Slot Treatment. Corona prevention in the slot parts of stator coils is accomplished with the use of a low-resistance conducting finish. Varnish-base conducting compounds are in present use. In the past, colloidal graphitic water suspensions have been used. In general, this treatment has successfully eliminated slot corona and has been used by Westinghouse for over 20 years. In a few cases, however, difficulty arising from a loss of electric contact between the conducting coil finish and the core has been encountered. This loss of contact has been limited entirely to top coil sides (those nearest the bore). Under these conditions, at voltages in the range of operating stress, a rather intense capacitive discharge occurs between ungrounded coil sides and the core. This phenomena, which is called a “slot discharge,” may be seriously destructive to the coil surface finish and to the basic ground insulation if it continues unchecked.