The spindle _p_ carries a leaden ball _t_, which is supported upon the rest _v_, and is secured in position by the screw nut _w_; _x_ is an india rubber ring, the object of which is to prevent any damage being done to the spindle should the ball when set in action by a heavy blow from a passing vessel be brought into contact with the dome; 2 is a brass disc attached to the spindle carrying an ebonite disc 4, connected to it by screws; 6 is a brass contact ring also fixed to the ebonite disc 4, provided with a screw 8, for the attachment of one of the base plug wires, and with platinised projections 3, 3, 3, Fig. 56. The contact ring 6 is completely insulated from the spindle and brass disc 2. Three contact springs 5, are attached to the circular ebonite piece _m_, and the faces opposite to the platinised projections of the disc 2 are also platinised. 7 shows the contact screws of the connecting pieces, which serve also as adjusting screws to regulate the sensitiveness of the apparatus, the points of which as well as their bearings on the springs are platinised. The springs are connected together by means of the wires 9, Fig. 55, one end of which is secured to the connecting piece by the screw 10, and the other passes through to the top of the ebonite piece, and is attached to the top of the spring next in succession to that to which it is fixed below. One terminal of a coil of 1000 ohms resistance (which is used for testing purposes) is attached to the line _L_, terminal of the ebonite base plug, which latter is also connected to the screw 8, on the circumference of the contact ring 6; the other terminal of the resistance coil is connected to the earth, _E_ terminal of the base plug. A bare copper wire of No. 16 B. W. G. connects the top of the last contact spring with the set screw _s_; a piece of similar wire jointed to it is passed round one of the brass collars and connected to the screw _r_. As a precaution against bad contact, the contact springs are connected together by bare wires _A_, _B_, _C_. This completes the connections for the signalling circuit, the earth being formed by the body of the instrument; _D_ is a hole left in the metal base for the passage of the insulating wire which connects the earth plate to the earth _E_ terminal of the base plug. _Testing Current._--For testing purposes the current from the test battery arrives by the line wire _L_, and passes thence through the resistance coil to earth by means of the wire _E_, which is attached to a zinc earth plate placed in a recess in the jacket of the circuit closer. _Action of the Circuit._--The action of the apparatus is as follows:--