This article is designed to convey a number of practical hints connected with the various branches of applied electricity, such as cannot be found in any existing treatise. Attention will be confined to strictly useful matter, the subject being divided into convenient heads.
In Fig. 13, a is a small piece of brass tubing, having a brass disc b, soldered on to one end of it. This is drilled and countersunk for screws, as at c; d is a small brass knob, either screwed or soldered on to the brass rod f, and protruding about half-way through a hole in the disc b; e is a circular disc of ebony fitting into the tube d, and having a hole through its centre to allow / to slide freely. The ebony is held in its place by screws. To one end of the ebony disc is fitted a brass circle j, of rather smaller diameter than e, also having a central hole to admit of f sliding. h is a spiral spring, which keeps the brass cross piece i (passing through a hole on f) in contact with the brass circle j. k is a brass disc closing the end of the tube, and also allowing / to slide. One wire l is attached to this; the other wire m passes through a hole in k, and is attached to the brass circle j. On pressure being applied to d, the points of contact are forced apart, and no current can pass. Immediately, however, on the pressure being removed (as by opening a door or window), the spring recovers itself, and brings the cross-piece into connection with g, thus closing the contact.
These contact-makers are sunt in the lintel of the door or in the groove of the window, as shown in the figure. One of these most be used for each door or window to be guarded. The wires l and m should be guttaperchs-covered, as, in fact, all wires used in this job should be. The Test of your work is comparatively easy. Yon must hare an indicator in your bedroom to show which room is attacked. This will be precisely similar to those used in ordinary electric-bell work. At some point in the battery wire which goes to the indicator, you may insert an interrupter, which is merely a brass arm pivoted at one end and resting on a brass stud at the other. When you desire to throw the arrangement out of gear, you have only to remove the arm from the stud.