Another form of circuit-closer is shown in the door-trap (Fig. 9). This is a wooden block that rests on the floor close to the bottom of a door, and is held in place by means of four sharp-pointed nails driven down through the corners of the block. The points should project a quarter of an inch or more, according to whether the block is on a hard floor or on a carpet. The front edge of the block is bevelled so that the bottom of a door that fits closely to the floor will pass over it. The block is five by seven inches, and three-quarters of an inch thick. At the left side a strip of metal (A) is held close to the block with straps or wide staples driven over it, but not so close but that it can move freely back and forth. To the front end a round piece of wood is made fast. This is the bumper against which the door will strike when opened. At the middle of the strip a screw is riveted fast; or it may be a machine-screw let into a threaded hole in the metal. At the right side of the block another strip of metal (B) is attached, but this is held fast with a screw at the middle and a screw-eye and washer at the rear end to act as a binding-post. The front end of this strip is turned up so as to form a stop; then a movable lever (C) mounted over both strips, with one end bent up, is attached to the block with a screw. A slot is cut at one end so that the screw in the movable strip (A) will move freely in it, and near the other end a small hole is made to receive the end of a spiral spring (D). To set the trap, the block is placed on the floor and the wires from battery and bell are made fast to the binding-posts. The spring D keeps the lever C away from the strip-end B, while at the same time it throws the strip A forward. When the door is opened it shoves the bumper and strip A back through the staples, while the screw operates lever C and causes its loose end to come into contact with the end B, thereby closing the circuit and ringing the bell or buzzer. When the door is closed again the spring draws lever C away from B, and the circuit is opened. 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.