Figure 2.

Additional images of critical aspects of the human magnetic exposure at Caltech. A, Partially complete assembly of the Faraday cage (summer of 2014) showing the nested set of orthogonal, Merritt square four-coils (Merritt et al., 1983) with all but two aluminum walls of the Faraday cage complete. B, Image of a participant in the facility seated in a comfortable, non-magnetic wooden chair and wearing the 64-lead BioSim EEG head cap. The EEG sensor leads are carefully braided together to minimize electrical artifacts. The chair is on a raised wooden platform that is isolated mechanically from the magnet coils and covered with a layer of synthetic carpeting; the height is such that the participant’s head is in the central area of highest magnetic field uniformity. C, Schematic of the double-wrapped control circuits that allow active-sham experiments (Kirschvink, 1992b). In each axis of the coils, the four square frames are wrapped in series with two discrete strands of insulated copper magnet wire and with the number of turns and coil spacing chosen to produce a high-volume, uniform applied magnetic field (Merritt et al., 1983). Reversing the current flow in one of the wire strands via a DPDT switch results in cancellation of the external field with virtually all other parameters being the same. This scheme is implemented on all three independently controlled coil axes (Up/Down, East/West, and North/South). D, Fluxgate magnetometer (Applied Physics Systems 520A) three-axis magnetic field sensor attached to a collapsing carbon-fiber camera stand mount. At the start of each session, the fluxgate is lowered to the center of the chamber for an initial current/control calibration of the ambient geomagnetic field. It is then raised to a position ∼30 cm above the participant’s head during the following experimental trials, and the three-axis magnetic field readings are recorded continuously in the same fashion as the EEG voltage signals. E, Air duct. A 15 cm in diameter aluminum air duct ∼2-m-long connects a variable-speed (100 W) electric fan to the upper SE corner of the experimental chamber; this is also the conduit used for the major electrical cables (power for the magnetic coils, sensor leads for the fluxgate, etc.). F, G, An intercom/video monitoring system was devised by mounting a computer-controlled loudspeaker (F) outside the Faraday shield on the ceiling North of the chamber coupled with (G) a USB-linked IR video camera/microphone system mounted just inside the shield. Note the conductive aluminum tape shielding around the camera to reduce Rf interference. During all experimental trials a small DPDT relay located in the control room disconnects the speaker from computer and directly shorts the speaker connections. A second microphone in the control room can be switched on to communicate with the participant in the experimental chamber, as needed. An experimenter monitors the audio and video of participants at all times, as per Caltech IRB safety requirements. H, LED lights, 12 VDC array, arranged to illuminate from the top surface of the magnetic coils near the ceiling of the chamber. These are powered by rechargeable 11.1-V lithium battery packs (visible in E) and controlled by an external switch. I, Ferrite chokes. Whenever possible, these are mounted in a multiple-turn figure-eight fashion (Counselman, 2013) on all conductive wires and cables entering the shielded area and supplemented with grounded aluminum wool when needed. J, Image of the remote-control area including (from left to right): the PC for controlling the coils, the DPDT switches for changing between active and sham modes, the fluxgate control unit, the three power amplifiers that control the current in the remote coil room, and the separate PC that records the EEG data. Participants seated in the experimental chamber do not report being able to hear sounds from the control room and vice versa. Additional guidance for the design of biomagnetic experiments is given by Kirschvink et al. (2010) and Schwarze et al. (2016).