The next thing required in order to complete the apparatus is a relay. You may use the relay described in Chapter X or build one according to the plan shown in Figure 235. This relay consists of a single electro-magnet mounted on a wooden base, two inches wide and four inches long. The armature is a piece of soft iron rod one-quarter of an inch in diameter and one-eighth of an inch long, riveted to the end of a thin piece of spring brass, about No. 34 B. & S. gauge in thickness. [Illustration: Fig. 235.—The Relay.] The other end of the spring is fitted to a bracket and provided with a thumbscrew to adjust the tension of the spring. The under side of the armature and the upper side of the magnet core are each fitted with a small silver contact. The contacts should meet squarely when the armature is drawn down on to the core by a current of electricity passing through the electro-magnet. By turning the adjusting screw, the armature can be raised or lowered. It should be adjusted so that it almost touches the core and is only just far enough away to slip a piece of thick paper under. The terminals of the magnet are connected to the two binding-posts on the base marked _S_ and _S_. One of the binding-posts, _P_, is connected to the brass upright, and the other is connected to the core of the magnet. Figure 236 shows how to connect up the outfit. It will require some very nice adjusting before you will be able to get it to working properly. [Illustration: Fig. 236.—The Complete Coherer Outfit.] If you wish to use the outfit for demonstration purposes or for sending messages for very short distances, as for instance across a room, you do not need an aerial but merely a pair of "catch-wires." The "catch-wires" are two pieces of stiff copper wire, about two feet long, placed in the lower holes in the double binding-posts forming part of the coherer. In order to set the apparatus for operation, raise the adjusting screw of the relay until the armature is quite far away from the core. Then push the armature down against the contact on the core. The decoherer should then immediately operate and begin to tap the coherer. Then turn the thumbscrew until the armature is brought down to the core in such a position that it is as close as it is possible to get it without ringing the bell. The transmitter should consist of a spark coil, battery, key, and a spark-gap. The gap should be connected to the secondary of the coil and adjusted so that the electrodes are only about one-eighth of an inch apart. The key is placed in series with the primary of the coil and the battery, so that pressing the key will send a stream of sparks across the gap. Fit the spark-gap with two catch-wires similar to those on the coherer and place the transmitter about four or five feet away from the coherer outfit.