This work studies the effect of varying air-gap length on performance and behavior of electromechanical relays. When the air-gap length changed due to mechanical movement producing from air or liquid pressure in pneumatic or hydraulic system, respectively therefore; the magnetic reluctance is change, that is the inductive properties be effected and as a result of that, the switching value of voltage and current are changed drastically in electromechanical relays. It can be controlled on the min. and maximum value for air pressure in a pneumatic system by controlling on the minimum and maximum value corresponding to voltage and current in the system relay. Two models of transducers are used in this study; the first is variable reluctance displacement transducer studies the intensity of magnetic field lines in 2-D, the intensity of magnetic field vectors and summation of magnetic field intensity contours estimated. The magnetic force is determined with air- gap length changing from (0.8 to 0.2) cm, gradually. Second model is variable differential reluctance transducer. The same results obtained in first model are determined in second model with two cases, armature in mid-way and armature shifted to right side by (0.4) cm. The magnetic force is determined along the line of axisymmetric and in Y direction of armature location. The results show from ANSYS 5.3 and MATLAB - 7 - simulation, the magnetic field increases with decreases of air-gap length in first model and for the second model the magnetic field concentrated in mid - way armature position and increase in the side where the air - gap length reduced and reduced in the side where the air – gap length increasing.