Here is the point that you will find very important in the near further --a coil of wire carrying a current as a magnet. if a length of wire carrying a current bent to form a loop, the lines of forces around the conductor all leave at one side of the loop and another side. thus the loop of wire carrying a current acts as a weak magnet having a north pole and a south pole. the north pole on the side at which line of the force leaves the loop and the south pole on the side at which enter the pole. 

If you desire to make the magnetic field of the loop stronger, you can from the wire into a coil of many loops as shown.  now the individual field of each loop is in series and from one stronger magnetic field inside and outside the loop. in the space between the turns, the lines of the force are in opposition and cancel each other out. the coil act as a strong bar magnet with the north pole is the end from which the line of the force.

A left-hand rule also exists to determine the direction of the magnetic field. if the finger of the left hand is wrapped around the coil in the direction of the current flow, the thumb will point toward the north pole and of the coil. 

Electromagnets:-

Adding more turns to the current-carrying coil increase the number of lines of force, causing it to be at a stronger magnet. an increase in current is also strong than the magnetic field so strong electromagnets have a coil of many turns and carry a large current as the wire size permits.in comparing coil using the same core or similar cores, a unit called the ampere -turn is used.

This unit product of the current in the ampere and the number of turns of the wire. although the wire strange of the electromagnet is increased both by using a large current flow. and many turn to from the coil, these factors do not concentrate the field enough for use in a principal generator. to further increase the flux density, an iron core is inserted into the coil. because the iron core offers much less reluctance  (opposition) to the line of force than air, the flux density is generally increasing.

If the iron core is bent to form a horseshoe and two coils are used, one on each leg of the horseshoe-shaped core as they illustrate, the line of the force travel around the horseshoe and across the air gap, causing a very concentrated field to exist across the air gap.