Setting Up the Light

The simple light should be setup per the schematic. Make sure that they are using a 2-way switch. The light doesn't work with the white wire cut, because there is no path back to ground. Without a path back to ground, the current will not flow.

The Voltage

Because the current doesn't flow, the voltage will be 12V on the brass screw, 12V on the silver screw, and 0V across the light bulb. 

This is because voltage measures potential or difference. So, there is a 12V from the brass screw to ground and 12V difference from the silver screw to ground. Across the light bulb, there is 12V on the silver screw and 12V on the brass screw, which means there is no difference. It will show 0V between them.  

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