182. What do the soft iron spheres mounted on arms on the binnacle compensate for?

Soft iron spheres on the binnacle, known as Kelvin's balls, compensate for induced magnetism in the horizontal soft iron of the vessel, which causes quadrantal deviation. These spheres are made of soft iron, which becomes magnetized and demagnetized by surrounding magnetic fields, and are adjusted to cancel the effect of induced horizontal soft iron, unlike permanent magnets or fields from electrical currents.