the second and third hydraulic volumes 12 13 are so much smaller that the thermal effects are compensated the first hydraulic volume 11 in this case remains constant secondary effects such as the rigidity of the actuator being negligible no leakage occurs as the entire hydraulic system 18 is enclosed in metal bellows the pressure in the hydraulic fluid remains constant at least as long as the spring 14 operates in a proportional area the thermal longitudinal compensation is an advantage but compensation is also provided for such longitudinal alterations which are not thermally generated included therein are ageing processes in the solid state actuator for example which can alter the polarization thereof and thus the length thereof as a result of the compensation for play all elements remain in contact with rapid processes a quite different behavior of the system is exhibited during the brief actuation period of the actuator the flow resistance in the annular gap 5 is sufficiently high for practically no fluid exchange to take place between the first and the second hydraulic volumes 11 12 typical injection processes during fuel injection in the motor vehicle however only last a few milliseconds thus the two desirable properties u201c hydraulically sealed with compensation for play u201d and u201c metallically sealed in a leakage free manner u201d are combined in one arrangement the separation of the functions takes place over the different time constants in this case the time constants of the compensation for play may be set by the dimensioning of the size of the annular gap 5 and the viscosity of the hydraulic fluid only metal bellows are moved these require no particular guides and also are not particularly.