L = 12 cm the field is about 
10 Oe.  This is about right for the coercivity of a "soft" iron rod.  Try 
a big nail.  It may have a lower coercivity than the rod you are using.  
You can get them at any hardware store.