MAGNETISM 



149 



The other method depends upon an effect of electric 

 currents, which may be shown by an experiment. 



Experiment 101. Pass a wire ab through a card, as in Fig. 

 107, and sprinkle iron filings on the card. Now send a strong 

 current through the wire. Note any change in the filings; without 

 disturbing the card, study their positions closely; stop the cur- 

 rent, watching the 

 filings carefully. 

 Place a small com- 

 pass c on the card 

 and note any sign 

 of a force acting. 



i 



I 



FIG. 107 



This experi- 

 ment shows that 

 while a current is 

 passing through 

 a wire, the latter 

 is surrounded by a field of force. If the wire is covered 

 by an insulator and is coiled around a bar of iron or 

 steel, this bar will be magnetized when a current is 

 passed through the wire coil. 



As a general rule, a piece of soft iron remains magnet- 

 ized only so long as it is being acted upon by the force 

 in the magnetic field ; a piece of steel, however, remains 

 a magnet for a long time after it is removed from the 

 field. 



173. Electro-Magnets. A piece of soft iron wound 

 about with a coil of insulated wire is called an electro- 

 magnet. When a current flows through the wire it 

 affects the particles of the iron in such a way as to make 

 the whole bar a magnet. The intensity of such a magnet 



