An inexpensive optically isolated nanoammeter, which permits digital logging of dc currents from an electrospray colloid thruster operated in vacuum has been designed for using with micro-Newton electric propulsion technology. The two-stage optically isolated system is intended to safely measure and record a current generated at high voltage, facilitating data acquisition with a grounded logging device such as a personal computer. It operates by converting the current into a frequency in a battery-powered, floated, high-voltage stage. Agilent Technologies fiber-optic kits were selected for their low cost and reliability. The fiber-optic cable passes between the high voltage and the grounded receiver stage via grommeted holes drilled in the metal housing. The use this optically isolated system permits accurate monitoring of micro-Newton thruster currents directly rather than through inaccurate proxies, giving physical insight into the behavior of the system at high accuracy and low cost.