A robust floating nanoammeter
  A circuit capable of measuring nanoampere currents while floating at voltages up to at least 25kV is described. The circuit relays its output to ground potential via an optical fiber. We particularly emphasize the design and construction techniques which allow robust operation in the presence of high voltage spikes and discharges. 
B. E. Sauer, D. M. Kara, J. J. Hudson, M. R. Tarbutt, E. A. Hinds