Solenoid
= Solenoid
{title2=$B=\mu_0NI$}
A solenoid is a helical winding of conducting wire. For an ideal very long winding with $N$ turns per unit length and negligible pitch, <Ampère's law> gives a uniform axial <magnetic field> $B=\mu_0NI$ inside and zero field outside. Finite-length end fields and the pitch correction are excluded by this approximation. The <self-inductance> of a length $l$ and cross-sectional area $A$ is $L=\mu_0N^2Al$.