Subject: Electromagnet field strength vs current

A standard experiment is to vary the current through an electromagnet made 
from 
a soft iron core about 8 mm. diameter and 12 cm. long with 100 turns of wire.  
With currents below about 1 amp, there is very little effect (measured by 
counting paperclips picked up or by measuring the force on a soft iron rod)  
Over one amp the relationship is clearly linear.  It is not an effect due to 
the clips or the rod becoming magnetised) Why is this?

Re: Electromagnet field strength vs current