MAGNETIC FIELD OF FORCE 39 



Magnets are at the present time ordinarily made by 

 electrical action and not by rubbing on other magnets. 

 Magnetized steel bars are called permanent magnets. 

 Electromagnets have become of almost inestimable use in 

 modern life. The telegraph, telephone, magnetic crane, 

 electric motor, and almost innumerable other mechanical 

 devices are dependent largely upon the principle of elec- 

 tromagnetism for their usefulness. 



19. The Magnetic Field of Force. Experiment 15. Place a 

 thin piece of cardboard about the size of a sheet of writing paper 

 above a bar magnet and carefully sprinkle iron filings over it. De- 

 scribe the behavior of the filings. Sketch on a piece of paper their 

 arrangement. Move a small compass about above the cardboard 

 and note the directions the needle assumes. How do the actions 

 of the needle and of the filings compare ? 



Holding the small compass two or three inches above the magnet 

 move it parallel with the magnet from end to end. Gently tap the 

 compass occasionally so that the needle will move freely. How does 

 the needle act when it is over the ends of the magnet? How does 

 the direction of the compass needle compare with the direction of the 

 bar magnet ? 



In the above experiment we found that when iron 

 filings were sprinkled above the magnet they arranged 

 themselves in definite lines. The small compass needle 

 also arranged itself along these lines when brought under 

 the influence of the magnet. There is, then, around a 

 magnet a magnetic field of force which affects magnets 

 and magnetic substances brought within it. It is found 

 that magnetic intensity, like the intensity of sound and 

 light, varies inversely as the square of the distance. 



When the compass was placed above the ends of the 

 bar magnet one of the ends of the needle was pulled down 

 toward the magnet, or it might be said to dip toward the 

 magnet. When moved near the middle of the magnet it 

 assumed a horizontal position, and when it approached the 



