Radium Exposure Calculator (late 1940s, 1950s)
A circular slide rule used in industrial radiography to calculate the optimal exposure time. Input parameters include the source (radium) activity in milligrams, film speed, film to source distance and the thickness of the steel being imaged.
It is identified as having been developed by the New York Naval Shipyard which was located along the East River in Brooklyn, N.Y. I suspect that the indicated date, 1948, refers to the patent and not necessarily the year of manufacture.
The center knob can be loosened or tightened—a nice feature that prevents an accidental misalignment during use.
See what the source might have looked like.
The back side of the calculator is used to determine the equivalent thickness of steel when X-raying other materials (e.g., aluminum, concrete).
Size: ca. 6" diameter