Over 300,000 can find room upon a square centimeter of surface.
One barn is equal to 10-24 square centimeter, which is approximately the cross-sectional area of a typical atomic nucleus.
The intensity of a magnetic flux is expressed by the number of magnetic lines of force in a square centimeter or square inch.
The practical limit of the magnetization of soft iron (called saturation) is 18,000 lines to the square centimeter.
This hole measures exactly one square centimeter in area, and is circular.