Answer:
When we have a current I, we will have a magnetic field perpendicular to this current.
Then if we have a wire in a "spring" form. then we will have a magnetic field along the center of this "spring".
Now suppose we put an iron object in the middle (where the magnetic field is) then we will magnetize the iron object.
Of course, the intensity of the magnetic field is proportional to the current, given by:
B = (μ*I)/(2*π*r)
Where:
μ is a constant, I is the current and r is the distance between to the current.
Now remember that for a resistor:
R = ρ*L/A
R is the resistance, ρ is the resistivity, which depends on the material of the wire, L is the length of the wire, and A is the cross-section of the wire.
If we increase the area of the wire (if we use a thicker wire).
And the relation between resistance and current is:
I = V/R
Where V is the voltaje.
Now, if we use a thicker wire, then the cross-section area of the wire increases.
Notice in the resistance equation, that the cross-section area is on the denominator, then if we increase the area A, the resistance decreases.
And the resistance is on the denominator of the current equation, then if we decrease R, the current increases.
If the current increases, the magnetic field increases, which means that we will have a stronger electromagnet.
(length of a tennis court)