One wire from the secondary coil is caught under a screw that holds one end of the brass bridge to the base; and the other to a screw, which may be placed at one corner of the block, and from which a short wire leads to the lead ring. Binding-posts may be arranged to serve the same purpose, and, of course, they are much better than the screws, because they can be easily operated by the fingers and do not require a screw-driver every time the interrupter is placed in series with an induction-coil. An interrupter on this same order may be made from a straight strip of lead with the pins driven through the middle of it. One wire from the secondary coil is made fast to the lead plate, and the end of the other wire is passed along the pins, thus making and breaking the circuit in a primitive manner. Chapter V ANNUNCIATORS AND BELLS A Drum Sounder A unique electric sounder that is sure to attract attention is in the shape of an electric-bell apparatus, with a drum sounder in place of a bell, or knockerless buzzer. Fig. 1. The outfit is mounted on a block four inches and a half wide and seven inches long. The cores and yoke are made as described for the electric buzzer (chapter iv.) and are wound with No. 22 cotton-insulated wire. The magnet is then strapped fast to the block by means of a hard-wood plate having a screw passed down through it; and between the coils and into the block an armature is made and mounted on a metal plate, which in turn is screwed to the block. Another block, with a contact-point, is arranged to interrupt the armature, and the series is connected as shown in the drawing Fig. 1. 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