such that their total resulting eccentric moment will be low or substantially zero when the shaft 10 is stationary or rotating at low speed utilizing the interplay between the eccentric masses described above it is possible to select three different vibrational amplitude positions for the present apparatus b the configuration in which the mass 19 is displaced to the limit of its travel fig3 and c the configuration in which the mass 19 locked together with the electromagnet 16 is displaced to the limit of its travel fig4 configuration a is obtained when shaft 10 is stationary or rotating at low speed as explained previously this configuration preferably has substantially zero eccentricity which is an advantage when starting rotation of the shaft configuration b is obtained when the electromagnet 16 is not energized and when the rotation of the shaft 10 exceeds the shaft speed at which the centrifugal force acting on the mass 19 overcomes the force of the springs 26 27 this produces the vibrational characteristics of low amplitude and high frequency configuration c results when the electromagnet 16 is energized and consequently attracts the mass 19 since the eccentric moment of the mass 19 is greater than that of the electromagnet the mass 19 pulls the magnet along with it under centrifugal force when the rotational speed of the shaft 10 is such that the centrifugal force on the combined eccentric mass i e the electromagnet 16 and the eccentric mass 19 is greater than the force of the springs 26 27 the composite mass is flung radially outwards against the wall of the casing 12 into the position shown in fig4 since the composite mass is greater than the eccentric mass 19 the composite mass is flung outwards under centrifugal force at a lower.