Since we all know that to increase the magnetic field we can do these of the following things:
(i) Increase the number of turns in a coil,
(ii) Increase the current following through conductor,
and there would be more but I know only these (upto my standard - Xth )
now if I use the first method to increase the magnetic field then the length of conductor will also increase because I am increasing the number of turns; say I am increasing the length of conductor by $x$ meters then as we all know that:
$R = \rho l/a$
you know resistance is equal to resistivity multiplied by length of conductor and then divided by the area of cross section.
According to this $R$ is directly propotional to length of conductor and by Ohm's law $R$ is inverously proportional to current
Therefore if I increase the length by $x$ m of conductor then the resistance will also increase at a rate of $x \Omega $ and current will decrease at $x$ Ampere, then how will the magnetic field increase if the current is being reduced?