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trouble. These are some of the reasons why the magneto set has become widely used as a signalsending and -receiving device.
Typical Forms of Magneto Bells are shown respectively in Figs. 21 and 23: The first one presented is a Stromberg-Carlson polarized ringer wound to a resistance of 1,600 ohms. Adjustment of the armature with respect to the poles is secured,
as shown in Fig. 22, by having the armature CC supported upon the spring plate D D, which is provided with two holes to receive the ends of the magnet cores EE. A cross-bar FF is mounted over the poles, and into this is threaded the screw B which projects through a hole in the permanent magnet A and has a shoulder bearing on the plate DD. Turning the screw B to the right or left moves the plate D D toward or away from the mag
net cores and thus enables the proper adjustment of the armature to be secured. The clapper G is bolted to the armature as shown. The coils are wound with silk-insulated wire and are covered with heavy linen cloth to protect them from injury.
Fig. 23 shows a Kellogg magneto bell designed somewhat differently from the one shown in Fig.
21. This form of ringer is called a "biased” bell, and is used where a selective signal is wanted, that is, where there is more than one bell connected to a circuit and it is desired to be able to ring one or the other of them at will. Its construction differs from the polarized bell only in the use of a spring s, which is fastened to either side of the armature e so as to pull that end of it toward the magnet. The result is that the bell will not respond to cur
rents of one polarity, but will respond to currents of the opposite polarity. It is necessary in any biased bell to determine by trial the side on which the spring should be placed to make it operate on a positive or a negative current.
A Typical Form of Magneto Generator is shown in Fig. 24. This 3-magnet Acme generator will be readily understood from the general description previously given. At the left end, however,
an automatic shunt s is fitted, the operation of which will be explained later on. The three horseshoe magnets h, etc., are held to the frame by the bolts a and c; each magnet is capable of lifting 5 pounds. The armature core on which the windings are placed is built up with sheet-iron punchings, each shaped like those shown at a, etc., Fig. 25. These punchings are threaded on the armature
shaft c and bolted together as there indicated, after which the core has the appearance shown in Fig. 26. Silk-insulated copper wire, either No. 34 or 36 B
Fig. 24.—Magneto Generator & S. gage, is then wound lengthwise over the core. The completed armature, Fig. 27, has a resistance of from 300 to 1,000 ohms and develops from 80
to 120 volts. The resistance of the armature winding must not be confused with the resistance rating of a magneto generator. This latter refers to the number of ohms resistance through which the generator can ring its own bell. A 10,000-ohm generator, for instance, is one which will ring the bell through 10,000 ohms resistance. Magneto generators are also rated according to the number
of permanent horseshoe magnets they contain; as previously stated, 3- or 4-magnet generators are the usual sizes employed.
Connections of the Talking and Signaling Apparatus.—Telephone apparatus must always be connected so that the following conditions are satisfied: (1) When the talking circuit is not in use, the bell must be connected so as to receive
a signal. (2) When a signal is to be sent, the generator must be connected to the line wires. (3) When the talking circuit is in use, the receiver must be connected to the line wires, and the transmitter, battery, and the primary windings of the