In electronics, a comparator is a device that compares two voltages or currents and outputs a digital signal indicating which is larger.
The Projection Blink Comparator (PROBLICOM), invented by amateur astronomer Ben Mayer, was a low-cost version of the professional tool.
The first commercial comparator was a creation of James Hartness and Russell W. Porter.
He defended his doctoral thesis in University of Turku in 1933 on the metrology of length by means of the Väisälä comparator.