Can someone please tell me how this device can possibly work? Is it just an electro scope our something more? Can you really measure a potential without accessing it. http://www.plesseysemiconductors.com/epic-plessey-semiconductors.php

 

EPIC is a noncontact electrometer,.....................................
..........................The capacitor Cext models the capacitive coupling to the measurement target.

That works by capacitive coupling, like a touch switch.

 

That works by capacitive coupling, like a touch switch.

What does the advertised "ultra high impedance" have to do with it?

 

If you (you being a sensor) don't need much current to sense something, you can sense things that have very little ability to supply current.

 

The capacitor value is very low so its impedance is very high, since the impedance is proportional to the reciprocal of capacitance.

 

when measuring with an electrometer, either tube or fet, what is called impedance is actually resistance, they dont really measure ac, more likely the charge of a capacitor. the electrometer tubes are evacuated to a high vacuum, and the electrometer fets are designed for very low leakage. often used to measure radiation, as particals go through the partial atmosphere of an ion chamber., some more info here, http://www.techlib.com/
several ion chamber projects and explations.