The end of the wire projecting above the armature is provided with a hard-wood knocker which operates upon the head of the drum. The drum is made from a small tin can, having one or two small holes punched in the bottom. Over the top a thin membrane, such as a bladder or a piece of sheep-skin or cat-skin, is drawn and lashed fast with several wraps of wire, having the ends twisted together securely. The membrane must be wet when drawn over the can end, and great care should be taken to get it tight and even. Then, when it dries, it will stretch and draw, like a drumhead, and hold its elastic, resonant surface so long as it does not become moistened or wet.
This drum is arranged in the proper position and lashed fast with wires passed over the box and down through holes in the block; where, after several turns, the ends may be securely twisted together. In place of the drum a small wooden box may be lashed fast with its open end against the block, so as to form a hollow enclosure. The raps of the knocker against its sides will give forth a resonant xylophone tone.
An Annunciator
A simple annunciator may be made from a core, a helix, and some brass strips. A soft iron core, made of a piece of three-eighth-inch round iron and threaded at one end, is converted into a magnet by having a spool and wire coil arranged to enclose it. This in turn is screwed into a strip of brass bored and threaded to receive it. [Fig. 2].
This brass strip is shaped as shown at [Fig. 3] A, and the ears are bent to serve their several purposes. The lowest ears are turned out and the lower part of the plate is bent forward so as to form the hinge on which the drop-bar turns. The drop-bar is only a strip of metal turned up at one end, on which a numeral or letter can be attached; while at the other the metal should be bent over so as to form a core into which a pin or wire may be passed and the extending ends bent down, after being caught through the holes in the ears. Above the magnet the strip is bent forward and the top or end ears bent up, so as to form the hinge on which the armature swings. Holes are made in the long ears, through which screws pass to hold the annunciator fast to the box or wood-work.
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.
ANNUNCIATORS AND BELLS