Harper's Electricity Book for Boys

by Joseph H. Adams

The block acts as an obstruction as well as an alarm, for the pins will hold in the floor and the little block will stand its ground. A simple form of contact for doors is shown at [Fig. 10]. This is simply two strips of spring-brass bent as shown, and screwed fast on either side the crack of a door, at the hinge side, so that when the door is opened one piece of metal bears on the other and the circuit is closed. This is to be operated in connection with a switch, so that the circuit may be opened in the daytime when the door is in use.

Signals and Alarms

There are many different kinds of electric call-signals used in and about the house; among these are some that a boy can readily make—for example, the ordinary call-buttons and the signals between house and stable or other out-buildings.

A portable call-bell, or alarm, is one of the most convenient things in any home. It may be temporarily rigged up from one room to another, or from one floor to the next, the small, flexible wire being run over the tops of door-casings, where it is held by slim nails or pins driven into the wood-work.

The main terminal of this portable outfit consists of a wooden box that will hold a large dry-cell, and to the side of which an electric bell or buzzer may be attached. Binding-posts are arranged at another side, to which the ends of the flexible wire-cord can be made fast, and a cover fitted to the box to hide the battery and wiring. The complete outfit, except the flexible wire-cord and push-button, will appear as shown in [Fig. 11]. No definite size can be laid down for the construction of this box, as dry-cells vary in size and shape, some being long and thin, while others are short and fat. By removing the cover and looking into the box, it will appear as shown in [Fig. 12]. The carbon is connected with one binding-post and the zinc to one of the poles of the bell. The other bell-pole is connected with the remaining binding-post, and it requires but a switch or push-button to close the circuit between the two binding-posts. This is done by the long line of flexible wire-cord, which may be of the silk or cotton covered kind, having the two strands twisted together as is customary with flexible electric-light wire. A pear-shaped push-button may be connected at the end of the line, and this may be arranged at the head of a bed or on a chair placed conveniently near an invalid’s couch.

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.