A moving charge creates a magnetic ﬁeld. If this moving charge takes place as current in a wire, the magnetic ﬁeld ﬂows around the wire. When the wire is formed into a coil the magnetic lines of ﬂux reinforce each other and thereby increase the ﬁeld within the coil (Figure 11.1a), which is given by
magnetic ﬁeld strength
the number of turns within the coil
In what used to be the more common centimeter-gram-metric (cgs) units,
was expressed in oersteds when
was in amperes and
was in centimeters. The System International (SI) units for
is ampere turns per meter, more often expressed as amperes per meter (A/m).