Also known as a variable autotransformer. This type of dimmer is now virtually obsolete. It consists of a single winding autotransformer and a carbon brush on a rotating shaft. This allows dimming the input voltage while maintaining the sine wave of the line. These devices are bulky and heavy, and not suited to remote control except via cumbersome fixed-speed motor drives.
A common variant of autotransformer, the Variac, also allows boosting the input line voltage to about 140V by placing the input tap on the transformer between the maximum end-of-travel of the carbon brush and the 0V neutral tap. This type of device was used on the 1975 Broadway production of A Chorus Line to boost the voltage to the 1000W FEL lamps in the Strong Trouperette III followspots, to gain additional brightness.
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Superior Electric (Luxtrol) and Ward-Leonard were the primary manufacturers of autotransformer dimmers for the theatre.
The model shown below, a Luxtrol "six pack," contains six 10A dimmers, which could be either electrically mastered by a 60A dimmer, OR the 10A dimmers and the 60A could be independent from one another.
The model below contains six dimmers of 10A-60A capacity. The handle on the far right (without a breaker), is a mechanical master. The Tee Handles are rotated 90° horizontal to lock into the master, or vertical to be independent. The oval plastic Tee handles identify this unit as one manufactured by Altman, using Superior Electric Luxtrol autotransformers. Complete units manufactured by Superior Electric had tubular plastic handles.