Abstract :
A theoretical discussion of the electromagnetic properties of aluminium-steel cored cables is given, in which the general cases of a solid and a stranded cable are discussed and the argument is extended to the more complicated case of a composite cable with a magnetic core, showing how the axial flux set up in the core by the spiralling of the outer layers will cause an increase in a.c. resistance and inductance depending on the magnetic properties of the iron and on the arrangement of the strands in the cable. For the special case when, owing to the spiralling of the two outer layers being in opposite directions, the flux in the core becomes negligible, the inductance may be calculated from the dimensions of the cable and the a. c. resistance from the conductivities of the components. The experimental work in verification and extension of the theoretical analysis consisted of the measurement of the d. c. and a. c. resistance and inductance, at commercial frequencies, of a short length of transmission line, short-circuited at the end over a wide range of currents by means of the Gall-Tinsley alternating-current potentiometer. This instrument was made the basis of an arrangement designed to make such measurements a matter of routine, while attaining an accuracy of the order ± ¿ per cent. Further tests were made to determine the flux existing in the steel core by measuring the voltage induced in a search coil. The results are discussed in the light of the theoretical analysis and of American practice, and definite figures are established for the British Standard conductors.
