Operation (Figure 298). When the hydrostatic clock completes its delay period, fuse L blows, seating the needle on its knife edges. Rotation of F allows E to rotate on its pivot, and the A.L.A. clockwork starts to run. Oscillation of escapement lever D drives C in a clockwise direction, with M and G being restrained by fuse K. Clockwise rotation of C dri-ves B clockwise, and, since A is geared to B, A rotates counterclockwise. Counterclock-wise rotation of A decreases tension on the needle hairspring to which A is attached.

When hairspring tension on the needle axis decreases so that the forces acting on the needle are equal and opposite to the force of the earth's magnetic field, the needle ma-kes its contacts, blowing fuse K. When K blows, a small hairspring rotates G until it en-gages A. Blowing of K also releases M, allowing the pawl to lock the gear system. Since C is still rotating clockwise, the entire assembly consisting of A, B, C, G, and M rotates clockwise. This puts tension back on the needle spring and opens the needle contacts. To assure that the needle contacts open at this point, rotation of M closes switch T no-mentarily, thereby energizing the hold-off circuit.

The assemnly rotates as noted above until M comes up against H, the distance tra-veled by M before contacting H determining the amount of hairspring tension on the needle and thereby the sensitivity setting of the unit. If, at any time during the above process, a countermining shock is received, closure of the pendulum switch energizes J, which holds D inoperative until the pendulum switch reopens.

Figure 298– Mechanical A.L.A.