Question

how can you increase strength of an electromagnet

Answer 1

As current runs through a straight wire, a circular magnetic field is generated around it. When a wire is made into a circle, the current generates a magnetic field parallel to its axis. If you pile loops on top of each other, as in a coil or solenoid, you increase the magnetic field strength.

The formula for the magnetic field inside a coil is the current multiplied by the loop-count density multiplied by a constant.

Answer 2

by increasing the amount of current in the circuit
and by increasing the number of loops in the coil