The armature is made from a strip of brass and is shaped like B in Fig. 3. The two ears at the top are bent down and fit within those at the top of the first strip. A screw or wire passed through the holes in the ears will complete the hinge. The strip is bent down so as to fall in front of the magnet, and to its inner side a button or disk of sheet-iron is riveted fast, so as to form an attraction-plate to be drawn against the magnet when the current is passing around it. The lower part of the armature is bent in hook fashion so as to hold up the drop-bar.
A slot is cut in the drop-bar through which the hooked end will project. A short spring is arranged at the top of the annunciator so as to keep the bar and the hook in place when not in action. The current passing around the soft iron core magnetizes it and draws the iron button on the armature towards it. This action immediately releases the hook from under the edge of the metal at the forward end of the slot, and the bar drops, bringing the figure down and into plain sight. It is necessary, of course, to mount this annunciator so that the bar will not drop down too far. This may be done by having a platform or wire run along under a series of the drops, so that they will rest on it.
For loud ringing, and to get the benefit of both the forward and backward stroke of the knocker, a double bell, similar to the one shown in Fig. 4, may be constructed upon the general principle of the single-stroke buzzer already described (chapter iv.).
Two soft iron cores are made, as described for the other bells, but instead of being yoked together with iron, so that the three parts will form a horseshoe magnet, the yoke is of brass or copper. Each core will then be an independent magnet.
The spools are wound with No. 22 insulated wire and the ends left free, so that the coils are not connected together. If the drawing is examined closely you will see that the armature swings on a pivot at the base of the knocker-bar. When the bell is not in action the knocker might rest naturally against one bell or the other; or it might stand in the centre and not touch a contact-point, were it not for the small spring which draws it to the left. Directly the current is run through the coils it alternately magnetizes first one and then the other. This action is due to the making and breaking of the circuit by the spring on the armature. It first comes into contact with one point, and then is drawn away from it to come into contact with the other. Fig. 4 shows the knocker-bar at rest between both bells and the armature unattracted by either magnet. This position is purposely given so as to indicate the balance of the armature and the spaces between it and the cores and also the contact-points above it.