liSS 



ELECTRICAL MEASUREMENTS 



which flows only in the sheath produces a field which has no 

 longitudinal component and, therefore, stray currents cause no 

 disturbance of the telephone. 



Referring to Fig. 429, when a current flows out along the spi- 



FIG. 429. Diagram for Durgin exploring coil. 



railed conductor ab and returns along the sheath, in effect along 

 cd, the case is entirely different for there is a loop twisted so that 

 it alternately presents its positive and negative side to the obser- 

 ver as he passes along an element of the sheath ; that is, positions 



of maximum and minimum magnetic 

 potential succeed each other in a 

 definite order. 



When the longitudinal exploring 

 coil is used in the case shown in Fig. 

 428, it will be found that no indica- 

 tion is obtained at points near the 

 signalling apparatus, for at these 

 points there is little return current in 

 the sheath and the effect of a twisted 

 loop is not obtained. On approach- 

 ing the ground, more and more cur- 

 rent flows in the sheath and the 

 signals increase in intensity until the 

 ground is reached; beyond this point 

 there is silence, for only sheath cur- 

 rents are present and they produce 

 no longitudinal field. 



Another use of the exploring coil is 

 in identifying a particular cable in 

 the distributing system. When deal- 

 ing with three-phase cables, there are three signalling loops 

 which may be utilized. 



I. Two conductors, the current flowing out by one and return- 

 ing by the other. 



FIG. 430. Showing mag- 

 netic equipotential lines 

 around a three-phase cable 

 when steady current flows in 

 the signalling loo 

 conductors A and B. 



