Wheatstone Bridge
The Wheatstone bridge is used to calculate unknown resistances. The variable resistance R3 is adjusted until the galvanometer reads zero with the switch closed. This simplifies the circuit, allowing Rx to be calculated based on the IR drops.
Source
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"OpenStax College, College Physics. October 26, 2012."
http://cnx.org/content/m42362/latest/?collection=col11406/latest
OpenStax CNX
CC BY 3.0.