The T4900 will continuously measure and compare the reactive current on the generator to an external DC reference signal connected to the "parallel line" input. The T4900 will increase or decrease the voltage of the generator (through the AVR) if it finds the reactive current level to be either too low or too high. Two built-in relays provide the increase and decrease pulses for the automatic voltage regulator (AVR). The strength (width) of the pulses is proportional to the voltage deviation. The E7800 Motorised Potentiometer can be used to adapt the contact pulses to a signal suitable for most AVR's.