Zenith variac type 100r - carbon brushes

[boater sam]

There are lots of articles on the net about this type of carbon, here is one,   https://sound-au.com/articles/variac.htm

another  https://www.eevblog.com/forum/reviews/va...-material/

The brush must short windings together as it traverses, ordinary carbon causes sparking which damages brush and winding.

There is a suggestion that the carbon rod from a dry cell may be more suitable material.