Dimmers are mechanical or electromechanical devices used to vary the amount of electrical power being sent to each lighting instrument. In most modern theaters, changing the dimmers' output is the primary means of controlling the intensity of the lighting on stage.
The construction of dimmers has evolved considerably over time. Some of the oldest dimmers used in the theater, known as 'salt water dimmers,' consisted of a glass jar filled with salt water with two metal leads placed inside. When this was wired in series with the light to be controlled, an operator could control the light's output by varying the distance between the leads. This was a difficult (and often hazardous) process.
Modern dimmers are much more technically complex, and a great deal safer. They control the amount of voltage in the circuit using thyristors, simple electronic circuits that rapidly turn themselves on and off, allowing precisely the amount of current needed to pass through.