The soft iron core helps in concentrating the magnetic lines of forces through the solenoid, so that the magnetic field is almost uniform at the end face of the core. In the absence of a core, the magnetic lines of forces start to diverge by curving sharply immediately outside the coils. The smaller the space between the coils and the core, the stronger is the magnetism induced in the core. Therefore, the best electromagnets have coils wound tightly around the core.