Fig. 588.--Diagram showing arrangement and connections of Leeds and Northrup fault finder. It is used to measure conductor resistance, fault resistance, to locate faults by four different tests, and when used with a buzzer and telephone, to locate opens. The essential feature of the instrument is the uniform resistance AB, which lies in a circle and which has a value of about 100 ohms. By a special construction, it is so arranged that the contact can be made at any point along it, and it is therefore equivalent to a very high resistance slide wire. It has a moving contact C and a uniform scale of 1,000 divisions. In series with this, there are the two resistances E and R which may be short circuited by the switches U and V. E has exactly the same resistance as the wire AB. R has a resistance of 100 ohms, and is the fixed resistance of the bridge arrangement for resistance measurements. The resistances of 1,000 ohms and 9,000 ohms connected to the battery post are to protect the battery and the apparatus from excessive current. The 9,000 ohms may be short circuited by the switch W.
Ammeter Test with Queen Acme Set.--Connect a low resistance in series with the ammeter and run leads from it to the testing set, the positive lead to the + battery post and the negative lead to the line post C ([fig. 577]). Insert a standard cell between the battery posts, with positive terminal to + battery post, and negative terminal to - battery post. Plug commutator as shown in [fig. 582]. Remove 10,000 ohms from rheostat, and 100 ohms from bridge arm B. Determine a balance in the usual way by changing the value of the resistance in the rheostat. This operation will balance the difference of pressure at the terminals of the shunt resistance against the standard cell, and its value is equal to
(1.40 × 100) / (R + 100) = 140 / (R + 100)
To determine the current flowing, divide the value of the difference of pressure thus obtained by the value of the shunt resistance.
Fig. 589.--Resistance measurement with Leeds and Northrup fault finder. The diagram shows the proper connections and switch settings for measuring conductor resistance. As in the ordinary slide wire bridge, the resistance X between the two posts 1 and 2 is obtained from the formula X = A ÷ (1,000 - A) × R. To avoid the necessity of solving in each case the fraction A ÷ (1,000 - A), a table is furnished with the instrument, giving the value of this fraction for each value of A. The resistance is accordingly determined in each case by simply setting the contact C for a balance and reading from the table the resistance opposite the number corresponding to the scale reading and multiplying by 100, the value of R. To use an outside battery, remove the inside battery and connect the outside battery between the posts Gr and Ba. The pressure of this battery should not exceed 110 volts. If it exceed 25 volts, open switch W.
EXAMPLE--With an unknown resistance connected between the posts 1 and 2, the galvanometer showed a balance for a dial reading of 387. The number opposite 387 in the table is .6313; hence, X = .6313 × 100 = 63.13 ohms.
