This thesis deals with the prototype development of a floating potential probe (FPP) instrument to measure spacecraft charging. The measurement concept is based on the works at the Naval Research Laboratory in Washington DC and is in this thesis further developed using modern components and with an added multi gain stage for a larger measurement range. This thesis explains the measurement principles, the design, and the test and verification of the instrument. The concept for this instrument is to have a spherical Langmuir probe, iso- lated from the spacecraft’s platform ground, and floating with the plasma po- tential. By adapting the probe sensor’s input impedance to the plasma environ- ment, the probe instrument is able to measure the spacecraft’s charging, relative to the plasma potential. The electronics inside the probe splits the signal into a low gain and a high gain signal, thus increasing the range while maintaining a good signal-to-noise ratio for charging levels ranging from 10 mV to 1.5 kV. Because of rapid high level charging events in the ionosphere, this instrument was designed with a time constant less than 1 ms. Spacecraft charging simu- lations in both the electronics laboratory and inside the UiO plasma chamber confirmed the instrument’s performance according to the requirements. The calculated time constant from these measurements was around 4-500 μs, show- ing that the instrument is more than capable of measuring these rapid changes in spacecraft charging. Due to design and laboratory instrument constraints, the instrument was only tested for up to ±50 V. For the positive 50 V plasma chamber test, the time constant was found to be increasing (around 700 μs) which is probably due to some unexpected behavior in the plasma chamber due to the expanding sheath. There were also made two probes with different input resistance. The small resistor probe did not manage to track the changes of the potential while in the plasma chamber. It was discovered that the connection between the probe surface and the PCB was not sufficiently secured, and fixing this should result in the probe performing as expected in future tests. The conclusion of the work in this thesis is that the concept seems to work as expected, with some limitations that can be addressed in future designs.