MORSE'S TELEGRAPH.

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current, as the paper continues to advance without interruption. The lines actually employed are of two lengths, one being made as short as possible (-) and called a dot, the other being about three times as long (-) and called a dash. The opposing spring D restores the armature to its criginal position the moment the current

ceases.

Morse's key (Fig. 568) is simply a brass lever, mounted on a

line-wire L. When the key is up, the projections ab are in contact, and currents arriving by the line-wire pass by the wire R to the indicator or the relay. By keeping the key down for a longer or shorter time, a dash or a dot is produced at the station to which the signal is sent. The dash and dot are combined in different ways to indicate the different letters, as shown in the following scheme, which is now generally adopted both in Europe and America:

A space about equal to the length of a dash is left between two letters, and a space of about twice this length between two words.

In needle-telegraphs, the dot is represented by a deflection to the left, and the dash by a deflection to the right.

843. Relay.-Fig. 569 represents Morse's indicator in connection with what is called a relay; that is to say, an apparatus which, on receiving a feeble current from a distance, sends on a much stronger current from a battery on the spot. The key B being up, a current arriving by the line-wire passes through the key from c to a, thence

through another wire to the coil of the electro-magnet belonging to the relay, and through this coil to earth. The electro-magnet of the relay attracts an armature, the contact of which with the magnet completes the circuit of the local battery, in which circuit the coil belonging to the indicator is included. The armature of the indicator is thus compelled to follow the movements of the armature of the relay.

Relays are used when the currents which arrive are too much enfeebled to give clear indications by direct action. They are also frequently introduced at intermediate points in long lines which could not otherwise be worked through from end to end. The analogy of this use to change of horses on a long journey is the origin of the name. Relays are also frequently used in connection with alarums when these are large and powerful.

844. Hughes' Printing Telegraph.-The employment of Morse's alphabet requires on the average about three currents to be sent per letter. The extension of telegraphic service has stimulated the

HUGHES' PRINTING TELEGRAPH.

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industry of inventors to devise means for obtaining more rapid transmission. Hughes, about 1859, invented a system which requires only one current to be sent for each letter, and which, accordingly, sends messages in about a third of the time required by Morse's method. Hughes' machine also prints its messages in Roman characters on a strip of paper. These advantages are, however, obtained at the expense of extreme complexity in the apparatus employed. It is only fit for the use of skilled hands; but it is extensively employed on important lines of telegraph. We will proceed to indicate the fundamental arrangements of this marvellous piece of ingenuity.

Fig. 570 is a general view of the machine. It is propelled by powerful clock-work, with a driving-weight of about 120 lbs., and with a regulator consisting of a vibrating spring acting upon a 'scape-wheel. A travelling weight on the spring can be moved towards either end to regulate the quickness of the vibrations. The clock-work drives three shafts or axes: (1.) the type-shaft, so called because it carries at its extremity the type-wheel T, which has the letters of the alphabet engraved in relief on its circumference at equal distances, except that a blank space occurs at one place instead of a letter; (2.) the printing-shaft, which turns much faster than the type-shaft, making sometimes 700 revolutions per minute, and carrying the fly-wheel V. These two axes are horizontal, and are separately represented in Fig. 571; (3.) a vertical shaft a, having the same velocity as the type-wheel, which drives it by means of bevel-wheels,

This vertical shaft consists of two metallic portions, insulated from each other by an ivory connecting-piece. In the position represented in Fig. 571, these two metallic parts are electrically connected by means of the screw V, but they will be disconnected by raising the movable piece v.

The revolving arm composed of the pieces v' v is called the chariot. It revolves with the vertical shaft, and travels over a disc D pierced with as many holes as there are letters on the type-wheel, these holes being ranged in a circle round the base of the shaft, and at such a distance from the shaft that the extremity of the chariot passes exactly over them. In these holes are the upper ends of a set of pins g, which are raised by putting down a set of keys BN resembling those of a piano. When the chariot passes over a pin which is thus raised, the piece v is lifted away from v', and the current from the battery, which previously passed from the pin through v and v

HUGHES' PRINTING TELEGRAPH.

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to the earth, is now cut off from v', and passes through v to the electro-magnet, and thence to the line-wire.

This is the process for sending signals. We will now explain how a current thus sent causes a letter to be printed by the type wheels at both the sending and receiving stations, the sending and receiving instruments being precisely alike.

The current traverses the coils of an electro-magnet E (Fig. 570), beneath which is a permanent steel horse-shoe magnet, having its

poles in contact with the soft-iron cores of the electro-magnet. When no current is passing, the influence of the steel renders these cores temporary magnets, and enables them to hold the movable armature p against the force of an opposing spring. The current is in such a direction that it tends to reverse the magnetism induced by the steel. It is not necessary, however, that it should be strong enough to produce an actual reversal, but merely that it should weaken the induced magnetism of the cores sufficiently to enable the opposing spring to overpower them. This is one of the most original parts of Hughes' apparatus, and is a main cause of its extreme sensibility.