It is sometimes desirable to arrange two bells and two push-buttons, so that a return signal can be sent. In that case the circuit shown in Figure 124 may be employed. It is then possible for the person answering the bell to indicate that he has heard the call by pushing the second button. For instance, one push-button and bell might be located on the top floor of a house and the other bell and button in the basement. A person in the basement wishing to call another on the top floor would push the button. The person answering could return the signal by pushing the button on the top floor and cause the bell in the basement to ring. A Burglar Alarm A simple method of making an efficient burglar alarm is shown in Figure 125. The base is a piece of wood about five by six inches, and half an inch thick. A small brass strip, _A_, is fastened to the base by means of two round-headed wood screws and the ends turned up at right angles. The lever, _B_, is also a strip of brass. One end is bent out, so as to clear the strip and the screws that are under it. The lever is pivoted in the middle with a screw and a washer. A small hole, _D_, is bored in the lower end through which a spring and a string are passed. The other end of the spring is fastened under a screw and washer, _C_. [Illustration: Fig. 125.—Burglar-Alarm Trap.] In order to set the alarm, first fasten the base in any convenient place. Carry the string across the room and fasten it. Adjust the string so that the lever is half-way between the two ends of the strip, _A_. If the string is disturbed, it will pull the lever over against the strip, _A_. If the string is cut, the spring will pull the lever over to the opposite side. In either case, if the alarm is properly connected to a bell and battery, the circuit will be closed if the string is disturbed, and the bell will ring. One wire leading from the bell and the battery should be connected to _A_, and the other to the screw and washer, _C_. The alarm may be arranged across a window or doorway and a black thread substituted for the string. Any one entering in the dark and unaware of the existence of the alarm is liable to break the thread and ring the bell. An Electric Alarm It is often desirable to arrange an electrical alarm clock so that a bell will ring continuously until shut off. [Illustration: Fig. 126.—An Early-Riser’s Electric Alarm Attachment for a Clock.]