Interviewed in the 1960s, Calder listed the variable elements of his sculpture: ‘You have weight, form, size, colour, motion and then you have noise.’ All of these elements had been brought together in Small Sphere and Heavy Sphere, made in 1932/3, a work that could be classified as sculpture, installation, musical instrument and performance.

The red sphere is pushed so that the white sphere arbitrarily knocks against an arrangement of bottles, a box, a can, and a gong on the floor. Calder would allow viewers to organise and reorganise these objects as they wished (for conservation reasons this is no longer possible). They were chosen for the range of different sounds that they could produce, resulting in what Calder described as a series of ‘thuds, crashes etc.’ The different weights of the spheres mean that the pattern of movement can not be predicted, so that a unique aural composition is generated each time the work is set into action.

Underlying the design is the precise sense of equilibrium that is fundamental to Calder’s sculpture. This work also proposes affinities with some of the avant-garde composers that Calder befriended throughout his life, from the percussive scores of Edgard Varèse to John Cage, whose experiments with chance were influenced by Calder.