Loading coil cores and their magnetic stability

The primary requisites of a loading coil are a minimum stray field, low resistance, and an inductance independent; of current. The use of some form of iron core of high permeability is essential, but the difficulty arises in the variation of the permeability with the polarising current. Cores made up of very fine hard-drawn iron wire (4 mils. diameter) of permeability 60-90 have been used with success, but remain unsatisfactory when telegraph and phantom circuits are superposed. The variations in permeability have been greatly reduced by the introduction of two diametrical air gaps in the iron circuit. The demagnetisation effect of the air gaps increases the magnetic stability at the expense of the permeability. This idea is extended in the use of powdered iron. The difficulty, then, is in hardening the iron and insulating the particles from one another. The iron is first obtained electrolytically and pulverised in a ball mill to a size which will pass through an 80-mesh sieve. The iron may then be annealed and re-pulverised. The powder is then treated with shellac and compressed to shape in an hydraulic press under a pressure of 200,000 lbs. per sq. in. The resulting product is of three grades, and from tests on these grades the author deduces a theory which explains the constancy of the permeability and its value in cores produced under different conditions.