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THE POPULAR SCIENCE MONTHLY. 



It revolves with the vertical shaft, and travels over a disk (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 dis- 

 tance 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 (B N) resembling those of a 

 piano. When the chariot passes over a pin which is thus raised, the 

 piece v is lifted away from v' t and the current from the battery, which 

 previously passed from the pin through v and v' to the earth, is now 

 cut off from v', and passes through v to the electro-magnet, and thence 

 to the line-wire. 



Fig. 14. 



Type-shaft and Printing-shaft. 



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 in- 

 struments being precisely alike. 



The current traverses the coils of an electro-magnet (E, Fig. 13), be- 

 neath which is a permanent steel horseshoe 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 tem- 

 porary 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 pro- 



