CABLE TESTING 



II. Two conductors in parallel and in series with the third. 



III. One conductor and the sheath. 



For purposes of explanation take the first case; the equipoten- 

 tial lines on a plane perpendicular to the axis of the cable, and 

 due to a steady current, are shown in Fig. 430. 



It is evident that because of the position of the conductors in 

 the cable, the magnetic potential varies from point to point 

 around the circumference of the sheath and that the maximum 

 and minimum points are 180 apart. This means, that on ac- 

 count of the lay of the cable, the distance between the points of 

 the maximum and minimum magnetic potential when measured 

 along an element of the sheath is one-half the lay of the cable. 

 If the iron core of the exploring coil has a length equal to one- 

 half the lay and is applied longitudinally to the cable between 

 these points, it will be traversed by a considerable flux, and a 

 signal sent into the cable will be audible in the telephone. Sheath 

 currents of any sort are not effective in producing sounds in the 

 telephone for they cause no inequalities of magnetic potential 

 along the length of the cable. 



Percent of Lay 



1 .,,,. 4;$!. Showing variation of magnetic potential when different 

 signalling loops are used. 



The two-conductor circuit, I, is best when a cable has to be 

 identified throughout its length because it gives the maximum 

 difference of magnetic potential and also because the maximum 

 and minimum points are equally spaced. 



With connection II, the maximum and minimum points 

 spaced alternately 40 per cent and 60 per cent of the lay. 



