screw thread and the closures 170 172 are formed with a complementary external screw thread and a screwdriver slot is formed in their outward facing ends by inserting a screwdriver blade in the slot a closure can be turned so as to move further into or in a direction out of the passage so as to compress its spring to a greater or lesser extent as the armature rocks into the position shown in fig1 the ball 174 is engaged by the armature end 118 and will be urged inwardly thereby compressing or further compressing the spring 162 until the armature end engages the inner face of 134 in doing so energy is stored in the spring 162 which is available to help accelerate the armature in an opposite sense when it is induced to rock towards its other home position the permanent magnets 112 114 are selected so as to provide sufficient magnetic flux when the armature is in either of the two home positions to retain the armature in the last position to which it has moved and to more than overcome any spring force in the compressed spring 162 or 164 in the pole 134 or 136 to which it has moved this is achieved by virtue of the closed magnetic path formed by the armature the two poles it contacts 134.