[Illustration: Fig. 612.--Diagram showing actual connections in the rheostat of Leeds and Northrup potentiometer. The figure corresponds to R of fig. 611. The rheostat is mounted in the end of the potentiometer as shown in fig. 610. Rough adjustment of the potentiometer current is made by means of the variable resistance R. Fine adjustment is made by means of the variable resistance R'. It will be noted that the 23 ohm resistance of this latter rheostat is shunted by a resistance of 6.1 ohms, making possible a very fine regulation. Further, there is in series with the moving contact a resistance of 400 ohms, which makes the effect of variable contact resistance negligible. Only one cell of storage battery should be used. When this battery is fresh, the plug shown in the figure at 2R should be inserted at R. This gives the greatest resistance in the rheostat circuit. As the cell runs down, the plug should be changed to 2R. When both plugs are in, the rheostat slide wires are in series with the potentiometer circuit.] EXAMPLE: A pair of wires containing one broken wire was connected with a good pair in a different cable as shown in fig. 607. The reading A was found to be 180. The good and bad wires were then joined at the distant end as in fig. 609, and the reading A was found to be 88. The total length of the bad wire MN was 1.44 miles. Required, the distance to the break. Substituting the values in the formula: d = 180 × 88 × 1.44 / 1,000(180 - 88) + 180 × 88 = .211 + mile. To Pick Out Faulty Wires in a Cable.--Short circuit the coils E and R with switches U and V. Set the pointer at 1,000. Connect the post Gr. to ground or the cable sheath and apply the wires one after another to the binding post 2. The galvanometer will deflect for a faulty wire. [Illustration: Fig. 613.--Diagram of the Crompton potentiometer. In this instrument the resistance consists of fourteen coils, each of 10 ohms, in series with a straight wire, also 10 ohms resistance, thus forming a system of fifteen equal steps. Across the whole a pressure of 1.5 volt is applied from a secondary cell, thus providing .10 volt per step. Any fraction is then tapped off by means of a radial switch on the resistance coils and a sliding contact on the wire. The standardization is performed by adjusting a resistance in series with the whole until the standard cell employed indicates, by means of the galvanometer G, a balance at the point which represents its electromotive force on the basis given above.] Ques. What is a potentiometer? Ans. An arrangement of carefully standardized resistances for measuring voltages in comparison with a standard cell. It is used for accurate measurement of voltages, currents, and resistances. In place of a series of standardized resistances, a slide wire may be used as in fig. 614. Ques. Describe one form of potentiometer.