The folding phenomenon of the comet tail rays is shown to be compatible with the Ferraro isorotation law if the comet tail magnetic field has no azimuthal component: Bφ = 0. Considering electric drift due to convectional electric fields one obtains a formula for the angular rate of a ray closure which reduces to that of Ness & Donn if the velocity profile across the tail is linear. The magnetic field B of about 20–40 γ in the coma and B ≲ 10 γ in the distant tail is estimated under typical solar wind conditions at 1 AU.