Testing tank

In progress.



FOOTNOTE 1 (in image above): Note that skull resistivity is 1-2 orders of magnitude higher than brain resistivity, which has been measured as 300-800 ohm-centimeter.
FOOTNOTE 2: Note that we need to be able to turn the electrode 90 degrees, so that we can measure the electric field along both axes of the tank. A hinge or rotating lock could do the trick.


Can build with hand tools, getting key parts commercially. No specialized fabrication needs.


1. Tap plastics
2. Tektronic
3. Digikey
4. A-M Systems


Tank (made of acrylic and epoxy)

thin insulated wires (In order to measure voltages, we need to have very high impedance electrodes. One idea, which also reduces the possibility that eddy currents in the electrodes themselves could heat locally, is to use silver chloride electrodes at the end of the wires. A-M systems sells silver chloride electrodes that are attached to wires, for simple soldering to wires. Another strategy, that I prefer, is to simply buy thin teflon-coated silver wire, insulated except for the very tips, and to bleach those tips overnight (commercial 5.25% bleach is fine). Wash the tips several times with distilled water, then let dry.)

wire stripper

oscilloscope, or Nidaq board (preferred)

voltage amplifier

analog low-pass filter (can also be done in software if Nidaq board is used)

XYZ translator: XYZ translators that can cover 15 cm in each direction, with ~ 1 millimeter resolution? Ideally can be computer driven, too (this is in principle many thousands of data points).
— Could program a CNC mill to do the 3D mapping.
— Could build one out of multiple DC servo motors.
— Could take three old inkjet printers and mount the inkjet head translators perpendicular to one another.
— Commercial systems exist, but may be overkill for this relatively simple and robust measurement. ASI Imaging


1. Get or make acrylic tank (e.g., from Tap plastics). Attach dowels or furniture feet below, to elevate the tank and make room for the test coil to be inserted underneath. Bleach the silver wire electrodes, if necessary.

2. Make a field probe by twisting the silver wire electrodes together and de-insulating the tips, spacing them 1 mm apart. Attach the ends to a voltage amplifier and then the oscilloscope or NiDaq? board. Voltages of interest are expected to be in the 0.01-20 mV range, and in the 1-100 kHz timescale; use that to set the amplifier, digitizer, and software settings.

3. Attach wire ends to the translation stage. Place coil under tank.

4. Map the electric field in the region of interest (probably some tens or hundreds of cm^3). Most likely you will want to map at a high-resolution in a single xy plane, or go up and down a particular vertical plane, since there are a lot of points.

5. Using estimates of rheobase and chronaxie, calculate the focality of the coil being tested. How deep into the brain will the coil drive neural activity?


Need to finish a prototype coil to test in the tank!

IF NIDAQ BOARD: Need to write a little script that will navigate the electrodes around.

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