Basic Electrical Theory
Magnetic Field and Polarity of a Coil
Bending a straight conductor into a loop has two results: (1) magnetic field lines become more
dense inside the loop, and (2) all lines inside the loop are aiding in the same direction.
When a conductor is shaped into several loops, it is considered to be a coil. To determine the
polarity of a coil, use the left-hand rule for coils (Figure 23).
Figure 23 Left-hand Rule for Coils
Adding an iron core inside of a coil will increase the flux density. The polarity of the iron core
will be the same as that of the coil. Current flow is from the negative side of the voltage source,
through the coil, and back to the positive side of the source (Figure 24).