Another important application of the space-charge tetrode was as an electrometer tube for detecting and measuring extremely small currents. For example, the General Electric FP54 was described as a "space-charge grid tube ... designed to have a very high input impedance and a very low grid current. It is designed particularly for amplification of direct currents smaller than about 10 amperes, and has been found capable of measuring currents as small as 5 x 10 amperes. It has a current amplification factor of 250,000, and operates with an anode voltage of 12v, and space-charge grid voltage of +4V."