Electricity!
This page describes the mechanical devices and circuits that control the Electricity! exhibit. The exhibit can be seen here.
Inside the green square, you can see the mechanism for the large rotary switch visible on the front panel. The spring-loaded lever provides engagements with the detents for positive positioning. The switches arrayed around the top are the voltage selectors.
In the red square are the voltage regulators. They are adjustable so that the indicator on the voltmeter can be made to precisely line up with the graduations.
In the blue square is the overload protection. If there is a rush current from being shorted, the voltage across the large resistor picks up the relay, which, in turn, activates a standard Elk timer. The display will simply go dark for two minutes. The large capacitor you see provides a slight delay to the process to allow for the connection of a motor, which naturally has a high inrush current.