The current, passing in at I, through J, K, the coil, and out at L, magnetizes the piece of pipe and draws the iron buttons or disks attached to the tin disk. But so soon as it does so it breaks the contact between J and K, and the buttons fly back into place, having been drawn there by the rigidity of the tin disk to which they are attached. Again the current is closed and the magnet draws the iron buttons. The brass rod moves but a very slight distance up and down--enough, however, to make and break the contact between J and K. As a result of this rapid movement and the consequent snapping of the tin disk, a loud noise is emitted through the bell, which can be heard a long distance and closely resembles a long blast blown on a fish-horn. Burglar-alarms A unique burglar-alarm trap may be made from a plate of wood, five by six inches and half an inch thick, a movable lever, and a brass strip having the ends turned out. These are arranged as shown in Fig. 8. The brass strip is fastened to the plate with screws, and the ends extend out for half an inch from the board. The lever is made from a strip of brass, and the upper part is bent out so as to clear the strip and screws that are under it. A hole is made at the lower end of the lever, through which a brass ring and the end of a spring may be fastened. The opposite end of the spring is attached to a screw, and a wire carried from it to a binding-post, A. Another wire connects the back plate with binding-post B. A string or piece of fine picture-wire is made fast to the ring and carried to any part of a room. To set the trap, make the block fast in any convenient place, such as the door-casing or the surbase, and carry the string out from the trap and fasten the end of it. Any one running against it in the dark will draw the lever over to the right side and connect the circuit. [Illustration: FIG. 8] [Illustration: FIG. 9] [Illustration: FIG. 10] When setting the trap, have the string adjusted so that the lever is in a vertical position, as shown in the drawing of Fig. 8. When the string is disturbed it will pull the top of the lever over to the right side; but if the string is broken by the person running against it, the spring attached to the bottom of the lever draws it over to the right side with a snap, and the top of the lever goes to the left side, when the circuit is closed and the alarm given. This trap is connected the same as a push-button, one wire leading to the bell, the other to the battery; then the battery and bell are connected together so that when the circuit is closed the bell will ring until some one throws a switch open to break it.