ensuring smooth application as to a specific combination with the hard material material for the rear ball the use of silicone rubber as the elastic material and any material selected from a group consisting of cemented carbide high density polyethylene polypropylene polyoxymethylene and tetrafluoroethylene resin as the hard material allowed in almost all the cases both the front and the rear balls to rotate without slippage thereby ensuring smooth application whether the balls rotate or not in each case would depend on a combination of the materials for the three factors the two balls and the writing is surface or the materials for four factors the three factors and also the ball seat though its mechanism has not been fully cleared a certain level of consideration can be provided presumably the reason why the front ball and the rear ball rotated almost certainly without slippage when silicone rubber was used as the elastic material is that silicone rubber used for the front ball has a low hardness hardness of 65 and has a large hardness difference from that of the hard material used for the rear ball further the use of ethylene propylene rubber having a hardness of 60 as the elastic material allowed both the front and the rear balls to rotate almost certainly without slippage by the above mentioned experiments the result in which both the front and the rear balls rotated without slippage though uncertainly was obtained when the front ball had a hardness below the hardness of urethane.