4 i2 HIE POPULAR SCIENCE MONTHLY. 



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 cir- 

 cuit of the local battery, in which circuit the coil belonging to the in- 

 dicator 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 en- 

 feebled to give clear indications by direct action. They are also fre- 

 quently 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 ser- 

 vice has stimulated the industry of inventors to devise means for ob- 

 taining more rapid transmission. Hughes, about 1859, invented a sys- 

 tem which requires only one current to be sent for each letter, and 

 which, accordingly, sends messages in about a third of the time re- 

 quired by Morse's method. Hughes's machine also prints its messages 

 in Roman characters on a strip of paper. These advantages are, how- 

 ever, obtained at the expense of extreme complexity in the apparatus 

 employed. It is only fit for the use of skilled hands ; but it is exten- 

 sively 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 power- 

 ful clock-work, with a driving weight of about 120 lbs., and with a regu- 

 lator consisting of a vibrating spring (/) 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 en- 

 graved 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 mov- 

 able piece (v). 



The revolving arm composed of the pieces v' v is called the chariot. 



