Membrane potential and voltage clamp

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lysmata
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Membrane potential and voltage clamp

 I am not very clear about the measurement of the membrane potential. Especially on the qualitative aspect of the phenomenon. What 's happened at the microelectrode? How a reference electrode inside a pipette, filled, for example, of 3M of KCl can "feel" the potential and can translate it in a current? 

And also about Voltage and current clamp...What is meant by current injection? There is an augmentation of ionic concentration? If so, what kind of ion? And this, doesn't affect the cytoplasmic concentration of that ion? 

Thanks to all.