Inductors
An inductor is, basically, a coil of wire. Its inductance value is determined primarily
by its physical dimensions, the
number of turns of wire and whether or not it has a core of some material (usually
brass or iron dust). Generally, coils designed to handle higher power levels tend
to be air-cored.
As the inductance is relatively difficult to adjust (compared with a variable capacitor), equipment designed to work on several frequencies usually has several fixed coils which are selectable via a panel-mounted switch to act as a course frequency adjustment with a variable capacitor for the fine tuning.
The small picture to the left shows the top of an iron-dust core inside a coil. The slot is to adjust the position of the core within the coil in order to change its value. These cores are always brittle and should not be adjusted without the proper tools and test equipment.