Answer: Using the same number of turns of wire for each electromagnet.
The formula of magnetic field strength of a solenoid or electromagnet is,
[tex] B= \mu n i [/tex]
Where, B is the magnetic field strength, μ is the magnetic permeability, n is the number of turns per unit length, and i is the current in the wires.
To study the change in magnetic field strength B, the current i, and number of turns per unit length should be same so that [tex] B \propto \mu [/tex] and we can study the change of quantity of iron as core.