also has an upper wall 48 a plurality of ribs 50 extend downwardly from wall 48 at their upper ends the ribs include arcuate portions 52 these ribs 50 cooperate to define the upper portion of the chamber 20 with contacts 22 24 extending into the chamber disposed in chamber 20 there is an inertial element in the form for example of a spherical ball 54 made of steel or other relatively dense material the ribs 50 are arranged and constructed to form a vertical as seen in fig2 and 4 tubular path for the ball 54 the blades 22 24 are made of a flexible conductive material such as steel copper or alloys thereof as shown in fig2 the blade 24 is arranged so that under normal conditions it applies a biasing force upward on ball 54 to maintain it in a rest position preferably the inside surface of tubular section 19a is threaded as at 60 to accept a screw in plug 62 the sensor is positioned in such an orientation that when the vehicle is involved in a crash which results on a deceleration exceeding a preset level the ball 54 overcomes the biasing force of blade 24 and starts moving upwards along the path defined by the ribs 50 as the ball moves upward it bends blade 24 until this blade comes into electrical contact with blade 22 as shown in fig3 the blades are part of an 10 electrical circuit or other means of initiating of the bag deployment for.