The accuracy of the location of both of the above methods depends on the good and broken pair, or the good and broken wires having equal and uniform capacity per unit length. It is not always possible to select wires that are alike in this respect. In such cases, as for instance, where there is no good wire in the cable containing the broken wire, and a good wire has to be selected from another cable, the method of _Case III_ may be used. [Illustration: Fig. 609.--Diagram of connections for reading in testing for opens with Leeds and Northrup fault finder, when broken wire and good wire are not in the same cable.] [Illustration: Fig. 610.--Leeds and Northrup potentiometer. It is direct reading from .000001 volt to 16 volts, and with accessories the range may be extended to 1600 volts, and currents may be measured up to 3000 amperes. The instrument has fifteen coils of 5 ohms each, which are in series with an extended wire about 190" long of equal resistance. The electrical circuits are shown in the diagram fig. 611. It is well for the user to open up the potentiometer and make himself familiar with its interior construction, in order to fully understand the operation of the rheostat and other parts. There are no contact resistances in the potentiometer circuit proper. The potentiometer has low internal resistance which gives it the maximum sensibility. Compared with high resistance potentiometer, this is especially advantageous in measuring the electromotive force of thermocouples, and the fall of potential across standard low resistances. As constructed, the last one-tenth volt is covered by the extended wire and the handle which carries the contact point on the wire may be manipulated rapidly so that a fluctuating voltage may be accurately followed. When used with any cadmium cell, the potentiometer is direct reading. The accuracy of the potentiometer resistances can be verified with the facilities of the ordinary laboratory.] Location of Opens.--_Case III_, in which the broken wire and good wire are not in the same cable. Connect the good wire and broken wire in the same way as shown in fig. 607, and set the pointer for a balance. Call the reading A. Then connect the good wire and the broken wire at the distant end and set the pointer for a new balance. Call this A'. The connections for this reading are shown in fig. 609. The distance to the break will be d = (A A' L) / (1,000(A - A') + A A') where L is the total length of the broken wire.