1. The possibility of fixing the skin elements on the spheres at great height.
2. The covering had to be rain-proof.
3. The exterior aspect had to conform with what the Atomium was supposed to represent.

There were very few similar constructions from which experience could be drawn for the covering of the Atomium. The construction to which it could best be compared was the Dome of Discovery at the London Exhibition in 1951.

The experience provided by the English engineers gave us two starting points for the choice of our solution: riveting the sheets did not make the skin suffuciently rainproof, and the connecting rod system had to be used, the more so as the framework of the Atomium was of steel, which meant taking into account the different coefficients of expansion of the aluminium skin and the framework of the sphere.

Spherical triangles, the three sides of which are great circles of the sphere, have the advantage of providing uniform radiuses for all the elements.

An added point was that it was to be possible at night, to light up the Atomium by circulating luminous spots along the 9 great circles of each sphere. With these points for a start, the Enghien St. Eloi Co. Ltd built a system of aluminium panels out of 12/10 mm thick sheet, the edges of which are mounted on a special welded aluminium frame.

Each of the 48 great triangles is made up of 15 of these small panels which are themselves spherical triangles. The 9 great circles which separate the 48 great triangles are 16-inch wide ribbon sheets, in rectangular sections.

All these panels are bolted together from inside and their intersection made rainproof by a plastic joint and a second one in rubber.

The junction points at the summit of the triangles are supported by circular plates which are attached to the secondary framework by means of a connecting rod made flexible by a sort of rubber Cardan.

The choice of disposition of the 15 secondary triangles was conditioned by the fact that, in certain spheres of the Atomium, large plexiglass windows had to be put in place of the aluminium panels.

The aluminium of the spheres is a very special metal. It is an alloy called "Peraluman 15" overlaid, by rolling, with a thin sheet of aluminium called "reflectal", which can be nearly specularly polished by a special process.

The exterior night-lighting is made by placing circular light-points at regular intervals (about 5 ft) along the nine great circles of each sphere. By means of rotary switch. gear, the lamps come on and off successivly; this gives the impression of luminous spots turning around the spheres at a certain speed. These luminous spots meet at the intersection of the great circles and thus gives the spectator an impression of pulsating light at different points of the sphere. The idea of this lighting is to represent the rotation of the electrons around each atom of the iron crystal.