119 as seen in fig1 in general when a dial knob 126 on controller 120 is turned then a contact 210 of the slave potentiometer fig4 is moved with respect to a resistor 212 which serves to vary the resistance more particularly when dial 126 is turned from a 34 o 34 or center position in one direction by the operator then the current in the armature of motor 202 in servo actuator 118 is varied so as to drive the spool of pump control valve 116 from its 34 center 34 position to its 34 right 34 position similarly when dial knob 126 is turned from its center position in the other direction electric motor 202 in servo actuator 118 moves the spool in pump control valve from its 34 center 34 position to its 34 left 34 position referring particularly to fig4 this represents an appropriate circuit which includes a transistor network indicated generally at 220 such transistor network functions to vary the potential in the armature of motor 202 which motor is of the permanent magnet d c type responsive to movement of contact 210 of a slave potentiometer along the resistor 212 when such contact 210 and the previously mentioned dial 125 which moves it fig2 and 2 a are centered the potential supplied by transitor network 220 balances the field and armature of motor 202 and the motor is stationary with valve actuating arm 204 in its centered.