It was in fact the window problem that led to the choice of the secondary triangulation subdividing each of the 48 great spherical triangles consituting the covering of the sphere. The problem was solved in that where windows were wanted, the aluminium panels were replaced by frames in which plexiglass sheets having the same spherical radius were fixed.

Three spheres are fitted with this system of windows which are on both sides of the sphere's equator: the top sphere with its restaurant and the two lower spheres where the circuit for the visitors to the technical exhibition ends. The other spheres are lit by a few elliptical port holes which give emergency lighting, the main lighting being artificial.

It was unadvisable, in fact, to provide too many view points in the lower spheres, as this would have help up the flow of the visitors to the part containing the technical exhibits.

The spheres being closed, it was necessary to aircondition and ventilate them.

Two different systems have been adopted for the Atomium:

1) For the reception pavilion and the base sphere, the air conditioning is made by one machine situated outside the Atomium, the cold or warm air - according to necessity - being led in at floor-level by metal sheaths. Use has been made of the new system called "heat pump" which uses refrigeration machinery to send warm air by inverting the cycle.

2) For the other spheres, an independent conditioning system is used. Each sphere has six cupboards containing the refrigeration, heating and ventilating systems.