referring to fig1 - 4 , there is shown an electromagnetic relay constructed in accordance with the principles of the present invention and indicated generally at reference character a . the relay a is shown mounted in a chassis 1 which is constructed of suitable insulating material . included among the components of the relay a are a coil 2 , a return pole piece 3 and an armature 5 . the return pole piece 3 has an outer wall 3 b , an inner wall 3 c and a pair of side walls 3 d , 3 d . the armature 5 is generally l - shaped , and includes a first leg portion 5 a , a second leg portion 5 c and a curved or arcuate corner portion 5 d which interconnects the leg portions 5 a and 5 c . return pole piece 3 is formed with an upper straight edge 4 which supports the armature 5 for pivotal movement , the edge 4 engaging an inner surface 8 of the corner portion 5 d . the armature 5 is clamped to the return pole piece 3 by means of an elongated curved mounting spring 7 . one end portion 7 a of the mounting spring 7 is bottomed on a lower wall 6 a of a notch or recess 6 formed in an outer surface 6 b of the armature corner portion 5 d . the mounting spring 7 extends upwardly and at an angle to the end portion 7 a as indicated at reference numeral 7 c and also includes an arcuate portion 7 d which leads to a vertical portion 7 e . at the lower end of portion 7 e is another arcuate portion 7 f which extends through an aperture 5 b and connects to the opposite end portion 7 b of the spring 7 . it is noted that the end portion 7 b extends through an aperture 3 a formed in the return pole piece 3 c and includes a lip or flange portion 7 g which abuts an edge surface 12 adjacent the aperture 3 a of the return pole piece 3 . as best shown in fig4 the line of the biasing force exerted by the mounting spring 7 between the return pole piece 3 and the armature 5 , as indicated at f , is slightly displaced or offset with respect to a line extending parallel thereto and passing through the supporting edge 4 of the return pole piece 3 , the distance between the parallel lines being indicated at reference character a in fig4 . this offset produces a desired armature resetting moment which may be defined by the formula m r = f x a , where m r = the resetting moment , f equals the biasing force produced by the mounting spring 7 between the ends 6 a and 7 b , and a equals the distance between the biasing force and the supporting edge 4 of the return pole piece 3 . during normal operation the pre - loading of the mounting spring 7 provides the necessary mounting or clamping forces between the return pole piece 3 and the armature 5 . under fairly severe impact or vibration loadings a contoured portion 9 of the chassis 1 permits only slight lifting of the armature 5 with respect to the return pole piece 3 . by virtue of this arrangement an over - extension of the mounting spring 7 is avoided . to prevent excessive lateral movement of the armature 5 along the length of the supporting edge 4 of the return pole piece 3 , the upper end portion 7 a of the mounting spring 7 is arcuately shaped , as best shown in fig3 at reference character 7 h . the recess wall 6 a in which the spring end portion 7 a resides is similarly curved . for example , in fig1 the reference character r indicates the radius of curvature of the lower wall 6 a of the recess 6 , whereas reference character r indicates the radius of curvature of the end wall 7 h of the mounting spring 7 . in accordance with the principles of the present invention , the radius of curvature r is slightly less than the radius of curvature r , whereby a centering effect is produced on the mounting spring 7 relative to the armature 5 . in addition , excellent stabilization of the armature 5 is achieved by constructing the lower end 7 b of the mounting spring 7 such that it abuts a substantial length of the abutment surface 12 of the return pole piece 3 . furthermore , fairly severe impact loading of the armature 5 in a lateral direction , that is , in the direction of the supporting edge 4 , is also limited by virtue of the confining wall 11 of the chassis 1 . referring again to fig4 it is apparent that the spring ends 7 a and 7 b are biased against one another by virtue of the spring force f , generating a mounting force indicated at reference character f l . the mounting force f l , as shown in the vector diagram , comprises a horizontal force component l h and a vertical force component l v , from which it is apparent that the mounting force f l returns the armature 5 back into engagement with the supporting edge 4 of the return pole piece 3 in the event severe vibration loading momentarily lifts the armature 5 from the supporting edge 4 . in the perspective view illustrated in fig5 the centering effect of the mounting spring 7 is clearly illustrated . thus the curved edge 7 h of the upper end 7 a of the spring 7 abuts the curved recess wall 6 a of the recess 6 . on the other hand , the lower end portion 7 b is of a width c which is only slightly less than the corresponding width of the aperture 3 a formed in the return pole piece 3 . in addition , the flange of the end portion 7 . sub . b engages the inner wall 3 c of the return pole piece 3 substantially along the entire width of the recess 3 a . thus the mounting spring 7 constantly urges the armature 5 to a centering position with respect to the return pole piece 3 . if desirable , the mounting spring 7 can be formed to engage the return pole piece 3 over even a broader base , as shown in the embodiment of the invention illustrated in fig6 . in that embodiment the aperture 3 a is eliminated and a pair of apertures 13 are formed in the side walls 3 d , 3 d of the return pole piece 3 , and the lower end of the mounting spring 7 comprises a pair of spring ends 15 , 15 which extend through the apertures 13 . in this arrangement the lower end of the mounting spring 7 also engages the outer wall 3 b across a distance indicated by reference character b . in the embodiment shown in fig7 the aperture 3 a of the embodiment shown in fig1 - 4 is replaced by a pair of smaller apertures opening to the side walls 3 d , 3 d of the return pole piece 3 , and the lower end of the mounting spring 7 comprises a pair of extensions 16 , 16 . it should be noted that the extensions 16 , 16 extend through the apertures 14 , 14 in a direction parallel to the side walls 3 d , 3 d of the return pole piece 3 , whereas in the embodiment shown in fig6 the lower end of the mounting spring 7 extends through the apertures 13 , 13 in a direction substantially transverse to the return pole piece side walls 3 d , 3 d . in some instances , therefore , the configurations of the mounting spring 7 shown in fig6 and 7 may be preferred to the arrangement shown in fig1 - 5 for improved centering capabilities of the mounting spring 7 relative to the return pole piece 3 and the armature 5 . although minor modifications might be suggested by those versed in the art , it should be understood that we wish to embody within the scope of the patent warranted hereon all such modifications as reasonably come within the scope of my contribution to the art .