Which type of core material decreases inductance when inserted into a coil?

When you insert a ferromagnetic material like ferrite or iron into a coil, it concentrates the magnetic lines of flux and increases the coil's inductance. However, inserting a non-magnetic, highly conductive material like brass has the exact opposite effect. When a brass core is inserted, the coil's changing magnetic field induces eddy currents within the brass. These eddy currents create their own opposing magnetic field, which cancels out part of the coil's original field. This opposition effectively reduces the coil's overall inductance. In many classic radio designs, brass tuning slugs were frequently used in variable inductors when the circuit required the inductance to be tuned downward from its base air-core value.