`==Approximations in dependence on length of solenoid==  The equation on the inductance of a solenoid states that it is inversely proportional to ``l`` (the length of the solenoid).  This implies that inductance would be infinite for a zero-length solenoid.  Certainly a single-winding solenoid could be made with a length arbitrarily close to 0.  So I'm guessing this equation is an approximation with assumptions (eg. ).  Could these assumptions please be stated?    `