One of the simplest forms of telegraph is the single-needle instrument. This consists of a magnetic needle fixed to a spindle at the back of an upright board through which the spindle is passed. On the same spindle, but in front of the board, is fixed a dial needle, which, of course, moves along with the magnetic needle. A coil of wire is passed round the magnetic needle, and connected to a commutator for reversing the direction of the current. By turning a handle to the left a current is made to flow through the coil, and the magnetic needle moves to one side; but if the handle is turned to the right the current flows through the coil in the opposite direction, and the needle moves to the other side. Instead of a handle, two keys may be used, the movement of the needle varying according to which key is pressed. A good operator can transmit at the rate of about twenty words a minute with this instrument. The Morse code, which consists of combinations of dots and dashes, is used, a movement of the dial needle to the left meaning a dot, and one to the right a dash. The code as used in the single-needle instrument is shown in Fig. 27. [Illustration: FIG. 27.--Code for Single-Needle Telegraph.] Needle instruments are largely used in railway signal cabins, but for general telegraphic work an instrument called the Morse sounder is employed. This consists of an electro-magnet which, when a current is passed through it, attracts a small piece of iron fixed to one end of a pivoted lever. The other end of this lever moves between two stops. At the transmitting station the operator closes a battery circuit by pressing a key, when the electro-magnet of the sounder at the receiving station attracts the iron, and the lever flies from one stop to the other with a sharp click, returning again as soon as the circuit is broken. A dot is signalled when the lever falls back immediately after the click, and a dash when it makes a short stay before returning. Fig. 28 shows the code of signals for the Morse telegraph. [Illustration: FIG. 28.--The Morse Code.] In passing through a very long wire an electric current becomes greatly reduced in strength owing to the resistance of the wire. If two telegraph stations are a great distance apart the energy of the current thus may be unequal to the task of making the electro-magnet move the lever of the sounder so as to produce a click, but this difficulty is overcome by the use of an ingenious arrangement called a “relay.” It consists of a very small electro-magnet which attracts a light bar, the movement of the bar being made to close the circuit of another battery at the receiving station. The feeble current works the relay, and the current in the local circuit operates the sounder.