describes the u201c apex angle u201d directly between the inner points of intersection of the side wall 13 and the surface line of the depression the greater angle 16 b about 20 u00b0 indicates the apex angle between the outer points of intersection of the side wall and the surface line of the projections the angle u03b1 16 a in fig4 which describes the angular range of the depressions in terms of their angular extent is in any event less than the angle u03b2 which describes the angular range of the angular extent of the projections preferably a ratio of u03b1 to u03b2 is about 1 to 2 in the illustrated embodiment the depth of the depressions 8 is chosen such that the ratio of the aforesaid diameter d of the outer envelope circle 10 to the diameter d of the inner envelope circle 9 in the illustrated embodiment is about 1 15 in the illustrated embodiment eight depressions 8 and eight projections 7 are provided with the depressions 8 and the projections 7 being evenly distributed over the periphery of the outer surface 5 respectively at a dividing angle of 45 u00b0 in concrete terms the diameter d of the outer envelope curve 10 in the illustrated embodiment is about 6 25 mil equivalent to about 0 1587 mm while the diameter d of the inner envelope curve 9 is about 5 5 mil equivalent to about 0 1397 mm for the illustrated embodiment this produces a circumference of about 0 60 mm to about 0 65 mm and a cross sectional area of about 0 015 mm 2 to about 0 20 mm 2 so that the ratio of the.