HISTORY AND PROGRESS. 45 



circulated in one or other of the escapement magnets, the 

 armature was held down on one side, and the index prevented 

 from moving farther round the dial. 



45. An improvement in the apparatus was made by dis- 

 pensing with one of the line wires, as well as one of the 

 contact springs, of the sending commutator, and one of the 

 electro-magnets of the indicator. This was a material step 

 in the right direction, and fulfilled the first condition of a 

 successful telegraph that of requiring only a single line 

 wire. 



In the improved indicator the duties of the one electro- 

 magnet were fulfilled by a spiral spring with an adjusting 

 screw for tightening or loosening it. This spring acted in 

 the contrary direction to the single electro-magnet, but, of 

 course, with inferior force. It had tension enough, however, 

 to separate the armature from the poles of the electro- 

 magnet, and to bring over the beam bearing the escapement, 

 whenever the current in the electro-magnet was interrupted. 



46. But the most important improvement introduced into 

 the construction of this apparatus was in the substitution 

 of magneto- electric currents for those of a voltaic battery. 



The sending apparatus, so modified, is shown in Fig. 22. 

 It consisted of a permanent horse-shoe magnet, or combina- 

 tion of magnets, fixed to the base board B, between the poles 

 of which was placed a vertical shaft, supporting, on opposite 

 sides, the coils c c of an electro-magnet. They were so 

 arranged that, on turning the shaft, the cores of c c at the 

 same moment approached, and left the poles of the perma- 

 nent magnet. The ends of the coils of wire round the 

 electro-magnet were connected,, by means of a sliding contact 

 underneath, with the terminal screws e and /. On the top 

 of the shaft was a pinion D, locking into the tooth- 

 wheel w. The number of teeth of the wheel w were in 

 relation to those of the pinion D, so that one complete revo- 

 lution of w would cause D to revolve half as many times as 

 there were letters, numerals, &c., engraved on the correspond- 

 ing dials of the sending and receiving instruments. It will 

 be evident, without further explanation, that a half-revolu- 



