Van de Graaff Generator
The voltage of this demonstration Van de Graaff generator is measured between the charged sphere and ground. Earth's potential is taken to be zero as a reference. The potential of the charged conducting sphere is the same as that of an equal point charge at its center.
Source
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"OpenStax College, College Physics. December 13, 2012."
http://cnx.org/content/m42328/latest/?collection=col11406/1.7
OpenStax CNX
CC BY 3.0.