misha82,
An electrometer is a device for measuring electric charge. In its most basic form, it consists of 2 pieces of metal foil.
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A picoammeter is a device for measuring low amplitude current.
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Both electron current and ion current are measures of flow of charge. The only difference is whether ions (charged atoms/molecules) or electrons are the vehicle for carrying the charge.
Regards,
Krasl
Added after 16 minutes:
misha82,
In the 3rd paragrph of my previous reply, please change "flow of charge" to "rate of flow of charge".
Regards,
Kral