Position of Search
Coil and Number
of Times Field is
Greater than with
Cable Unenclosed


erage Increase in
Id at Three Posi-
ions, A, B, C.
A.
B


C


A


B


c




A


Unenclosed


1.73


2.38


1.79


• • •


• • •


• • •


• • •


In C.I. trough -


3-04


4.61


2.38


1.75


1.92


1.33


1.66


'


Steel armour -


2.28


3.72 , 2.53

1


1.32


1-55


1.41


1.42


A


Unenclosed


1. 19


.198


2.71


• • •


• • •


• • •


• • •
•


In C.I. trough -


2.5


.228


4.6


2.08


I. II


1.7


1.63


8


Steel armour -


1.68


.34


3.73


1.41


1.69


1.38


1.47



It might appear at first sight that the above values obtained
for strength of field under various conditions are much smaller
than would be expected from the large currents traversing the
cores of the cable. The explanation is, however, not far to seek,
and the discrepancy is due to the fact that as the plane of the
coil was necessarily tangential to the circumference of the cable
the direction of the lines of force was in no case normal to the
plane of the test coil. Nevertheless a comparison of such
readings, taken with as nearly as possible identical positions of



26 Three-Phase Transmission

the search coil and the same current values first without and
then with the iron envelope surrounding the cable, should still
represent with close approximation the extent by which the
external field of the cable is augmented by the troughing or
armouring.
Coming now to the direct measurement of lead sheath losses
upon underground cables, this is in practice usually accompanied
by some difficulty arising from the following amongst other
points.
