A mixing device for highly viscous fluids with finite electrical conductivity is investigated theoretically. Stirring is performed by means of electromagnetic forces provided by inductor wires located outside the flow domain. The flow shows hyperbolic and elliptic singular points. Inductors are displaced in a periodic manner, leading to an efficient stretching and folding mechanism. The goodness of mixing is quantified by means of a geometrical analysis based on box-counting techniques. This analysis gives valuable information about advection of a spot of dye injected in the flow, in the limit of infinite Peclet numbers. A spatiotemporal criterion for mixing efficiency is derived, and characteristic scales are analyzed. The influence of various parameters on mixing efficiency is discussed by making use of both the geometrical analysis and Poincaré sections.