374 Introduction to the Study of Science 



Queen Elizabeth, and illustrated by an interesting woodcut in 

 a book published by him in the year 1600. The quaint picture 

 shows a blacksmith holding a bar of iron in alignment with the 

 meridian, north and south, and hammering it vigorously. The 

 iron bar is thereby magnetized. It was known also at that time 

 that a bar of cooling iron would become magnetized without 

 hammering, but that the process could be hastened by hammer- 

 ing. The same experiment may be performed to-day to illus- 

 trate the important facts. 



Exercise : Magnetizing by induction from earth. You will need a 

 compass, a hammer, and a piece of iron pipe or rod about three feet 

 long. If the iron has been lying about for some time, it may be mag- 

 netized. It should be tested for magnetism by holding each end suc- 

 cessively near the north-seeking pole of the compass. Observe the be- 

 havior of the needle. If the iron attracts the north needle at each 

 trial, it is not magnetized. 



Now hold the iron in the meridian, that is, north and south, with 

 the north end pointing a little below the horizon. Hammer it vigorously 

 for a moment. Test the ends of the iron by holding each successively 

 near the north-seeking pole of the compass. Note the effect of each 

 end of the iron on the needle. Does the end of the iron which was 

 pointed north attract or repel the north-seeking pole of the compass ? 

 How does the needle behave in relation to the south end of the iron? 



Reverse the iron, and holding it in the meridian as before, strike it 

 several sharp blows. Test each end again, as above. Compare the 

 results with those of the former series of tests. Is the magnetic in- 

 fluence of the magnet changed? How can you account for the facts 

 observed ? 



The earth a magnet. In 1600 William Gilbert explained 

 the facts observed in the experiment by the theory that the 

 earth is a huge magnet. He mentions the now familiar .fact 

 that an iron rod suspended horizontally and with an inclination 

 like that of the magnetic needle, pointing, that is, toward the 

 magnetic north pole, becomes magnetized in course of time 

 by the earth. This is done in the same way in which a horse- 

 shoe or a bar magnet magnetizes a needle or a nail. If we wish 

 to magnetize quickly a piece of steel, such as a needle, we stroke 



