The Annunciator.
The mechanism of an annunciator consists of electromagnets which allow shutters to drop or needles to move on the circuits being closed. A bell is also rung in most cases to call attention to the annunciator. The number of the circuit is marked on the shutter, or near the needle, either shutter or needle being replaced by a reset device, which may be mechanical or electrical.
Fig. 52
Annunciator drops are made in a variety of forms. [Fig. 52] illustrates the principle underlying nearly all of them.
When current flows through the magnet coils M, the armature A is attracted, and being pivoted at P, the lever hook H rises and allows the weighted shutter S to fall and display a number painted on its inside surface.
Fig. 53 Fig. 54
The needle drop in [Fig. 53] is one that has met with great favor and works as follows: the soft iron core of the magnet C has a hole drilled through it, in which turns the shaft S. An arrow or needle is attached at the front end over the face of the annunciator. A notched arm B is fixed on the rear end of the shaft and is held in a horizontal position by the end of armature A.
When the current flows around C, armature A turns on its pivot towards the core of C, as in [Fig. 54], unlocking B, which falls and thereby partly rotates shaft S and the arrow.