disc 15 this will tend to destroy the usefulness of the second gland this disadvantage may be eliminated by connecting the bellows to the inner margins of the seal plates rather than as shown in fig2 as a result an almost equal pressure balance is achieved on the plates with the outside of each plate 34 seeing 34 the intermediate pressure established between the glands however connecting the bellows in this alternative position requires more electrical power to achieve the intended plate motion referring to fig2 and 3 the various pairs of confronting plate projections 21p and 23p are incorporated in respective transducer assemblies 29 the typical assembly 29 includes a rectangular support 31 which is fitted between rectangular piezoelectric crystals or transducer elements 33 and 35 the crystals are chosen to respond to suitable electric excitation by repeatedly expanding and contracting in the direction of the axis of the shaft that is they are polarized so that they respond to an oscillatory excitation current by vibrating in the thickness mode as shown in fig3 the crystals 33 and 35 are in respective contact with the flat faces of plate projections 21p and 23p a ring 37 clamps these various elements together maintaining.