VIBRATION DAMPERS This invention relates to vibration dampers for suspended elongate objects such as cables. In the context of this specification the term cables will be used generally and should be interpreted 5 as meaning elongate objects in general and including, among other things, power cables, optical cables, ropes, and wires. The present invention is particularly, though not exclusively, usable for suspended optical cables. Suspended elongate objects such as cables are liable to vibrate under the action of the wind. 10 Such vibrations, if not damped, can become extremely destructive. A known type of vibration damper is the DulmisonTM Spiral Vibration Damper which comprises a polyvinyl chloride helically formed rod of cylindrical section. The vibration dampers are formed by extruding the polyvinyl chloride as a rod, wrapping the rod while still warm and pliable about a mandrel to form the helix and quenching the rod and mandrel in a water bath. The helical diameter of the 15 damper varies along its length to form a narrower helix and a wider helix. The narrower helix acts as a gripping section having a diameter selected to grip the cable to which it is attached by interwinding the cable and the helical rod. The wider helix acts as a damping section, to provide the action/reaction opposed to the cable vibration by mechanical interaction between damper and cable. Such dampers are available under Dulmison Inc. catalog numbers SVD 20 0441, SVD 0635, SVD 0830, SVD 1173, and SVD 1432 and come in lengths ranging from 1.23 metres( 48.5") to 1.68 metres (66") although the length may be varied to suit the vibrational characteristics of the cable concerned. Such vibration dampers were developed for use on metallic conductors and there have been 25 problems in using them on optical cables. Optical cables are frequently suspended from pylons carrying electrical cables. The optical cables are thus subjected to strong electromagnetic fields. As the optical cable is non-conductive this results in different electrical potentials along the length of the optical cable. The dampers exacerbate this problem and the varying electrical potential can result in discharges that damage the dampers or, worse, the optical cable. There 30 have been many efforts to reduce this problem, including adding conductive materials to the plastic materials from which these dampers are formed. For example, GB 2234830 discloses a rod of metal or metal alloy whose overall diameter over a part of its length increases smoothly. However, a potential problem is that the use of a cylindrical rod means that in high amplitude SUBSTITUTE SHEET (RULE 26) WO 98/53542 PCT/GB98/01361 2 vibration situations the damping section may strike the cable and such contact will be as tangential point contact where the inner diameter of the helix strikes the surface of the cable. The applicants have now realised that the problems can be solved by making the dampers of 5 metal strip as this will firstly provide a means to even out the varying potential and will provide line contact between the damping section and the cable in high amplitude vibration situations. WO 96/14176 describes methods and apparatus for manufacturing helical products from metal strip and provides a discussion of earlier methods of making such products. Such apparatus 0 may be used to form the dampers of the present invention by producing a helix that varies in diameter from fiat strip material. Further features of the invention will be apparent from the following description and the claims. 15 The invention is illustrated by way of example in the following with reference to the drawings in which: Fig. 1 is a part schematic view of a device in accordance with the invention; Fig. 2 is a part schematic view of apparatus as claimed in WO 96/14176 20 Fig. 3 is a graph illustrating the results of comparative damping tests. In Fig 1. region 1 is the gripping section and region 2 is the damping section of a spiral vibration damper. The spiral is formed from metal strip and typical, but not exclusive, ranges of dimensions are: 25 Typical Range Overall length 1 - 2 metres Grip length 0.2 - 0.4 metres Thickness of strip 1.2 - 15mm Width of strip 2.5 - 25mm Gripping section internal diameter 5 - 35mm Damping section internal diameter 15 - 50mm WO 98/53542 PCT/GB98/01361 3 It must be appreciated that the gripping section diameter must be chosen to match the diameter of the cable to be damped so as to be a reasonable grip. 5 The damper may be formed by using apparatus capable of making helixes and of selectively varying the diameter of the helix during production of a single article. Suitable apparatus is disclosed in WO 96/14176. The apparatus of WO 96/14176 is versatile and can be programmed as required and, as described in WO 96/14176, variation of the pitch and diameter of a helix being formed can be selectively varied. As shown in Fig 2 a pair of rollers or other 10 forming members 42A and 42B are used to bend and twist incoming strip material 26 to form a helix 46. By varying the angle and spacing of rollers 42A and 42B the pitch and diameter of the helix can be varied as required even during the formation of a helix so that switching from a helix of one diameter to a helical spiral of increased diameter is straightforward. By such means it is also possible to provide flared ends to the gripping section so that it does not dig 15 into the surface of the cable. This can be important for optical cables which are fragile in nature. Fig. 3 shows the results of comparative damping tests in which the Y-axis gives the bending amplitude in mm and the X-axis the frequency of vibration of a span of cable under various 20 driving conditions. A span of standard ADSS (All Dielectric Self Supporting) optical cable from PirelliTM having a diameter 14.6mm was placed under tension and vibrated at a range of frequencies. The amplitude of vibration was measured using a VIBRECTM vibration recorder which measured the bending amplitudes over a range of frequencies in 5Hz steps. The vibration frequency was automatically swept over the entire range with a uniform amount of 25 power applied to the span. Comparative tests were performed using an undamped cable, a cable damped with a Dulmison Inc SVD 1432 spiral damper, and a cable damped with a spiral damper in accordance with the present invention. Below is a comparison of the SVD 1432 spiral vibration damper and the above mentioned 30 vibration damper used in the comparative tests: 0 1 llnt-rlm rrr nt tr-r-r im~ ii r- A \ WO 98/53542 PCT/GB98/01361 4 SVD 1432 Current invention Overall length 1676mm 1675mm Grip length 330mm 330mm Thickness of material 19mm round section 2.5mm Width of material 9.5mm Gripping section internal 12.9mm 12.9mm diameter Damping section internal 25mm 26mm diameter Weight 0.79kg 0.2kg Material Polyvinyl chloride Aluminium alloy To fit conductor diameter 14.3 - 19.3mm 14.3 - 19.3mm In Fig..3 the symbol N indicates the results found for an undamped span of cable. The symbol O shows the results for a cable damped with the SVD 1432 spiral vibration 5 damper (made, as stated above, of polyvinyl chloride). It can be seen that the known SVD 1432 damper reduces the amplitude of vibration for frequencies from about 10Hz to about 40Hz. From 40Hz to 80Hz it only provides a small amount of damping and, effectively, is not doing its job of reducing vibration in this range. 0 The symbol @ shows the results of using a vibration damper according to the present invention and it can be seen that a considerably higher damping effect is achieved, and across a wider range of frequencies, than the conventional SVD 1432 spiral vibration damper. The present vibration damper gives effective damping from 10Hz to 80Hz. 15 Such a large difference in damping efficiency would not be expected given the broadly similar dimensions of the dampers. However the damper according to the present invention is less stiff than the conventional damper and this may provide the explanation for the improved efficiency of damping. Under vibration the damper according to the present invention appears to "come alive" and impact the cable along most of the length of the damping section. In contrast, the SUBSTITUTE SHEET (RULE 26) WO 98/53542 PCT/GB98/01361 5 stiffness of conventional spiral dampers means that they only impact the cable in their last third of the damping section. Its reduced stiffness, in combination with its lower weight, makes the damper of the present 5 invention easier to install than a conventional spiral damper of similar overall dimensions. The damper used in the above mentioned tests was made from a high tensile strength 6000 series aluminium alloy. The low weight of the resulting damper means that the damper is less likely than conventional dampers to migrate along the cable under vibration and so permits a less rigid gripping section to the damper. 0 The present invention is not limited to any particular metal and indeed specifically contemplates the use of steel for the material of the strip. SUBSTITUTE SHEET (RULE 26)