Coil with Variable Number of Turns per Unit Length For the investigation of magnetic flux density of cylindrical coils as a function of the number of turns per unit length. Measurement of the number of winding turns per unit length is carried out using a teslameter (e.g. 8533981) with Hall sensor. An insertable plastic strip is used to mount a compass in order to determine how the field lines align. The coil can be mounted on any 10-mm stand rod.