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.
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.
