The voltage induced in a coil of wire in a uniform time harmonic field may be estimated as follows:
Voltage ~ i x 2 x pi x frequency x Bz x pi x (Ro^3 - Ri^3)/(3 x (Ro-Ri))
~ i x 2 x pi x frequency x Bz x pi x (Ro^2 + RoRi + Ri^2)/3
where the lower case "i" is the imaginary number sqrt(-1), "Bz" is the component of magnetic field that is perpendicular to the face of the coil (parallel to the coil axis), Ri is the inner radius of the coil and Ri is the inner radius of the coil.