150 ELECTRICITY 



may be greatly increased by winding several layers 

 of the wire around the iron, like thread on a spool. 

 Of course the wire must be insulated, for if it were bare 

 the current would be conducted straight across the coil 

 without going through its many turns. 



Experiment 102. Wind a piece of soft iron (e.g. a cut nail) 

 with insulated wire, as in Fig. 108. Join one end of the wire to 

 a battery, holding the other end in the hand so as to close and 

 open the circuit at will. With the circuit open, touch one end of 

 the nail to a common tack. Can you lift the 

 tack in this way ? Now close the circuit and try 

 again. This device is a small electro-magnet. 

 What is needed in order that it may exert mag- 

 netic force ? Again opening the circuit, bring 

 the end of the nail down to within one sixteenth 

 of an inch from the tack lying on a desk ; close 

 tiie circuit, watching the tack. Lift the magnet 

 and tack a few inches, and open the circuit. How 

 long before the tack drops from the nail ? How 

 long a time is required for the soft iron nail to 

 become magnetized, and to lose its magnetism? 



Electro-magnets may be made very 



FIG. 108 r T i . 



powerful by increasing the number of 

 turns in the wire coil and the strength of the current. 

 Magnets of this sort are used in dynamos and electric 

 motors. The use of soft iron for the core of an electro- 

 magnet allows it to become magnetized almost instantly, 

 and it is demagnetized (loses its magnetism) when the 

 current ceases to flow ( 172). For this reason electro- 

 magnets are very useful in all electrical devices where 

 motion is to be produced at will ; for example, call 

 bells, motors, signals, telegraph systems, etc. 



