through a wire 21 to the solenoids of the valves 1 and 4 and another through a wire 22 to the solenoids of the valves 2 and 3 each pulse train is of the type shown in fig2 here the voltage v is shown as a function of the time t during equal periods t the voltage is v 1 during part t 1 of each period while it is zero during the remaining part of the period i e t t 1 a solenoid activated valve being influenced by the pulse train of fig2 will perform opening and closing movements as shown in fig3 here the valve travel is designated by s and a fully opened valve is obtained after a travel distance s 1 it appears that due to reactance and inertia of a solenoid governed valve a complete opening will be dependant on obtaining a certain proportion between t 1 and t the servosystem of fig1 is influenced in both directions in one direction by the pulses through the wire 21 and in the other by the pulses through the wire 22 thus it will be the difference between the pulse width t 1 of the two pulse trains and not the actual width which will determine the function of the servosystem as the piston 11 is always influenced in both directions the friction.