construction it is identical with ordinary submarine cables, and therefore presents no increased difficulty either in laying, picking up, or in repairs.

The copper conductor, which weighs 300 pounds per nautical mile, consists of a central copper wire surrounded by five copper tapes according to the method introduced by Willoughby Smith, and the inductance loading is obtained by surrounding the conductor with a close helically-wound layer of .012-inch steel wire. Cable of this form is also suitable for underground service between remote points, and is said to preserve sound-wave forms and reduce inductance losses.

TESTING TELEPHONE LINE

WIRES AND CABLES

Continuity Tests, for determining whether or not a line wire or cable wire is broken, are made by grounding the wire at one end of the line and with a 3-cell battery and buzzer in series and grounded at the other end, touching the remaining terminal to the wire undergoing test. If the buzzer operates, the wire is continuous; if the buzzer fails to work, the wire is broken.

Testing for Crosses between wires or between wires and cable sheath is done by means of a 3-cell battery and telephone receiver. Taking, for example, the case of a cable still upon its reel, the connections are made as in Fig. 98. N represents the near end and F the far end of the wire being tested, B the battery, and T a telephone receiver. All the wires of the cable at F are carefully separated from each other and from the lead sheath; those at N are stripped of their insulation except the one under test, and are connected together and to the lead sheath by the wire C leading to the battery. The tester rapidly taps the free binding post of the receiver with the conductor joined to

the wire undergoing test. The first tap, and perhaps the second, will produce a distinct click in the receiver, but if the wire tested is perfectly insulated no further sound in the receiver will follow the tapping. If, however, the wire is crossed with any other wire in the cable or with the sheath, every tap will be followed by a distinct click in the receiver. A partial connection caused by

FIG. 99.-A Portable Wheatstone-Bridge Testing Set

moisture in the paper insulation will be indicated by faint clicks. In the same manner other wires in the cable may be tested, care always being taken to see that the exposed ends of the wires at F are separate from each other and from the sheath.

Locating Grounds, Crosses, or Swinging Wires can conveniently be done by means of a portable

Wheatstone-bridge testing set, Fig. 99, connected as shown in Fig. 100. The troublesome line wire is denoted by m, Fig. 100; one end of this wire is connected to the Wheatstone bridge B at d, and its other end is connected to h either directly or by

FIG. 100.-Connections for Locating Grounds, Crosses, or

Swinging Wires

means of a perfect conductor n of the same resistance per unit length as m. The point y represents the location of the trouble, which we will suppose to be a leakage to ground. The ratio arms of the bridge are denoted by a and b, the adjustable resistance arm by r; and the circuit dm n h, consisting of the defective wire m and the perfect wire n, forms the resistance x. The junction of a and b is connected to ground through the key k and battery t. By introducing equal resistances in the r-arm, a balance is finally obtained; this is indicated by no deflection in the galvanometer g when the keys k and o are closed. Knowing the value of x from the size and length of the wires m and n, the resistance of the wire length hy can be found