. American telephone practice . e is continuous. The same precaution as previously TESTING. 855 pointed out must, however, be observed with the magneto bell. Thissame test for continuity is well illustrated in Fig. 625, in which casea vibrating bell instead of the receiver or galvanoscope is used. In testing a cable all defective wires should be marked crossed,grounded, or broken at the end at which they are tested. Thecorresponding ends of the tagged wires at the other end of the cableshould then be found and similarly marked. If there are not therequisite number of good wires in a new cable
. American telephone practice . e is continuous. The same precaution as previously TESTING. 855 pointed out must, however, be observed with the magneto bell. Thissame test for continuity is well illustrated in Fig. 625, in which casea vibrating bell instead of the receiver or galvanoscope is used. In testing a cable all defective wires should be marked crossed,grounded, or broken at the end at which they are tested. Thecorresponding ends of the tagged wires at the other end of the cableshould then be found and similarly marked. If there are not therequisite number of good wires in a new cable it should be rejected. It is often desirable to be able to pick out a certain wire at someintermediate point in an open cable, or in a large bunch of insulatedwires, in order to establish a branch connection. This is easily doneby the foregoing methods if the cable is to be cut, but frequentlythis is not the case. It may be done without cutting by the followingsimple method : Ground the wire or wires desired at the distant end,. FIG. 626.—DIAGRAM OF WHEATSTONE BRIDGE. being sure that these wires are free from all the others at both having loosened the bunch of wires at the point at which thebranch is to be taken off, test each by means of a needle-pointed in-strument, connected to ground through a bell or receiver and needle-point can readily pierce the insulation and make goodcontact with the conductor within. A knowledge of this very simpletest will often save an immense amount of trouble. In the second class of test—that is, those requiring quantitativemeasurements—there are three distinct subdivisions, which are asfollows: Tests for resistance or conductivity, tests for capacity,and tests for insulation. Tests for the location of faults in linesalways depend on the application of one or more of these. There are three principal methods of making resistance tests :First, by the use of a Wheatstone bridge, which is accurate for allresistances except
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