This same apparatus is a very convenient thing for a lecturer where a stereopticon is used. A buzzer takes the place of the bell, which would be too loud in a hall or lecture-room, and the cord, passing from the apparatus close to the operator, is hung over the lecturer’s stand, or the button held by him in the hand, to be pressed whenever he desires the pictures changed.
This apparatus can be used also in connection with an alarm-clock, where the winding-key is exposed at the back, as it is in most of the nickel-cased clocks that are operated by a spring and which have to be wound each day. For this purpose obtain a piece of hard rubber or fibre, one-sixteenth of an inch thick, an inch long, and half an inch wide. A piece of stout card-board or a thin piece of hard-wood soaked in hot paraffine will answer just as well, if the fibre or rubber cannot be had. Bore a small hole at the two upper corners and one at the middle near the lower edge. Obtain three garter-clips, with springs, and rivet one of them fast to the little plate of non-conducting material. Cut two lengths of old brass watch-chain, four inches long, or obtain eight inches of chain at a hardware-store, and divide it in half. Attach a garter-clip to one end of each piece, and make the other end fast in the holes at the corners of the small plate as shown in Fig. 13. This will be the connector and will close the circuit when the alarm goes off.
When the clock is wound and the alarm-spring is tight, catch one binding-post with a clip at the end of a chain and the other post with the remaining clip. Place the clock near the box and grasp the alarm-key with the clip on the plate. When the alarm goes off the bell on the clock will begin to ring, and when the key has made one revolution it twists the two pieces of chain together, closes the circuit, and the electric bell rings until some one unfastens one of the clips on the binding-posts and breaks the circuit. The great advantage in this double-alarm outfit is that it keeps the bell ringing until the attention of the sleeper is attracted. The bell on the clock will stop ringing directly the spring is unwound or run down; but in so doing it twists the chain and sets the electric mechanism in motion, to run until it is stopped, or until the battery polarizes or is exhausted.