Drift velocity ﹩v_{Bx}﹩ of the magnetic field as a function of the density ﹩N_{\mathrm{H}\,}﹩ and the field strength B for a collapsing cloud in model A. The dotted curves represent the contours of constant ﹩v_{Bx}﹩ normalized by the free‐fall velocity ﹩u_{\mathrm{ff}\,}﹩, and its logarithmic values are labelled on those curves. The solid curve shows the locus along which ﹩v_{Bx}=u_{\mathrm{ff}\,}﹩ is satisfied. The dashed line represents the locus ﹩\vert \tau _{\nu }\omega _{\nu }\vert =1﹩ for electrons. The dot‐dashed line represents the critical field ﹩B_{\mathrm{cr}\,}﹩, given by eq. (3).