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just mentioned. These small currents are often called “sneak” currents.

The protective apparatus just mentioned must be connected so that the fuses protect the lightning arrester and heat coils. On a grounded circuit, but

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Fig. 41.

Fig. 42
Fig. 41.-Combination Set of Protective Apparatus for
a Grounded Telephone Circuit, Showing the Necessary
Wiring

FIG. 42.-Combination Set of Protective Apparatus for a Complete Metallic Telephone Circuit, Showing the Necessary Wiring

one set of these devices is used, connected between the line wire and the telephone set. Figs. 41 and 42 show D. and W. combination protective sets

for a grounded circuit and a complete metallic circuit, respectively. In the former, the circuit runs from the line through the 2-ampere fuse A to the o.3-ampere heat coil B and the lightning arrester C, as indicated by the dotted lines. The other terminal of the heat coil is connected to the telephone set, and the other terminal of the lightning arrester is connected to ground. The ground wire from the telephone set is fastened to the binding post s which is also directly connected with the grounded side of the arrester. For a complete metallic circuit, the protective apparatus in Fig. 41 is simply duplicated, and the connections are made as in Fig. 42. In both cases the apparatus is mounted on a porcelain block.

The Fuse should be of the inclosed type, in order to more securely protect the fuse wire from mechanical injury, and also to enable the arc, which forms when the fuse blows, to be more readily extinguished. The arc is extinguished either by placing in the tube a compound that will suppress the arc or by allowing the vapor, which forms inside the tube when the fuse melts, to blow out the flame.

The Heat Coil comprises a small coil of Germansilver wire of from 5 to 50 ohms resistance, depending upon the sensitiveness desired; this surrounds a pin and is fastened to it by an easily fusible solder. A spring bears upon the pin so that when released by the fusing of the solder it comes in contact with the ground connection and opens the circuit passing through the coil. The heat coils in Figs. 41 and 42 are rated to operate at 0.4 ampere in less than 15 seconds, and are inclosed in cases of non-conducting material.

The Location of the Protective Apparatus depends largely upon whether it is all combined on one base as in Figs. 41 and 42, or whether the fuse is separate from the lightning arrester and heat coil. In the former case, it is usual to mount the apparatus immediately inside the building at the point where the wires enter, so that the inside wiring will be exposed as little as possible to abnormal currents; in the latter case it is considered best to mount the fuse on the house wall outside the building, and the lightning arrester and heat coil close to the telephone set, so that the entire inside wiring is guarded, and the delicate parts of the protective apparatus can be easily reached in case of trouble.

The Operation of Series Stations.—When a party ai one of the telephone sets on a series line desires : to communicate with a party at another set, he turns the crank handle of his magneto generator at intervals so as to give a certain number of rings throughout all the bells, ringing once to signal one station, twice to signal another, and so on according to a predetermined code. The party he wishes, recognizing the number of rings as corresponding to his instrument, goes to the telephone and removes his receiver from the hook, whereupon he is in telephonic communication with the party who desires to speak to him. Ob

viously, but two parties can use the line at one time, and any of the others can, if they so desire, hear all that is being said over the line by simply listening at their receivers. Another disadvantage, which increases with the number of stations connected, is the annoyance caused by all the bells ringing whenever a signal is sent.

An Extension Bell in a Series Station is often of great convenience, enabling two parties in different rooms, for example, to utilize the same telephone set and each have their own signaling code.

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Fig. 43.—Diagram of Wiring for an Extension Bell in a

Series Station

Fig. 43 shows the wiring for an extension bell b in a series-connected station A. No. 18 B. & S. gage rubber-covered wire should be used for the leads m and n. In the telephone set, Fig. 32, the extension bell would be connected between the binding post 7 and the wire leading to this binding post from the magneto generator. The coils of the extension bell should be wound to a resistance of 80 ohms.

The Bridging Connection.—The bridging connection of the signaling apparatus with respect to the line is shown in Fig. 44. Comparing this diagram with the series connections, Fig. 28, the following points of difference are to be noted. The magneto generator a is connected directly across the line wires m and n. One end of the armature winding terminates in an insulated pin

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c in the armature shaft, and the other end on the armature core. When the generator is not in use, the armature circuit is automatically opened at e by the pin c being separated from the end spring. When the crank handle of the generator is turned, a spring mounted in the hub of the gear wheel forces the armature shaft forward so that c makes contact with the end spring and closes the circuit. The mag

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