C. Wilhelm Wessman caliper, patent Oct 7 1884

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An improved Caliper that almost no one cared about. A 140 years ago C. Wilhelm Wessman of Chicago, IL. had what he thought was a better idea for a sliding caliper that would make it easier to read both outside and inside measurements. On Oct. 7 1884 (#306,123) he was issued a patent that had all kinds of complicated features. It would have looked really cool if it were made that way but was probably way too expensive. The production version looks almost like a regular caliper as we know it today but look closely. The simplest part of his idea was to read the inside and the outside measurement from the same scale lined up with the same line. Most calipers of the day had separate scales or multiple pointer lines. On Wilhelm’s caliper the “fixed jaw” was not so fixed, it slid back and forth to compensate for the thickness of the jaws when taking an inside measurement. To keep the “fixed jaw” lining up correctly there are adjustable stops inside the bar.

The photo clearly shows the caliper set for 1”, both inside and outside. It sounds simple, always read any measurement from the same place, but the tool buyers of the day certainly didn’t think so. This is based are how few surviving examples exist, less than you can count on one hand. When Ken Cope wrote his book on machinist tools he knew of only one.

So thank you Mr. Wessman for at least entertaining our minds after almost 140 years since your neat idea.