The rolling sphere method enables designers to determine where air terminals or strike termination devices are required on a building. Assuming the electrically-charged field produces a lightning strike with a 150 ft. radius, the designer visualizes a sphere with that radius rolling over the surface of the building.
As seen in this video, lightning can strike the building any place the red sphere touches it. To prevent the sphere from touching the building, air terminals (shown in green) are installed to divert electrical charges through the lightning protection system into the ground.
The rolling sphere method is included in National Fire Protection Association document NFPA 780 — Standard for the Installation of Lightning Protection Systems for having performed successfully for more than two hundred years.