Hi,
read below "similar threads"
In my eyes measuring this voltage makes no sense, because it is "floating". The voltage ins inpredictable an may change any time. (Caused by airflow, humidity, touching, connections of other floating devices...) Additionally the measurement device usually has a finite input impedance, so the measurement itself affects this voltage. This means the voltage during measurement may be different as when mesurement devise is disonnected.
Another problem is that this voltage usually is a mixture of DC, low frequencies and high frequencies. How do want you handle this?
--> Maybe it is more interesting to ground one point and measure the GROUND current.
--> Instead of "undefined floating" you could pull it defined to GND. Maybe with 1MOhms for DC and 1nF for AC.
Klaus