The Resistance of a Montage

I am of the opinion, like some other people in the early adopter tDCS community, that a single 9V battery is not sufficient to provide the full regulated current (particularly 2mA) for all the well known montages. So I did a small experiment to give some evidence to this.

 (she was not at the party, pilfered the pic from www.diytdcs.com, hope ok John!)

At an Easter party, on five people, I measured the resistance of the R Anode F10 to L Cathode upper arm, also known as the Darpa accelerated learning montage. The people may have been stuffed with turkey and ham and wine, but I do not think that would have affected their resistance very much.

As the method, I used the Cognitive kit at a setting of 1mA. The Cognitive kit comes with sponge electrodes and the electrodes were soaked in saline of 1/4 tsp per cup of water. The current regulation of the Cognitive kit will be +/-5%, hence by measuring the voltage between the Anode and Cathode with a simple voltmeter, and knowing the expected current, the resistance can be calculated with some confidence. Just by testing the resistance with the meter may work, but the injected current is unknown and the accuracy is questionable.

Results were as follows:

1. 6.7kohm
2. 7.4kohm
3. 6.8kohm
4. 7.8kohm
5. 6.3kohm

Actually I was surprised at the consistency of the measurements, but I am confident of the method and then of the results. If this resistance is consistent with other people, then a single 9V battery will not be about to provide 2mA with this montage.

It may not make me popular to say it, but that means DIY devices and commercial devices that use only a single 9V battery would be inadequate to provide the 2mA for this particular montage.

Of course, I did this not just for curiosity sake, the Cognitive kit is designed with 2 x 9V batteries for this very reason. As a side note, the higher end tDCS devices (which includes Human Bioelectric's future device, MindStim) may have only one 9V battery, or even a lower voltage battery, but they include a boost circuit to raise the voltage to provide the potential needed to supply the 2mA in the high resistance circumstances.