Radiotone Single Layer Roller Contact Vertical Variable Autotransformer (colloquially known as Variac) construction looks like a conventional 3 phase transformer as can be seen from the picture.
Speciality of this Variable Autotransformer lies in winding Cu strip of appropriate cross section edgewise. This reduces the length of the column to 1/4 as compared to winding widthwise, thus making it compact. This column is then polished & roller carbons are made to make contact with winding for current collection. Due to circular column construction, a Double Carbon assembly can be provided giving two outputs 180 degrees out of phase (or as it is known + and - type).
Radiotone Single Layer Roller Contact Vertical Variable Autotransformer can give +/- type output with carbon assembly moving in opposite directions (up/down). For large current ratings these columns can be paralleled. Also in three phase construction, it can be used for higher Currents in delta (p) type connections. Radiotone Single Layer Roller Contact Vertical Variable Autotransformer finds application mainly in Plating rectifiers, high voltage Testing Equipment & high power automatic voltage regulation (Servo Stabilizers).