The ABC-M1A1 RADIAC Calculator is used by the US Army to determine the dose rates and doses to personnel after a nuclear explosion. It is the military version of the "Radiation Dosage Calculator" developed by William Orr.
If the exposure rate (rads/hr) is known at a given time after an explosion, the calculator predicts the exposure rate at any other time. It also estimates the dose to personnel who are in the area at specified periods of time after the explosion. The back of the slide rule has the (none too clear) instructions.
Size: 4.5" diameter
References
Instruction card, no date.
U.S. Army Chemical School Lesson 2 and Lesson 3 of "Calculate and Compute Nuclear Data" Subcourse CM2308.