The correct formula for the magnetic field strength inside a solenoid is:
$B = \mu_0 n I$
Where:
$\mu_0$ = permeability of free space ($4\pi \times 10^{-7} \, Tm/A$)
$n$ = number of turns per unit length (turns/meter)
$I$ = current through the solenoid (Amps)
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