Powdered-Iron Slugs

Both self-inductances and mutual inductances of a coil may be increased by inserting a slug of powdered iron inside the coil tube. Tuning a coil to a given frequency is often effected in this manner with fixed capacitors instead of tuning with variable capacitors. Such a coil is shown in Fig. 179, with the powdered-iron core hidden by the coil form.

Fig. 179. Coil inductance is varied by powdered-iron slug.

At the left end is the screw and lock by which the inductance can be adjusted and maintained at a given value. The mutual inductance of a pair of coils can be changed similarly. This is preferable to attempting to vary the distance between the coils, since it requires no flexible connections. Powdered iron is available in several grades, from ordinary powdered iron to powdered nickel alloy. Insulating compound reduces the permeability of the core to values ranging from 10 to 125, depending on the grade of iron and the frequency. In a given coil, the insertion of a powdered-iron slug raises the inductance from 2 to 3 times the value which it would have if no iron were present. Circuit Q increases similarly. Higher Q results from a powdered iron or ferrite magnetic path, closed except for small air gaps. For an untuned transformer, where high Q is not essential, the air gap may be zero to reduce magnetizing current.