The Magnetostriction Receivers (Figures 227, 228). This two receivers make use of the magnetostriction effect. They consist of packs of nickel laminations in which a permanent magnetic field is introduced by flashing them, and a winding consisting of a few turns of robust insulated wire. The demensions of the laminations are chosen so that they have a mechanical resonance at 22.5 kilocycles per second. The vertically focussed receiver consists of roughly rectangular laminations, (figure 228), one wave length long, assembled together to form a pack having a square cross section. The top side of the pack it the receiving surface, while the other sides are screened by wrapping them up in a sheet of closed-cell sponge rubber, two millimeters thick, held in place by rubber bands. The small air bubbles in the rubber reflect and absorb the sound. The nickel pack, together with its windings and rubber screening, is held in place by casting them in pa-raffin wax. The exposed surface of the pack lies in the bottom of a funnel-shaped re-flector, also made from closed-cell sponge rubber. The orifice diameter and internal angle of the reflector are carefully chosen to give the directional characteristics shon in figures 225 and 226.

The horizontally  rocussed receiber (see figure 228) is made from ring-shaped lamina-tions pierced with 18 holes, allowing a toroidal winding. It is mounted round the base of the funnel-shaped reflector, and the lower side of the latter, together with the upper surface of the buoy, act as a ring-shaped reflector, and give the receiver the directional characteristics shown in figures 225 and 226.

The two receivers and their cable gland are assembled together in a single unit which could be mounted on the top of a float or buoyant mine as required. A drawing of this assembly is shon in Fig. 227.

Figure 225

Figure 226

Figure 227

Figure 228