Loop Test.--This is a method of locating a fault in a telegraph or telephone circuit when there is a good wire running parallel with the defective one. In the process, the good and bad wires are joined at their distant ends and one terminal of the battery is connected to a Wheatstone bridge, while the other terminal is grounded. There are different ways of making loop tests as by:

  1. The Murray loop;
  2. The Varley loop;
  3. Special loop.

Fig. 595.--The Murray loop test. The apparatus is connected as in the figure. The rheostat of the bridge is used in place of the second arm to permit large adjustment. X and Y are the resistances of the cable between the fault and the points 1 and 2 respectively.

The Murray Loop.--In this test only one of the two regular bridge arms is used, the other being replaced by the rheostat giving an arm of large adjustment.

The connections are shown in [fig. 595]. In making the test, close key and note the deflection of the needle due to pressure of chemical action at fault, if any. This is called the false zero.

Now apply the positive or negative pole of the battery by depressing the battery key, and balance to the false zero previously obtained by varying the resistance in arms A or B. Then by Wheatstone bridge formula: RX=AY, and L=X+Y; Y=L-X, whence

X = A/(R+A)
Y = L( R/(B+A) )

Fig. 596.--Murray loop method of fault location with Leeds and Northrup fault finder. Case I where there are two wires having equal resistance, in one of which there is a fault. Connect and set switches as shown; join the good wire to post 1 and the faulty wire to post 2. The resistance of E is equal to that of AB. From the symmetry of the arrangement, it is evident that, if the fault were exactly at the junction between the good and bad wires, the contact point C would rest for a balance at 1,000 on the scale, or at 500 if the fault were half-way along the bad wire; hence, at whatever point it comes to rest, the reading divided by 1,000 and multiplied by the length of the bad wire is the distance from the instrument to the fault.

EXAMPLE--In a pair of equal wires, 5.8 miles long, one is grounded. With the connections made as above, and the galvanometer balanced for the dial reading 124, the distance to the fault is (124 × 58) ÷ 1,000 = .7192 miles.