Some biopotential measurement systems (especially those without active electrodes) require the impedance at the electrode-electrolyte-skin interface to be as much as 100 orders of magnitude lower than the amplifier’s input impedance (e.g. 10 kOhm at the skin into 10 MOhm at the amplifier) to make accurate, noise-free biopotential measurements.  There is more than one way to reduce contact impedance, and these skin preparation products can make the job easier for you and more comfortable for the participant.  Note that measurements with active electrodes generally do not require preparation of the skin by means of abrasion to reduce skin contact impedance, but most active electrodes do require the use of a conductive gel.  Depending on the input impedance of the recording apparatus, active electrode sysems may tolerate skin-contact impedance as high as 300 kOhm.  Unprepared skin at the scalp typically has an impedance of between 50 and 250 kOhm.