by substituting the values of x and r in the for

for hy by the resistance per foot or mile of the wire gives the distance of the fault from h in feet or miles. The portable testing set, Fig. 99, contains, in addition to the Wheatstone bridge B, the galvanometer g, the battery, and the keys o and k as indicated.

For locating crossed or swinging conductors, one of the defective wires must be grounded, and the other joined by means of a perfect wire to the points d and h of the bridge as in the previous case. This test is also conducted like the preceding one. If the crossed or swinging conductors make good contact with each other, the location of the cross can be found by measuring the resistance of the

a

circuit thus formed (it being equal to r), and

dividing the result thus resistance per unit length.

b

obtained by twice their

For this test the battery

t and key k are connected from the junction of a and b, Fig. 100, to h instead of to ground. The values of a and b in this case may be made respectively 10 ohms and 100 ohms, for greater

accuracy.

Testing Out the Separate Pairs of a Cable requires two men, one at each end of the cable, provided with a battery, electric bell or buzzer, and a portable telephone set. A wire at

en

of the cable is first connected through the buzzer to ground. At the other end of the cable the battery and buzzer are joined in series, and one terminal of the connection is grounded while the other terminal is scraped over the bared conductors. When contact is made with the conductor previously grounded at the further end of the cable, both buzzers will operate. This con

ductor, together with its mate which is easily distinguished on account of being twisted with it, afford a complete metallic circuit through the cable. If the portable telephone sets be now connected to this circuit and the test continued as before, the separate pairs can be tested and numbered with but little trouble.

An Acme telephone set adaptable for line testing is shown in Fig. 101. The working parts are

completely inclosed within the box a when not in use, even the receiver r and the crank c of the magneto generator being thus housed. By means of the strap s the set can easily be carried from place to place, connection being made to the line at the binding posts m and n.

WIRING AND OPERATION OF

SPECIAL TELEPHONE

SYSTEMS

Party-Line Systems. In these, two more telephone stations are connected to one line, and where a central office or exchange is provided, the line is connected to one set of switchboard terminals. If there is no exchange the system is operated by the parties who use it. Party-lines have been devised to reduce cost of installation and operation over that necessary when each telephone station is connected to a separate line circuit. They may be divided into series party-line systems and bridging party-line systems. Although the regular wiring for both these cases has already been illustrated and described under Series Connections and Bridging Connections of telephone instruments, there are several modifications of these which have proved adequate in meeting special requirements and merit consideration here. Details regarding size of wire, etc., follow the instructions previously given.

Series Party-Line Systems.-The simplest of

these systems is shown in Fig. 102. Magnet telephones are used for transmitting instead of the usual carbon transmitters, and batteries are employed instead of magneto generators. In each of the 3 stations shown, t represents the trans

FIG. 102.-The Simplest Form of Series Party-Line
System

mitter, the receiver, h the hook switch, b the battery, v the battery bell or buzzer, and s a switch. When a station is not in use the receiver hangs on the hook switch, causing the latter to connect the line with the lower contact c, and as the switch s is normally held against the upper contact a by a spring, the bell circuit is closed. When a signal is to be sent, the receiver is removed from the hook switch and the switch s pressed down upon the lower contact n; the current from the battery in that station then rings the bells in the other stations and, according to the number of rings, signals the desired party. Conversation is then carried on as usual, one of the magnet telephones being used as the transmitter and the other as the receiver. The extreme simplicity of this arrangement is accompanied by little cost and trouble for apparatus and wiring, but the system is