quantity to inflate the air bag 34 causing the air bag to break out of the cover 42 and expand into the passenger compartment one of the features of the present invention is a safety interlock arrangement which prevents the rollers 76 and 78 from moving inadvertently when the safety unit is not in use the interlock is automatically released when the safety unit is mounted in position on the steering wheel as shown in fig2 the safety interlock includes a generally l shaped lever 90 one leg of which extends along the outside of the sensor housing 50 and terminates in a projecting pin 92 which extends through an opening 94 in the housing the pin 92 is sufficiently long to project into an opening 95 extending through the center of the roller 78 the outer leg of the l shaped safety lever terminates in a projection 96 which is positioned to engage the hub portion of the steering wheel when the safety system is properly mounted the l shaped lever 90 is pivotally supported by a pin 98 so that a pressure against the projection 96 causes the lever to pivot in a direction to withdraw the pin 92 from the hole 95 in the roller 78 a wire coil spring 100 wrapped around the pivot pin 98 normally urges the pin 92 down into the hole 95 to lock the roller 78 when the safety unit is in position against the hub of the steering wheel the action of the spring 100 is overcome and the pin 92 is automatically withdrawn freeing the rollers 76 and 78 to move in the manner described above when the unit is dismounted the pin 92 automatically locks the roller 78 again the sensor element as described.