comparator

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various instruments for comparing a measured property of an object, such as its shape, color, or brightness, with a standard.

from The Century Dictionary.

• noun A vessel in which liquid is kept rapidly stirred and at a uniform constant temperature so that thermometers immersed in it must all be maintained at the same temperature. Simultaneous readings of the secondary thermometers and the standard are made, and the corrections to the former are thus determined for a wide range of temperatures.
• noun An apparatus for making comparisons; especially, an instrument for comparing the lengths of nearly equal bars, either from end to end or between lines engraved upon them.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Physics) An instrument or machine for comparing anything to be measured with a standard measure; -- applied especially to a machine for comparing standards of length.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Any device for comparing a physical property of two objects, or an object with a standard.
• noun An electronic device that compares two voltages, currents or streams of data.