A simple method of changing the direction of the current in a two-instrument circuit is shown diagrammatically in Fig. 57. The _principle_ is used in the Wheatstone needle instrument. The battery terminals at each station are attached to two brass plates, A B, A^1 B^1. Crossing these at right angles (under A A^1 and over B B^1) are the flat brass springs, L R, L^1 R^1, having buttons at their lower ends, and fixed at their upper ends to baseboards. When at rest they all press upwards against the plates A and A^1 respectively. R and L^1 are connected with the line circuit, in which are the coils of dials 1 and 2, one at each station. L and R^1 are connected with the earth-plates E E^1. An operator at station 1 depresses R so as to touch B. Current now flows from the battery to B, thence through R to the line circuit, round the coils of both dials through L^1 A^1 and R to earth-plate E^1, through the earth to E, and then back to the battery through L and A. The needles assume the position shown. To reverse the current the operator allows R to rise into contact with A, and depresses L to touch B. The course can be traced out easily. In the Wheatstone "drop-handle" instrument (Fig. 54) the commutator may be described as an insulated core on which are two short lengths of brass tubing. One of these has rubbing against it a spring connected with the + terminal of the battery; the other has similar communication with the - terminal. Projecting from each tube is a spike, and rising from the baseboard are four upright brass strips not quite touching the commutator. Those on one side lead to the line circuit, those on the other to the earth-plate. When the handle is turned one way, the spikes touch the forward line strip and the rear earth strip, and _vice versa_ when moved in the opposite direction. SOUNDING INSTRUMENTS. Sometimes little brass strips are attached to the dial plate of a needle instrument for the needle to strike against. As these give different notes, the operator can comprehend the message by ear alone. But the most widely used sounding instrument is the Morse sounder, named after its inventor. For this a reversible current is not needed. The receiver is merely an electro-magnet (connected with the line circuit and an earth-plate) which, when a current passes, attracts a little iron bar attached to the middle of a pivoted lever. The free end of the lever works between two stops. Every time the circuit is closed by the transmitting key at the sending station the lever flies down against the lower stop, to rise again when the circuit is broken. The duration of its stay decides whether a "long" or "short" is meant. TELEGRAPHIC RELAYS. [Illustration: FIG. 58.--Section of a telegraph wire insulator on its arm. The shaded circle is the line wire, the two blank circles indicate the wire which ties the line wire to the insulator.]