Figure 1.

A simple circuit for introducing blind conditions in experiments using static or low-frequency magnetic fields, adapted from Kirschvink (1992b). When building the coil systems, wrap them carefully with two identical strands of wire, placed adjacent to each other and held firmly in place with high-conductivity thermal epoxy (to minimize vibration). These ‘double-wrapped’ coils can then be configured with a silent DPDT switch that will allow the current to flow in series through the two coils, but permits the direction of flow to be reversed in one of them. (a) Circuit with the DPDT switch set so that the current in the two coils is in opposition, yielding no external magnetic field (sham mode). (b) Same circuit with the switch set to produce parallel current flow, yielding the external magnetic field (active mode). We recommend using quasi-random Gellermann (1933) orders to set the active/sham states by a person external to the experiment. If that is impractical, a second DPDT switch can be inserted in the circuit, and two separate investigators can each control one of the switches with separate random orders to ensure fully blinded experimental conditions. Recommendations for multiple-coil designs to produce uniform magnetic stimuli within an experimental chamber are also given by Kirschvink (1992b).