Pencil and crayon on J. Green’s mould-made paper watermarked ‘JG’, 793 x 714 mm (31 1/4 x 28 ¼ in)
Inscribed in pencil ‘Kenneth Martin’ bottom right; on back trial screenprint, inscribed by the artist in pencil ‘NOT THIS SIDE’ bottom right
Purchased from the Angela Flowers Gallery Ltd., London (Knapping Fund) 1972
British Drawings 1952-1972, Angela Flowers Gallery, London, November-December 1972 (81, as for Screw Mobile)
Tate Gallery Report 1972-4, London 1975, p.193 (reproduced)
Drawing for Screw Mobile is a late example of one stage of the process of making a mobile that Kenneth Martin had maintained since the early 1950s. Indeed, the form projected in this drawing resembles that of Small Screw Mobile of 1953 (Tate T00552), although with more bars increasing to a greater breadth. The artist told the Tate in 1974: ‘I have made it [the mobile] of brass and it is unfinished.’ The dimensions make it likely to be identifiable with Screw Mobile, 1973 (Estate of Kenneth and Mary Martin).
The drawing, which is full-scale, has the quality of a technical or engineering drawing. It was the subject of a detailed discussion in Tate Gallery Report 1972-4 which benefited from the artist’s own comments. As well as preliminary sketches - including (at the centre) a curve generated by twenty-five radii which was not used for the mobile - there are two main diagrams. That in the upper right produced five ellipses by drawing within a string looped around two pins - different ellipses being achieved by slightly changing the pins’ spacing. Subsequent lines radiating from the pin-points were measured for translation to the second diagram. In the top left quadrant of this second (and larger) diagram, these measurements were aligned horizontally to produce extended curves, each outlined in a different colour for ease of identification. Martin chose the shallowest, in effect ‘plotting a roulette which would come close to being rectilinear without ceasing to be a curve’. In explaining the process, he stated: ‘The curve is produced by a focus of an ellipse which rolls along the vertical centre of the drawing’. This is compatible with the mathematical definition of a ‘roulette’ (essentially the path of a rolling point) also used for Small Screw Mobile. The shallow curve was reflected in both directions in order to achieve the final form; ‘then’, the artist continued, ‘the central axis was tilted to go from a point at the top of one side of the curve to the bottom of the other side. Then lines were drawn at right angles to the centre line at 1/2 inch intervals ... these were then marked off to give the 1/4 inch bars and spaces.’ The bars were shades and given right-angled ends.
The figures annotated on the drawing gave either the numerical sequence of lines (in the case of the ellipses) or their length (in the case of the horizontals) which allowed for the calculation of the length of brass rod required for the mobile (61 feet or approximately 18.6 metres). There were to be fifty-eight bars of brass (not fifty-six as stated in Tate Gallery Report) each 1/4 inch high and 1/8 inch thick (6 x 3mm), with 1/4 inch spaces. According to the Tate Gallery Report the central rod was to be 61 inches long and of 1/4 inch diameter (155 x 6 mm); with the structure occupying 28 1/2 inches (724 mm) in height. The numbers on the right side of the largest diagram are measurements from the horizontal lines and relate to the lengths. They differ, with the top sequence opening: 1 1/4, 1 1/2, 2, 2 3/4, 3 1/2, and the bottom sequence opening: 1, 1 1/2, 2 1/4, 3, 3 3/4. Although they culminate in a pair of 23 inch bars at the centre, if transferred faithfully to the mobile this would allow for a slight weighting of the lower half. As the earlier Tate entry noted that ‘when the drawing was complete it was traced onto a sheet of asbestos upon which the pieces of brass were braised [sic] to the central rod’, it may be presumed that this was the case.
Although Martin often used graph paper for the accurate plotting of his structures, Drawing for Screw Mobile was made on the reverse of a screenprint proof. He recalled that it was one of several sheets which ‘Chris Prater had used to try out some different coloured lines’. These are linked to the contemporary Chance and Order group of drawings, paintings and prints, although as trials they do not relate to any eventual prints.
 Kenneth Martin, letter to Tate, 14 May 1974, Tate Gallery cataloguing files
 Reproduced in Kenneth and Mary Martin, exhibition catalogue, Annely Juda Fine Art, London 1987, no.77, p.93, 1600 x 584 mm (63 x 23 in).
 Tate Gallery Report, p.193.
 Letter, 14 May 1974.