a) The dimensionless torque G normalized by G₀ ≡ G(Re,R_Ω = 0) for different values of the radius ratio η nicely collapses when plotted as function of R_Ω. The data from Wendt (1933) are shown as cyan (η = 0.68), blue (η = 0.85) and green (η = 0.935) triangles. Additional radius ratios are examined using the data from Taylor (1936), shown as orange diamonds, Richard (Richard 2001; Dubrulle et al. 2005), given by magenta squares, Paoletti & Lathrop (2011) indicated by black circles and van Gils et al. (2011b) by red circles. The solid lines are fits given by Eq. (12). Keplerian flow states are indicated by the dashed line at R_Ω = −4/3. In b) we focus on the peak of the curve shown in a), but only present the data from Twente (van Gils et al. 2011b) (red) and Maryland (Paoletti & Lathrop 2011) (black). The solid line is the fit (12).