Fig. 137.—A Diagram showing how to connect two Complete Telegraph Sets, using one Line Wire and a Ground. The Two-Point Switches throw the Batteries out of Circuit when the Line is not in use.
The diagram for connecting the instruments is self-explanatory. In cities or towns where a "ground" is available by connecting to the gas or water pipes, one line wire may be easily dispensed with. Or, if desirable, a ground may be formed by burying a large plate of zinc (three or four feet square) in a moist spot and leading the wire to it.
How To Build a Telegraph Relay
In working a telegraph over a long line or where there are a large number of instruments on one circuit, the currents are often not strong enough to work the sounder directly. In such a case a relay is used. The relay is built on the same principle as a sounder, but the parts are made much lighter, so that the instrument is more sensitive. The armature of a relay is so small and its movement so little that its clicking is scarcely audible. It is therefore fitted with a second set of contacts and connected to a battery and a sounder, which is to set in operation every time the contacts close. The principle of a relay is that a weak current of insufficient strength to do the work itself may set a strong local current to do its work for it.
There are many forms of relays, and while that which is described below is not of the type commonly used on telegraph lines, it has the advantage of being far more sensitive than any instrument of the regular line relay type that the average experimenter could build.
Fig. 138.—Details of the Relay Parts.
Make the magnets from one-quarter-inch stove-bolts, and cut them off so that they will form a core about two and one-quarter inches long. Fit each of the cores with two fiber heads to hold the wire in place. Insulate the legs with paper and wind each with about fifty layers of No. 30 B. & S. gauge single-cotton-covered magnet wire. The winding space between the magnet’s heads should be one and one-eighth inches.
The upper ends of the magnet cores should be allowed to project about one-quarter of an inch beyond the fiber head. The end of the core is filed flat, as shown in the illustration.
The magnets are mounted upon an iron yoke, three-sixteenths of an inch thick. The holes in the yoke should be spaced so that there is a distance of one and one-half inches between the centers of the magnet cores.