Loading Coils

Figure 14. Midsection termination of a loaded cable.

by the heavy black lines. The magnetic cores are continuous, although cores with air gaps have been used.¹¹ Since the instantaneous currents in the two line wires are in opposite directions, the flux produced by each winding adds to that of the other, producing the maximum inductive effect.

Figure 15. Midcoil or midload termination of a loaded cable.

The first loading coils had continuous ring-shaped cores consisting of a coil of many turns of fine iron wire. The wires were insulated to reduce eddy-current losses. In 1916 an important improvement was made when a core of powdered and compressed iron was perfected. A core of powdered and compressed Permalloy was developed¹⁷ about 1926. For about the same efficiency, the Permalloy core gives a coil very much smaller than the iron-core type, and hence it permits reductions in size. A further reduction in the size of loading coils is made practicable by the use of the molybdenum Permalloy dust core shown in Fig. 17. A detailed summary of recent improvements in cable loading coils and a description of the method of winding the coils is given in reference 20.