Screw Mobile

Screw Mobile (Unlimited) 1969/70 was conceived as a development of Small Screw Mobile, 1953 in the Tate Collection.

Various versions were considered between 1969 and 1973, by increasing or decreasing the spaces between the bars, the dimensions of the bars - thickness, width and length - and the diameter and length of the central rod. The first version that Kenneth Martin (hereafter KM) made used similar elements to that of the Tate version – a central rod with a number of horizontal bars silver-soldered, or brazed, at regular intervals to complete two revolutions from top to bottom. Small Screw Mobile, 1953, (hereafter SSM1953), has 33 bars, whose dimensions are determined by the width of the brass strip, and which are brazed on to the central rod, with spaces between also equal to the width of the bar.

Screw Mobile 1969/70 (SM1969/70) has 37 bars, each with a thickness of 1/8 in. (c.3mm) and which has spaces between the bars of 1/8 in. (c.3mm), equal to the thickness. The width of the bar is 3/8 in.(9mm) while the lengths vary from 1 7/16 in. (36mm) top and bottom, to 11 ¼ in. (286mm) at the centre. The variation in the length of the bars is determined by radii emanating from a point in an ellipse on its major, or long, axis meeting the circumference.

Although the whereabouts of the drawing for this screw mobile is unknown, it almost certainly followed that of known drawings of similar, closely related screw mobiles, including SSM 19531. The drawing in the ellipse begins (in other drawings) with the first radius from the point from which the ellipse is partially constructed on the major, or long, axis. Radii set at 10 degrees are then drawn successively outwards to the circumference from the predetermined point on the axis. The increasing lengths of the radii become the lengths of the bars. Theoretically for SM1969/70 the major axis may have measured 12 11/16 in. (322mm): the shortest radius + the longest radius, or the length of the top (or bottom) bar + the middle bar - which can be seen to be the longest. As there are many methods for constructing ellipses with combinations of major and minor axes, and it is not known which method was used, the shape of the ellipse cannot be determined exactly. Accounting for the lengths of the bars of the SM1969/70 is thus conjectural. The physical measurements of the bars do not otherwise correspond with any other known system or sequence that KM is known to have used, such as the Fibonacci series. KM used the shape of ellipses in paintings and linkage constructions from the early 1950s2. As confirmation of an ellipse being the construction method for this screw mobile, when SM1969/70 is viewed directly from above, or from underneath, the profile (or plan) of the extremities of the bars is that of an ellipse, not a circle, as might be expected.

In SM1969/70 there is also the matter of placement and displacement of the horizontal bars on the central rod. One end of each of the top and bottom bars terminates precisely at the centre of the tangent to the vertical rod. The next bar is moved sideways so that the end of the bar is in line with the right-angled tangent to the rod. The next bar projects beyond the rod by half the thickness of the rod and so on, until the centre of the middle bar, No.19, is placed exactly on the vertical centre of the central rod.

When the first version was made, each angle for each successive bar was set up and brazed in position. The work was then rotated 10 degrees and reset up for brazing, and so on. At the same time, as the bars became longer, the work had to be gradually lifted higher off the table and supported accordingly. It was an exacting and time-consuming process. As the work progressed, the bars already completed were in the way of the new bars and the brazing became progressively more difficult to execute. One complete revolution of the bars was thus achieved by the 37 bars set at 10 degrees, the top and bottom bars being set directly above each other, overlapping in plan, ie. 360º. While at first looking it may appear that there is a revolution and a half of the bars – they appear from the side to look as though they are set 180º to each other – the effect is achieved because the bars also ‘travel’ across the central rod as they move down the rod, moving from left to right.

On completion of the making in 1969/70 there was excess silver-solder at the joints where it had been difficult to access the centre, because the bars were in the way of each other. The excess had to be carefully filed away (with needle files), then re-surfaced in the direction of the extrusion marks on the bars so that the filings and finishing were no longer to be seen. The joints needed to look perfect and unobtrusive so that there was no interruption to the ordering of the bars around the rod. Then the whole SM was polished with successively finer grades of emery paper until there was a highly reflective, smooth surface. It has to be remembered that each bar has six separate surfaces: two long faces (front and back), two ends and a top and a bottom of each bar – a total of 222 surfaces - and that there is only 1/8 in. (c.3mm) between each bar.

Several versions in brass were made: one was nickel plated, one was chromium plated and a small number were stove lacquered with a clear finish. Those in brass that Kenneth retained were hung up in the studio by nylon fishing twine. They rotated in intermittent currents of air, clock and counter-clockwise, aided by a tiny fishing swivel to facilitate the movement. The final screw mobile that eventually became the prototype for the multiple was made by the same principle, but by a slightly different method. It was made from brass and then chromium plated. The Unlimited Series (between c.50 and 100, confirmed by the Martin Estate) was made by engineers from Rotork, an engineering company in Bath, Somerset, owned by Jeremy Fry who was interested in making art-works available to the general public in the same way that books and records were.

In the accompanying drawings for a possible solution to the ellipse construction for SM1969/70, the overall length of the major axis has been taken as 12 inches in length. The first marked off points have been selected as 1/8 of the length from each extremity as KM often used the thickness of commercially available brass to determine further measurements and proportions in a work.

Beautifully simple to look at yet infinitely elegant, rotating gently and effortlessly in the breeze while light ripples slowly up and down the reflecting surfaces of the bars: Screw Mobile 1970 holds one’s attention, inviting one to participate in its timeless journey.

'The spiral gives no return, the ellipse does' (Kenneth Martin, unpublished notes, exhibition catalogue, Tate, London, 1975, p. 27).

We are very grateful to Dr Susan Tebby for preparing this catalogue entry.