ate bells of similar and opposite polarity. Each bell is wound to a resistance of 2,500 ohms and is fitted with a biased spring to hold the armature against one of the pole faces, thus making it sensitive to one polarity of current, but not to current of the opposite polarity, the wrong polarity simply causing the armature to be attracted in the same direction as that produced by the tension of the biased spring. This feature, together with the connections of alternate bells to the "tip" and "sleeve" sides of the line as in Fig. 105, enables any one of the 4 stations, A, B, C, and D, Fig. 106, to be signaled without ringing the bells in the remaining stations. The terminals of each telephone set are joined to the lines wires and ground in the same manner, but the leads from the bells to the terminals are connected differently as shown, to bring about the results described. The line wires terminate at the exchange switchboard in a low-resistance magnet drop, as in the preceding case, so that a party signaling the exchange does so without ringing the bells in the telephone sets.

The wiring in the exchange for a party-line circuit of this kind is shown at the left of Fig. 106, and includes, besides that for the line drop, jack, and plug, the connections of the generators giving the positive and negative pulsating current, in addition to the alternating current used for regular ringing, and of the generator switch, special selecting key, and regular ringing key. In calling

a party the operator first sets the special selecting key to correspond to the station desired, pressing it perhaps as indicated by the dotted lines so as to make contact between the proper springs; after inserting the calling plug in the line jack, she depresses the regular ringing key into its dotted position, thereby allowing the ringing current to flow through the signaling circuit. When the special selecting key is in its upright or normal position, alternating current is available, but when pressed to the right or left, places positive or negative pulsating current at the disposal of the operator.

Intercommunicating Systems. In these, 3 or more, usually not over 15, stations in the same or adjacent buildings are connected together, so that a party at one of them can call up and converse with a party at any other without the use of an exchange switchboard and operator, but with more privacy and as satisfactory selective signaling as supplied by the best of such systems; also at a comparatively small cost of operation. At each station there must be at least one wire to every other station and a common return wire which runs through all the stations. If battery bells and one common ringing battery are used, there must be at least two more wires than there are stations. No. 18 B. & S. gage rubber-covered copper wires are preferably used; for connecting the stations together these wires may be formed into a cable, if desired, and covered with friction. tape.

A Common Form of intercommunicating system is shown in Fig. 107. Four stations, 1, 2, 3, and 4, each fitted with series polarized bells, magneto generators, and the usual talking instruments, are there indicated connected together by the four wires 1, 2, 3, and 4 and the common return wire r. At cach station there is a switch s, the contact points of which are joined to the wires connecting the stations. With the contact arm of each switch on the contact point corresponding to its station, a party at any station can signal any other station. by moving his switch arm to the contact point corresponding to the station desired and turning his generator handle; this action will ring the bells only at the station called and the station calling, and the talking current will likewise pass only through these two stations. In the diagram the switch at Station 1 is set for signaling Station 4. Each of the telephone sets in this system has the same interior wiring, and the number of wires. between the stations is one in excess of the number of stations connected. Although only four stations are shown in Fig. 107, any number desired may be connected by developing the plan along the lines indicated. In order that an instrument in this system be in circuit for receiving a call it is necessary that its switch arm be on the contact point corresponding to its own station; this necessitates that each party after making a call returns the switch arm to its original position after his conversation is finished.

An Improved Form of intercommunicating system is that in which a station can be called even

though its switch arm is not on the proper contact point; the wiring for such a system is shown in Fig. 108. The telephone sets contain the same.

FIG. 108.-Method of Wiring an Intercommunicating System so that a Station can be Called Regardless of the Position of Its Contact Arm

apparatus as in the preceding case, connected up in the same way. One terminal of each set, however,