Technical conductors are inherently polycrystalline and do not behave as a single crystal in most cases. In MgB2 wires, the changing crystallographic orientation of the grains leads, together with the upper critical field anisotropy of this material, to a variation of the local properties when a magnetic field is applied. In the cuprate superconductors on the other hand, this effect is negligible compared to a grain-to-grain misalignment, which drastically reduces the critical current. The strategy for overcoming this limitation was the development of highly textured conductors, most successfully by the coated conductor technology. However, granularity effects are still relevant in some architectures. The grain boundary limitation is weaker in the iron-based superconductors, which may enable cheaper production techniques.
Recent scanning Hall probe measurements are reported addressing granularity effects in different superconductors. While granularity effects can be directly visualized for two-dimensional films, average inter- and intra-granular currents can be obtained from bulk samples. Strategies for optimization of granular materials will be discussed.