A coil of wire that has a long axial length relative to its diameter. The coil is usually tubular in form and is used to produce a known magnetic flux density along its axis.
At a point on the axis inside the solenoid, ignoring any end effects, the magnitude of the magnetic flux density, B, is given by
where μ0 is the magnetic constant, n the number of turns per unit length, I the current flowing through the solenoid, and θ1 and θ2 the semiangles subtended at the point by the ends.
A solenoid is often used to demonstrate electromagnetic induction and a bar or rod of iron that is free to move along the axis of the coil is usually provided for this purpose.