The stainless steel test sphere, a cloud chamber used to study static microbial aerosols, is a four-story high, 131-ton structure.

Its 1-inch-thick (25 mm), carbon steel hull was designed to withstand the internal detonation of "hot" biological bombs without risk to outsiders.

The device was designed to allow exposure of animals and humans to carefully controlled numbers of organisms by an aerosol (inhalational) route.

Human volunteers breathed metered aerosols of Q fever or tularemia organisms through ports along the perimeter of the sphere.

Herbert G. Tanner, the head of Camp (now Fort) Detrick's Munitions Division, had envisioned an enclosed environment where biological tests could be conducted on site, rather than at remote places like Dugway Proving Ground, Utah and Horn Island, Mississippi.