The Comparator

We bought this machine on eBay from The Radio Guy, Steve Erenberg, who finds some wonderful things and offers them on his web site and on eBay.

The machine is very well made: all of the motions are very precise. The engraved tag says:

As you rotate the silver dial on the side, the slides (visible from the top) move back and forth. The motion is cosine-like, in that the closer the slide is from the center, the more displacement there is for a given amount of turning of the input dial. The dials are labelled "Observations" and calibrated from + 500 yeards to -500 yeards. The slides can be set in different horizontal positions, against a scale that runs from Left 500 years to Right 500 yards. The slides themselves have major and minor markings, but no calibrations. The circular glass plate on the top has a reticle line running the diameter. The glass rotates smoothly as the little know turns. There is a scale, again from -500 yards to +500 yards.