Differential amplifier (difference amplifier) also see –> Instrumentation amplifier
http://en.wikipedia.org/wiki/Operational_amplifier_applications
above found via ::: https://www.google.com/search?q=voltage+difference+circuit&hl=en&gbv=2&oq=&gs_l=
Next search should be ::: https://www.google.com/search?q=Instrumentation+amplifier+circut&hl=en&gbv=2&oq=&gs_l=
http://www.ti.com/lit/ds/symlink/ina128.pdf
http://www.analog.com/static/imported-files/design_handbooks/5812756674312778737Complete_In_Amp.pdf
http://www.ecircuitcenter.com/Circuits/instamp1/instamp1.htm
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http://circuit-zone.com/?cat=test_and_measurement
http://embedded-lab.com/blog/?p=396
http://mcs.uwsuper.edu/sb/Electronics/Vmeter/