EXAMPLE--A wire having a resistance of 16.46 ohms per mile is grounded. This was looped with a wire of unknown resistance and the resistance of the loop was found to be 54.07 ohms. Connections were made as in the figure, and the reading A was found to be 234. From the table T = .3055, and substituting: d = (54.07 - 30.55) ÷ (1.3055 × 16.46) = 1.094 + miles.
Special Loop.--This method may be used to advantage where the length of the cable or faulty wire only is known and where there are two other wires which may be used to complete the loop. It is not necessary that the resistance of the faulty wire and the length and resistance of the other wires be known. Figs. [601] to [604] show the connections and method of testing.
EXAMPLE.--All the wires in a cable 10,852 ft. long were found to be grounded so that none of them could be used as good wires. Two wires were selected out of another cable going to the same place by a different route and securely joined to one of the grounded wires at the distant end. This grounded wire and one of the good ones were connected as shown in figs. [601] and [602] and the reading A was found to be 307. Connections were then made as shown in figs. [603] and [604] and A was found to be 610. What is the value of d?
According to formula
d = AL/A = (307 × 10,853)/610 = 5,461 ft.
Figs. 601 and 602.--Special loop test with Leeds and Northrup fault finder. For the first measurement connect the faulty wire to 2, either of the good wires, as Z, to 1, the post Gr to ground, and short circuit the coils R and E by closing switches U and V as in the figures. Balance in the usual way and call the dial reading A. For the second measurement, connect the post Gr. (disconnected from ground), to the other good wire y as shown in figs. [603] and [604], and get another balance; call this reading A'. The distance d, to the fault is determined from the simple formula d = AL ÷ A' where L is the length of the cable or faulty wire.
Figs. 603 and 604.--Special loop test as made with the Leeds and Northrup fault finder. Diagram showing connections for the second measurement. The special loop test may be used to advantage where the length of the cable or faulty wire only is known, and where there are two other wires which may be used to complete the loop. To use an outside battery, connect one pole to Ba, and ground the other. The pressure of this battery must never exceed 110 volts; if it be over 25 volts, see that switch W is open.
The Potentiometer.--For the rapid and accurate measurement of voltage, current, and resistance, the potentiometer can be recommended. Those in charge of electric light and power companies, and also those who purchase large amounts of electrical energy are realizing, more and more, the necessity of having satisfactory primary standards with which to check their volt-, ampere-, and watt-meters.
When it is realized that an error of one per cent. in a commercial instrument means an error of one dollar one way or the other in every one hundred dollars charged, the need of such standardization apparatus becomes at once apparent.
The potentiometer, it should be noted, relies for its accuracy, only upon the constancy and accuracy of resistances and upon standard cells.