shading ring

To simplify the sealing issues, the plugnut, core, springs, shading ring, and other components are often exposed to the fluid, so they must be compatible as well.

Material has implications for shading ring: copper is satisfactory for many materials, but it is unsatisfactory for coffee and urea; silver is satisfactory for those fluids.

Typically this is done with a small copper "shading ring" crimped around a portion of the core that creates the delayed, out-of-phase component, which holds the contacts during the zero crossings of the control voltage.