THE TELEGRAPH 



139 



should expect that a piece of soft iron placed inside a coil 

 of the wire would become a temporary magnet, just as 

 when it is in the field of a bar magnet (cf. 137). 

 Such magnets are called electromagnets. The 

 coil must be made of insulated wire, so that 

 the successive turns may not touch one an- 

 other. If they do touch, the current will not 

 pass through all the wire, but will be "short 

 circuited." The greater the number of turns in 

 the coil the stronger the magnetic field will be, 

 and the stronger the electromagnet. But as 

 soon as the circuit is broken, the electromagnet 

 is demagnetized. 



The coil of wire is called the helix; the iron is called 

 the core. A core made up of a bundle of small iron wires 

 makes a much stronger electromagnet than one in a single 

 piece. Telegraph instruments, telephone bells, electric 



doorbells, fire alarms, 



!j- JR. and many other de- 

 \ H vices are operated by 

 electromagnets. 



FIG. 128. 

 Iron placed 

 in the field 

 of a coil 

 carrying a 

 current 

 becomes a 

 magnet. 



Key 



FIG. 129. 



A Simple Telegraph Instrument. When the key is 

 pressed, the circuit is made complete, and the sound- 

 er is drawn down by its electromagnet. 



155. The Tele- 

 graph. The tele- 

 graph instrument 



(Fig. 129) consists essentially of a key for making and 

 breaking the circuit in an electromagnet, and thus pro- 

 ducing "clicks" in the sounder. The sounder is a small 

 lever (cf. 198) which is held by a spring against its upper 

 stop. Opposite the core of the electromagnet is a piece of 

 soft iron called the armature. When the key is pressed 



