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IntroductionPassive electrodes have no inbuilt circuitry.
Saline ElectrodesSaline electrodes' main component is a sponge full of salt water which is next to your skin.
Paste/Gel electrodesThese are the traditional kind. Basically, you find some bare skin on the appropriate part of your head (ie. the bits usually covered with hair), and paste these things to your head.
You may want to try the Electrode Howto
Paste/GelYou can get some from the Electrode Store listed above. Additionally, Andreas Robinson once made the following comment on the mailing list, in response to a message about homemade alternatives to 10-20 electrode paste:
About electrode paste: I can't help you there. I am using essentially the same thing: home made starch/gelatine gel with salt water. Cheap, but without any adhesive properties.
Other materials for electrodes
Jim Peters says:
Good skin contactThe results of using passive electrodes depend on a number of factors all being just right. One of these is the quality of the skin contact. The following are some tips on ensuring good skin contact.
Electrode CablingEach channel has two electrodes. Each channel has + and - inputs (electrodes), and calculates the difference of the input voltages. This cancels out or attenuates common mode signals like mains hum that (ideally) have the same signal strength on + and - inputs.
With 2 channels 5 electrodes (including DRL) are required (or at least 4 when 2 channels share one reference (-) electrode)
DRL is an output that acts as an active earth and suppresses mains hum (at least in theory) even more than a simple connection to VGND.
ShieldingYou will need to use shielded cabling for all passive electrodes. The cables from the electrode store are not suitable. This is necessary to get the DRL working effectively. Andreas said:
First, any cheap shielded cable should work. Not too cheap though. Very low cost audio cabling can be worthless, that is have very little shielding.
Assume you want a 1.2 m electrode cable (longer than 1.8 m is not recommended). Have both leads for one EEG channel run in the same cable, sharing the same shield, for about 1 meter. Then have the cable branch into two leads the last 20 cm (or longer if you have a big head. ;-). The branches are also shielded of course, up to the electrode.
In short, it should look like a Y.
By doing this, you reduce the capacitive loading on the electrode leads, and get more managable cabling too.
Just remember to only attach the cable shield to the amplifier board, and not the electrode itself.