4 2 -2 PUSH TO SET Sheave pulley (top) Mass shafts Control cord Mass Main pulley Main pointer centering spring Driver arm Sheave pulley (bottom) Pointer reset shaft Pawl Auxiliary pointer return spring Ratchet Main pointer Auxiliary pointer (plus G indication) Auxiliary pointer (minus G indication) Figure 10-61.  A reed-type stall warning device is located behind  this opening in the leading edge of the wing. When the angle of  attack increases to near the point of a stall, low-pressure air flowing  over the opening causes a suction, which audibly vibrates the reed.  Figure 10-60.  The inner workings of a mass-type accelerometer.  the leading edge of the wing. The cavity has an open passage  to a precise point on the leading edge. In flight, air flows over and under a wing. The point on the  wing leading edge where the oncoming air diverges is known  as the point of stagnation. As the AOA of the wing increases,  the point of stagnation moves down below the open passage  that leads inside the wing to the reed. Air flowing over the  curved leading edge speeds up and causes a low pressure.  This causes air to be sucked out of the inside of the wing  through the passage. The reed vibrates as the air rushes by  making a sound audible in the cockpit.  [Figure 10-61]  Another common device makes use of an audible tone as  the AOA increases to near the point where the aircraft will  stall. This stall warning device includes an electric switch  that opens and closes a circuit to a warning horn audible in  the cockpit. It may also be wired into a warning light circuit.  10-34