Figure 126 shows an electrical alarm attachment. It consists of a wooden box, large enough to receive an ordinary dry cell. A bell is fastened on the outside of the box. Connect one terminal of the battery to one terminal of the bell. Connect the other bell and battery terminals, each to a short piece of brass chain, about four inches long. The ends of the chain are then fastened to a small piece of sheet fiber or hard rubber, so that they are insulated from each other. The opposite end of the fiber is fastened to a piece of wire spring having a garter or suspender clip soldered to the end. [Illustration: Fig. 127.—Details of the Chain Electrodes, etc.] The operation of this electrical attachment is very simple. Wind up the alarm key of an ordinary alarm clock and place the clip on the key. Place the clock in such a position that the two chains do not touch each other. Set the clock. When the mechanical alarm goes off, the key will revolve and twist the two chains, thus closing the electric circuit and causing the bell to ring. The bell will ring until the clamp is removed. The outfit can be attached to any ordinary alarm clock. An Annunciator Annunciators are often placed in bell and burglar alarm-circuits to indicate where the button ringing the bell was pushed, in case there are several. The separate indicators used on an annunciator are called _drops_. [Illustration: Fig. 128.—An Annunciator Drop.] A drop may be made from an electromagnet and some brass strips, etc. The frame is cut from heavy sheet-brass and shaped as shown in Figures 128 and 129. The drop bar is a strip of metal which is pivoted on the frame at its lower end and has the upper end turned up to receive a numeral or letter. The armature is made from a strip of sheet-iron. It is pivoted on the frame at its upper end. The strip is bent at right angles so as to fall in front of the magnet. The lower part of the armature is bent into a hook. The hook fits into a slot cut in the drop bar. A fine wire spring is placed between the frame and the upper end of the armature so as to pull the armature away from the core when the current is not passing through the magnet. The electromagnet should be wound with No. 25 B. & S. cotton-covered magnet wire. When a current is sent through the magnet, it will draw the armature in. This action releases the hook from the edge of the slot in the drop bar and permits the bar to drop and bring the number or letter down into view. [Illustration: Fig 129.—Details of the Drop-Frame and Armature.] A number of "drops" may be arranged on a board and placed in different circuits so as to indicate which circuit is closed at any time. It is a good plan to arrange a bar to act as a stop, so that the numeral will not drop down too far. Each time that any one of the drops falls, it must be reset by pushing the bar back into position. CHAPTER X ELECTRIC TELEGRAPHS