[Illustration: Fig. 607.--Diagram of connections in testing for opens with Leeds and Northrup fault finder. The apparatus required consists of fault finder, buzzer, dry cell to operate buzzer, small induction coil, and telephone receiver. Connect the battery to the primary of the induction coil, one terminal of the secondary to the post Ba, and the other to the connected wires as shown. Set switches U and V so as to short circuit the two resistance coils.] 2. The resistance between the two sections of the broken wire should be measured. This may be done by joining the broken wire and a good wire at the distant end of the cable and measuring the resistance of the loop. To ensure close locations, this resistance should be over 100,000 ohms. Fair locations can be made when the resistance is much lower and it is worth while to attempt it even if the resistance be as low as 10,000 ohms. The difficulty of determining the balance point increases as the resistance decreases. Ques. Describe the method of locating an open with a fault finder. Ans. (_Case I_) The broken wire will be one of a pair. Select another pair in the cable that will have the same capacity per mile and join together the mate of the broken wire and one wire of the other pair. Make the connections as shown in fig. 607, then depress the battery key and move the contact to the point of minimum sound in the telephone. The distance to the break is equal to LA ÷ (1,000 - A), where L is the length of the good wire. EXAMPLE: A cable 1.45 miles long contained a broken wire. It was found that the insulation resistance of the end of this wire was over 10 megohms, as was that of the good pair selected to test against it. The resistance between the two pieces of the good wire was also over 10 megohms. Connections were made as in fig. 607, and it was found that the balance point was 476. Accordingly from the table A / (1,000 - A) = 0.9084 and d = 1.45 × .9084 = 1.317 + miles. [Illustration: Fig. 608.--Diagram of connections in testing with Leeds and Northrup fault finder for open wire in telegraph and other cables in which the wires are not grouped in pairs. Connect the broken wire to 1. Select a good wire and join to 2. Connect all other wires and ground them, by connecting to the cable sheath. Connect the distant end of the broken wire to the others. Ground the end of the induction coil that is not connected to the post Ba.] Location of Opens.--(_Case II_) Open wire in telegraph or other cables in which the wires are not grouped in pairs. The connections are made as in fig. 608, and the measurement and calculation exactly as in the preceding case.