e and f at the end of the lever arm 31a the sensor element 22 is engaged by a tension spring 32 whose other end is secured to a single arm adjusting lever 43 which is rotatable about a pivot 44 in the direction of the arrows g and h the free outer end of the lever 43 may form a manually engageable handle the pivot 44 is secured to the base plate 39 in case the setting lever which may be immobilized by detents is moved from its position shown in fig6 a in the direction of the arrow h into the position shown in fig6 b the securing location of the spring 32 is changed whereby the bias and thus the spring force exerted on the sensor element 22 is altered the base plate 39 has detents 45 and 46 such as slots and bolts for determining positions for the setting lever 43 fig7 a and 7b show a single arm pivotal lever 47 which is swingable in the direction of the arrows i and k about a pivot 48 secured to the base plate 39 one end of a tension spring 50 is connected to the pivotal lever 47 at a location 51 while the other.