CHAP. IV.] 



ELECTRIC TELEGRAPHY. 



589 



already stated, are absolutely synchronous. The electro-magnet E (Fig. 

 382) has a special arrangement. It is formed of two pieces of soft 

 iron about which the bobbins are coiled, and which are placed on the 

 poles of a permanent magnet. When no current is passing the 

 tongue of the lever p is attracted by the armatures of the electro- 

 magnet, and presses against them ; but as soon as the current passes, 

 since it acts in a contrary direction to the permanent magnetism, the 

 soft iron is demagnetised, the lever p yields to the action of a spring r, 

 and leaves the armatures. In this movement, the tongue raises a 

 lever / which in turn acts upon the catch of the immovable part of 



% FIG. 382.-- Directions of the currents in Hughes's telegraph. 



the cog shaft and this latter finally participates in the motion of the 

 other shafts, till after a complete revolution the catch is disengaged 

 and the shaft is stopped. 



Let us see then how this shaft causes the printing of the letter cor- 

 responding to the key depressed together with the transmission of 

 the current and other effects just described. 



The printing shaft carries a sharp cog p (Fig. 383) which at each 

 rotation conies against the tooth b of a lever ab and raises it ; this 

 lever thus forces the printing roll M to come in contact with the band 

 of paper against the inked letter of the type-wheel which passes at 



