We have already laid down the principle which will enable us to design
electromagnets to act at a distance. We want our magnet to project, as
it were, its force across the greatest length of air gap. Clearly,
then, such a magnet must have a very large magnetizing power, with
many ampere turns upon it, to be able to make the required number of
magnetic lines pass across the air resistance. Also it is clear that
the poles must not be too close together for its work, otherwise the
magnetic lines at one pole will be likely to curl round and take short
cuts to the other pole. There must be a wider width between the poles
than is desirable in electromagnets for traction.