The magnetic force at a point on its axis due to a current in a helical coil of one turn

The incomplete circuit formed by a single turn ABC of a helix of pitch p, which lies on a cylinder of radius a, is completed by a straight link CA, of length p, parallel to the axis of the cylinder. Through any point O on this axis are taken rectangular axes Ox, Oy, Oz. Of these, Ox is the axis of the cylinder, and Oz is parallel to the shortest distance between Ox and CA. The components H1, H2, H3 of the magnetic force at O, due to a current in the circuit, are found for the case when p/a is small.