Page:Popular Science Monthly Volume 3.djvu/426

 412 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.

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 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's 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. 13 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 (l) acting upon a 'scape-wheel. A travelling weight on the spring can be moved toward 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. 14; 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. 14, these two metallic pai-ts 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.