44 LABORATORY EXERCISES 



text. Use an iron "cut" nail if possible; if you use a wire nail, 

 you may need to heat it red hot and then to let it cool slowly. 

 While the nail is near the magnet, bring it near small tacks or 

 iron filings. What is the result? Now hold the magnet farther 

 away from the nail, and note what happens. 



e. Place a horseshoe magnet under the middle of a smooth 

 piece of writing paper (or a pane of glass) and sprinkle some iron 

 filings over the paper. Tap the paper gently, and see how the 

 filings arrange themselves. Draw a sketch showing this 

 arrangement. Compare it with the arrangement in the case of 

 a bar magnet, as shown in the text, Fig. 114, 138. Is the 

 influence of a magnet cut off by paper or glass? 



EXERCISE 43 

 ELECTRIC CHARGES 



Apparatus and Materials. Glass rod or a slender bottle, silk pad, 

 silk thread, cork or pith, sealing wax, rubber ruler or comb, flannel 

 pad or fur muff. 



a. Hold a clean, dry glass rod (or a slender bottle) by one 

 end, and rub the other end vigorously with a pad of silk cloth. 

 Bring your knuckle near the rubbed part of the glass, but do 

 not let it touch the glass. What occurs? Do the same with 

 the rubbed part of the silk pad, and tell what happens. 



6. Make an " electric pendulum" by attaching a silk thread 

 to a piece of dry cork about 5 mm. in diameter. Instead of 

 cork you can use the pith found inside the stem of a burdock. 



Suspend the pendulum from a ring stand, a shelf, or a bracket; 

 then rub the glass and silk together, and hold the glass near 

 the pendulum. What happens at first? What further change 

 takes place? Now hold your hand near the charged pendulum; 

 what happens? 



