Switching in high Tc superconductor current transport measurements

It is pointed out that switching voltages can occur in four-wire current transport measurements of sintered high-T_c superconductors. These switching voltages are irreversible shifts in the voltage-current characteristic of the superconductor that result in multiple branches. The voltage along these branches can be very nonlinear as a function of current and can be positive or negative in polarity relative to the current direction. These voltages can interfere with the correct determination of resistivity and critical current density. Experimental data on unaligned sintered Y₁Ba₂ Cu₃O_7-δ which illustrate the complex nature of the voltages and the confusion they can create are presented. Models based on weak links and H_c1 and on other effects are discussed as are observations on NbTi and Nb₃Sn-based superconductors