As theatre lighting became more complex, there was an increasing need for the
lighting operator
to be able to see the performance in order to coordinate the lighting changes with the action on the stage. After the World War,
remote control
systems which employed mechanical
transmission
by wires and pulleys were developed for rheostats. These were known as control desks or control boards. The position of the
lighting operator
could now be separate from that of the rheostats themselves. The
control board
itself could be located for instance at the side of the
stage
or near the
stage
manager's corner.
Between the two world wars, Germany was the leading manufacturer and developer of stage
lighting control
systems, and German rheostats and control boards were in demand throughout Europe
However, dimming with rheostats was not always smooth. Since the rheostats had to match the
electric load
of the lamps they controlled, they could, when improperly loaded, lose sensitivity of control. A new method of
lighting control
based on variable transformers developed in Germany - the
autotransformer
dimmer - enabled a stable dimming curve. The
autotransformer
was not limited to a fixed electric load, and was able to smoothly control varying loads of up to 6000 watts. A small
lever
caused a change in
voltage
supply to the lamp, thus changing the intensity of the light. It became fairly simple to place the
control board
with levers at a distance from the
autotransformer
power supply unit, connecting them by regular electric wires. In terms of efficiency of handling the electric power, the
autotransformer
was a vast improvement on the rheostat, producing as much as 80% of the electric power fed to it.
An early auto-transformer
Variable transformers soon made their appearance in America under the trade name Variac. After World War II both types of dimmers, the
rheostat
and the variable transformer, were commonly used in both Europe and America.
A typical Autotransformer
control board
(right). The interlocking (left).
