How is voltage measured using non-metallic contact?

QI heard that you have a power meter that can measure voltage without making contact to metallic terminals. What is the measurement principle?


Hioki's non-contact voltage sensor contains an internal  electrode (metallic plate). 
The electrode and measurement target circuit are capacitive coupled and a very small current flows when there is an electric potential difference between the electrode and circuit.
Then, this current flow is detected, and voltage is generated internally to eliminate the current.
Since the internally generated voltage and the voltage to be measured are equivalent, measuring the generated voltage enables the instrument to provide accurate voltage measurement without making metallic contact.