Figure 1: Schematic diagram of the apparatus used by Gallet et al. to simulate magnetic fields created by an astrophysical dynamo. Two impellers counterrotate at frequencies $F_1$ and $F_2$. When $F_1=F_2$ the setup is invariant under rotation by 180 degrees about the $z$ axis. The vertical thick lines indicate the locations of the arrays of three-axis Hall probes, which measure the magnetic field close to each disk.