Comparator

·noun An instrument or machine for comparing anything to be measured with a standard measure;

— applied especially to a machine for comparing standards of length.

comparator
compărātor, ōris, m. [2. comparo], a purchaser (late Lat.), Paul. Sen. 2, 17, 15; Cod. Th. 10, 33...

A New Latin Dictionary by Charlton T. Lewis Ph.D. and Charles Short, LL. D.